POLITECNICO DI MILANO Facolt a di Ingegneria Civile, Ambientale … · 2013. 3. 5. · reazione di...

POLITECNICO DI MILANO

Facoltà di Ingegneria Civile, Ambientale e Territoriale

Corso di Laurea Specialistica inIngegneria per l’Ambiente e il Territorio

Engineering Transition Metal Borohydridesfor Energy Storage and Conversion

Relatore: Prof. Giovanni Dotelli

Laureando: Matteo Zattimatr. 721973

Anno Accademico 2010 - 2011

Engineering Transition Metal Borohydridesfor Energy Storage and Conversion

Matteo Zatti

Material Research Division

Risø - National Laboratory for Sustainable Energy

Technical University of Denmark

Roskilde 2011

Author

Matteo Zatti

E-mail: [email protected]

Supervisors

Tejs Vegge

Material Research Division - Risø - DTU


Didier Blachard

Material Research Division - Risø - DTU


Risø - National Laboratory for Sustainable Energy

Technical University of Denmark

Frederiksborgvej, 339

DK-4000, Roskilde

Denmark

www.risoe.dtu.dk


Phone: +45 4677 4677

Fax: +45 4677 5688

Title page illustration: Li2CdCl4 – 2 × 2 × 2 cell. Li in tetrahedral sites shaded in red. Li

and Cd distributed among the octahedral sites in green. Cl in blue.

One does not discover new lands

without consenting to lose sight

of the shore for a very long time.

André Gide

Abstract

Hydrogen storage properties of the mixture LiBH4 + CdCl2 were studied. The

samples were synthesized using planetary ball milling and characterized by x-ray

powder diffraction. The thermal desorption was investigated using a Sieverts

equipment and mass spectrometry. The main aim was to see if the LiBH4 de-

sorption temperature could be lowered by destabilizing the compound, and if the

dehydrogenation was reversible. The main results are that a new phase, with a

proposed nominal composition Cd(BH4)2, for which no available structural data

were found in the literature, was produced; this new phase releases mainly H2 at

temperatures of 90-105 ◦C. The rehydrogenation with 90 bar and 110 ◦C for 18

hours showed signals of absorption, but higher pressure and temperature should

be applied to fully test the reversibility of the dehydrogenation. This could ma-

ke this mixture interesting for usage as hydrogen storage media for transport

application coupled with fuels cells.

Resumé

Man har undersøgt brintlagringsegenskaberne hos blandingen LiBH4 + CdCl2.

Prøverne blev syntetiseret ved brug af en kuglemølle og var karakteriseret af x-ray

pulverdiffraktion. Den termiske desorption blev undersøgt ved hjælp af Sieverts

udstyr og massespektrometri. Hovedform̊alet var, at se om LiBH4-desorptions-

temperaturen kunne sænkes ved destabilisering af stoffet, og om afbrintning var

reversibel. De vigtigste resultater var, at der produceredes en ny fase med en fo-

resl̊aet nominel sammensætning Cd(BH4)2, hvis struktur der ikke er fundet data

p̊a i litteraturen. Denne nye fase afgiver hovedsageligt H2 ved temperaturer p̊a

90-105 ◦C. Afbrintning med 90 bar og 110 ◦C i 18 timer viste tegn p̊a absorption,

men der bør anvendes højere tryk og temperatur for at kunne teste afbrintningens

reversibilitet til fulde. Ovennævnte kan gøre denne blanding interessant til an-

vendelse som brintlagringsmedie ved transport kombineret med brændselsceller.

Sommario

Questa tesi constituisce il completamento della doppia laurea all’interno del pro-

gramma T.I.M.E., in conformità all’accordo tra il Politecnico di Milano e la Tech-

nical University of Denmark. Il lavoro di tesi è stato svolto presso la Material

Research Division del Risø DTU – National Laboratory for Sustainable Energy.

Il lavoro è costituito da un’indagine sperimentale nel campo dell’immagazzina-

mento dell’idrogeno. Il sistema studiato è LiBH4 + CdCl2, utilizzando le tecniche

di planetary ball milling, la spettroscopia ai raggi X, il Sieverts equipment e la

spettrometria di massa. La tesi è strutturata nel modo seguente:

Capitolo 1: vengono espresse le motivazioni per una ricerca nel campo del-

l’immagazzinamento dell’idrogeno. Il riscaldamento globale e l’incombente esau-

rimento dei combustibili fossili richiedono lo sviluppo di fonti di energia rinno-

vabili che non emettano gas serra. L’idrogeno è un vettore energetico che puó

ricoprire un ruolo fondamentale in questa ricerca. Esso può essere prodotto,

sfruttando le energie rinnovabili, attraverso l’elettrolisi dell’acqua. Quando viene

usato, ad esempio in una cella a combustibile, l’unico prodotto della reazione è

ancora l’acqua. Considerando l’utilizzo dell’idrogeno nel settore dei trasporti, l’a-

spetto più critico è l’immagazzinamento. Per garantire lo stesso raggio d’azione di

una vettura alimentata a combustibile tradizionale, il Dipartimento dell’Energia

statunitense ha definito i requisiti che un sistema di immagazzinamento dell’idro-

geno a bordo vettura deve soddisfare. Tra queste, spicca la temperatura, attorno

ai 100 ◦C, cui l’idrogeno deve essere rilasciato, temperatura di funzionamento

delle celle a combustibile PEM. Varie tecnologie vengono studiate e sviluppate

vi Sommario

al fine di soddisfare tali requisiti, ma la soluzione più promettente è costituita

dall’immagazzinamento dell’idrogeno in composti chimici allo stato solido.

Capitolo 2: viene fornito il background teorico per l’indagine sperimentale

condotta. Il composto studiato in questa tesi è il LiBH4. Esso appartiene alla

categoria degli idruri metallici complessi, in cui l’idrogeno è chimicamente lega-

to ad altri elementi per formare un composto che rilascia l’idrogeno in seguito

a decomposizione termica. Il LiBH4 è caratterizzato da un elevato contenuto

di idrogeno, pari al 18.4 wt%, ma è noto per rilasciare l’idrogeno tra i 380 ◦C

e i 500 ◦C, temperature troppo elevate per applicazioni automobilistiche. Per

poter essere utilizzato, il litio boroidruro necessita di essere destabilizzato. La

strategia di destabilizzazione seguita in questo progetto si base sulla relazione

di proporzionalità inversa esistente tra l’entalpia di formazione dei reagenti della

reazione di deidrogenazione e l’elettronegatività del metallo contenuto nel com-

posto. Facendo reagire il LiBH4 con un alogenuro metallico in cui il metallo abbia

un’elettronegatività maggiore del Li, è possibile rendere la natura del legame con

il gruppo BH4 meno ionica.

Capitolo 3: vengono descritte le procedure sperimentali adottate nello studio

del sistema LiBH4 + CdCl2. I campioni, in quanto sensibili all’ossigeno e all’umi-

dità, vengono sempre maneggiati all’interno di una glove-box riempita di Argon.

La ball milling, eseguita per mezzo di una Fritsh Pulverisette 6, ha lo scopo di

promuovere la reazione meccanochimica allo stato solido tra il LiBH4 ed il CdCl2.

I reagenti vengono posizionati all’interno di un recipiente in acciaio inossidabile

insieme a delle sfere di carburo di tungsteno. Le condizioni utilizzate sono 650

rpm con un Ball-to-powder ratio (Bpr) di 197:2 e 400 rpm con un Bpr di 158:2.

Durante il processo di milling, uno speciale coperchio permette di registrare i dati

di temperatura e pressione all’interno del recipiente. Un altro coperchio speciale

permette di analizzare tramite spettrometria di massa i gas liberati durante il

processo.

La spettroscopia a raggi X serve a caratterizzare i campioni ottenuti nel processo

di ball milling. La geometria adottata per gli esperimenti di diffrazione è quella

di Bragg-Brentano. Lo spettro di diffrazione prodotto permette di identificare le

fasi presenti nel campione. Il diffrattometro utilizzato è un Bruker D8 (voltaggio

40 kV e corrente 40 mA) con una radiazione Cu Kα e lunghezza d’onda di 1.542

Å. Viene utilizzato un Linx-Eye detector. Le misure vengono condotte nell’inter-

vii

vallo 10◦≤ 2θ ≤ 65◦, con un incremento di 0.04◦ e un’esposizione di 4 sec.Le proprietà di assorbimento e desorbimento dei campioni vengono studiate con

il metodo Sieverts. I boroidruri, infatti, rilasciano l’idrogeno in seguito a decom-

posizione termica. Un sistema chiuso, come il Sieverts, permette di registrare la

corrispondente variazione di pressione. Il processo viene esaminato grazie ai dati

di temperatura e pressione registrati simultaneamente nel sistema. Negli esperi-

menti di deidrogenazione, i campioni vengono scaldati da temperatura ambiente

fino a 500 ◦C, con un incremento di 1 ◦C/min, in un’atmosfera di 1 bar di He.

Negli esperimenti di reidrogenazione, la massima pressione di idrogeno raggiun-

gibile nel sistema viene applicata (90 bar) e la temperatura viene tenuta costante

al valore selezionato (110 ◦C).

La spettrometria di massa viene usata per analizzare i gas che si formano duran-

te il processo di ball milling e in seguito a decomposizione termica dei cam-

pioni. Lo spettrometro di massa utilizzato in questo progetto è un Pfeiffer

OmniStarTMGSD 320 O1, con un intervallo di 1-100 amu, dotato di un filamento

di tungsteno per la ionizzazione ed un analizzatore a quadrupolo QMA 200 M.

Le specie comprese nell’analisi sono i principali componenti dell’aria (utile per

verificare la presenza di perdite), H2, B2H6 più Cl e HCl.

Capitolo 4: vengono presentati e discussi i risultati ottenuti. L’indagine spe-

rimentale parte dall’ottimizzazione del processo di sintesi del sistema LiBH4 +

CdCl2. 8 ore di ball milling a 650 rpm con un Bpr 197:2 risultarono eccessive,

alla luce della cospicua quantità di gas rilasciato durante il processo. Il processo

di milling viene studiato fino a 4 ore, grazie alla caratterizzazione ai raggi X di

una parte del campione dopo 30 min, 1 h, 2 h, 3 h e 4 h di milling. Dopo 30

minuti si distingue la formazione di una nova fase, la quale inizia a degradarsi già

dalla prima ora di milling in poi. La comparazione degli spettri con i database

ICSD e ICDD - PDF non fornisce alcuna corrispondenza per la nuova fase. Lo

studio della reazione a condizioni di ball milling più moderate (400 rpm, Bpr

158:2) rivela la sintesi della nuova fase già dopo 30 min. In caso di macinazione

manuale dei reagenti, viene verificata l’impossibilità di promuovere la formazione

della nuova fase in seguito ad attivazione termica nel Sieverts.

La reazione di sintesi proposta è una reazione di metatesi che porta alla forma-

zione di Cd(BH4)2. Questa composizione è confermata dalla mancanza di LiBH4nei campioni in seguito a ball milling, mentre sono presenti LiCl e CdCl2. I si-

stemi LiBH4 + 0.11CdCLl2, LiBH4 + 0.2CdCl2, Li2CdCl4 + LiBH4 e xLiBH4 +

CdCl2 + (1− x)LiCl, con x = 0.5 e 1, confermano la formazione della medesima

viii

fase. Questa fase è caratterizzata da un rilascio piuttosto netto a 90 - 105 ◦C, in

sintonia con i requisiti del DOE.

Lo studio della reazione di deidrogenazione, effettuando caratterizzazione XRD

dei campioni a diverse temperature durante il desorbimento (80 ◦C, 100 ◦C e

160 ◦C) conferma la reazione di sintesi proposta alla luce del rilascio di Cd puro

parallelamente alla decomposizione della nuova fase.

L’analisi di spettrometria di massa dei gas rilasciati in seguito a decomposizione

termica attesta la presenza di diborano insieme all’idrogeno.

L’esperimento di reidrogenazione rivela che le condizioni applicate (90 bar di H2 a

110 ◦C per 18 h) non sono sufficienti per osservare una significativa ri-formazione

della medesima nuova fase. I deboli segnali di diminuzione del contenuto di Cd nel

campione in seguito a reidrogenazione suggeriscono che pressione e temperature

più elevate porterebbero a un’apprezzabile ricomposizione del Cd(BH4)2.

Capitolo 5: nelle conclusioni della tesi vengono presentate le prospettive per

ulteriori studi del sistema LiBH4 + CdCl2. Tra di esse sarebbe particolarmente

interessante variare il rapporto stechiometrico tra i due reagenti, in modo da

verificare la possibile formazione di un boroidruro multicationico, in grado di

regolare la stabilità del composto risultante. Analisi XRD in situ della reazione

di deidrogenazione permetterebbero di conoscere dettagliatamente il percorso di

desorbimento. Una back-pressure di pochi bar di H2 durante la deidrogenazione

limiterebbe la formazione di B2H6. Infine, ulteriori esperimenti di reidrogenazione

dovrebbero prevedere l’applicazione di pressione e temperatura più elevate.

Preface

This thesis was prepared at the Material Research Division at the Risø - Na-

tional Laboratory for Sustainable Energy - Technical University of Denmark in

fulfillment of the requirements for acquiring the M.Sc. in Sustainable Energy,

Hydrogen and Fuel Cells study line.

THis work will be presented both at the Technical University of Denmark and

at Politecnico di Milano, as provided for in the agreement for the Double Degree

program T.I.M.E..

I would like to thank the institutions and the individuals that I met along the

way and helped me in this challenge. I am grateful to Didier Blanchard and Tejs

Vegge, my supervisors, for their patient and inspiring guidance. Every discus-

sion was prolific and brought about fundamental ideas for this work. I sincerely

thank Dadi Sveinbjörnsson and Jan Kehres for their assistance with the labo-

ratory equipments, and all the people in the research group for their help and

the stimulating environment. Thanks also to Lars Lorentzen for helping on var-

ious practical things in the labs. Thanks to Lene Danielsen for being one of the

kindest persons at Risø. Thanks to Silvia, for her brilliant translation.

Of course, thanks to my family, for their long-distance support and love, and a

special thank to Gloria, for her joyful presence and love.

Frederiksberg, 21-July-2011

Matteo Zatti

Contents

Abstract i

Resumé iii

Sommario v

Preface ix

1 Introduction 1

1.1 Climate and Energy Situation . . . . . . . . . . . . . . . . . . . . . 1

1.2 Renewable Energies . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 Hydrogen Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Background 9

2.1 Lithium borohydride: stability and thermodynamics . . . . . . . . 9

2.2 Destabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3 Aims of the Project . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3 Experimental 15

3.1 Ball Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.1.1 Experimental Procedure for the Synthesis . . . . . . . . . . 16

3.1.2 Pressure and Temperature during the Ball Milling . . . . . 18

3.1.3 Estimation of the Gas Release . . . . . . . . . . . . . . . . 20

3.2 Powder Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2.1 Lattice structure and Bragg’s Law . . . . . . . . . . . . . . 21

3.2.2 The Diffraction Pattern . . . . . . . . . . . . . . . . . . . . 22

3.2.3 Experimental Procedure . . . . . . . . . . . . . . . . . . . . 24

3.3 The Volumetric Sieverts Equipment . . . . . . . . . . . . . . . . . 25

xii CONTENTS


3.3.2 Calculation of the Gas Desorption . . . . . . . . . . . . . . 28

3.3.3 Calibration of the System . . . . . . . . . . . . . . . . . . . 29

3.4 Mass Spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . 30


3.4.2 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4 Results and Discussion 35

4.1 Synthesis Optimization . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.1.1 8 Hours Ball Milling . . . . . . . . . . . . . . . . . . . . . . 35

4.1.2 4 Hours Ball Milling with Steps . . . . . . . . . . . . . . . . 36

4.1.3 Milder Conditions: 400 rpm - Bpr 158:2 . . . . . . . . . . . 40

4.1.4 Hand-Ground Sample . . . . . . . . . . . . . . . . . . . . . 44

4.2 A Reasonable Reaction . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.2.1 Doping effect? . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.2.2 Partial Anion Substitution . . . . . . . . . . . . . . . . . . 52

4.2.3 Cadmium Borohydride Structure . . . . . . . . . . . . . . . 59

4.2.4 The Other Possible Phases . . . . . . . . . . . . . . . . . . 60

4.3 The Desorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.3.1 Gas Composition of the Release . . . . . . . . . . . . . . . . 63

4.4 Rehydrogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

5 Conclusion and Outlook 73

A XRD Patterns 77

A.1 XRD of the cadmium chloride and the lithium borohydride used

in this work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

A.2 Some selected XRD patterns. . . . . . . . . . . . . . . . . . . . . . 80

Bibliography 87

Chapter 1

Introduction

The world energy situation together with the protection of the environment are

raising some challenges for our and next generations. The development of renew-

able energies is apparently the only way to maintain the current lifestyle of the

western countries and to ensure a sustainable development for the others. In this

framework, energy storage is going to play a pivotal role.

1.1 Climate and Energy Situation

Today, a vast majority of scientists agree on the existence of a direct connection

between the global rising of temperature and the increasing concentration of

Green House Gases (GHGs) in the atmosphere, as can be seen in figure 1.1 and 1.2

on the following page. The consensus is also on the prime role of the human

activities in the observed climate change since the post industrial revolution [1]. If

the temperature predictive models are confirmed, the global warming will change

the surface of the Earth, many inhabited costal areas will be submerged by the

rising ocean levels and a great part of the ecosystems, upon which the mankind

rely for its sustainment, will be corrupted. If the present and the future generation

want to keep the quality of the life as it is today and not to verify the accuracy

2 Introduction

Figure 1.1: The trend of the Earth surface temperature is clearly increasing inthe last 150 years, after the industrial development started increas-ing dramatically the burning of fossil fuels. Figure taken from [1]

Figure 1.2: The concentration of CO2 in the atmosphere has been estimated fora period of thousands of years. An enormous increase has beendetected for the last 150 years (Inset panel). A correlation with theincreasing temperatures has been proved. Figure taken from [1, 2]

1.1 Climate and Energy Situation 3

of these models, an intense effort needs to be done to promote GHGs-free energy

resources.

In addition to these humanitarian concerns, there is another relevant issue: the

fossil fuels are not renewable resources, at least on the human time scale. In fact,

the modern society is burning them much faster than the geological processes

required to build them up. Considering oil, the most important among the dif-

ferent fossil fuels for the contemporary mankind, the Hubbert peak theory [3]

predicted that future world oil production must reach a peak and then decline,

as the reserves are exhausted. It is difficult to predict precisely when the oil

peak is going to occur since there are many erratic factors to be considered, as

the economical trends and the actual global oil reserves. Indeed, many incorrect

prediction has been done until now [4]. Anyway, the intrinsic finitude of this

resource, implies that after the peak is reached, the oil economy – and thus the

whole economy – will be in serious difficulties: in economical terms the rigidity1

of the oil demand will lead to skyrocketing prices. People will have to find al-

ternative energy sources, with the risk of not finding them on time. Common

sense would suggests to look for a replacement of oil, but to date there is not a

unique source capable of covering, limitless, the energy demand. Even nuclear

energy, without mentioning the safety problems inherent to its utilization, does

not represent a definitive solution, since fission materials are limited in stock as

well. Recapitulating, it seems very clear that it is necessary to develop solutions

with two main characteristics:

• no emissions of GHGs in order to avoid the aggravation of the climatechange

• fossil fuel independency

Probably the time for the beginning of an energy revolution has come and the

Sun is likely to play an even more central role for us than ever.

1The rigidity of the demand of a product defines the willingness to pay for that product anygiven price. Oil is a product people will hardly accept to live without, because of the centralrole it plays in the modern society.

4 Introduction

1.2 Renewable Energies

All the energy consumed on Earth is provided by the Sun. The man is able

to harness it either directly, converting the light photons in electricity via the

photo voltaic technologies, or indirectly, through, for instance the fossil fuels or

the wind. A back-of-the-envelope calculation, indicates that the Sun is gifting

us with more than what we need. The solar constant (the intensity at the top

of the atmosphere) is 1366 W m−2 [5]. Assuming that half of it reaches Earth

surface [6] and half of the time is night, in one year one square meter receives

more than 1 × 1010 J. Accounting only for the total area covered by land, it isroughly 1.6× 1024 J year−1, the triple if also the oceans are considered, whereasthe worldwide primary energy consumption is 5× 1020 J year−1 [7].Wind and solar energy are the most attractive way of exploiting this abundance:

they do not emit GHGs during the conversion of energy and they are fossil fuel

free. But they are clearly subjected to the weather condition, therefore their

output is inherently fluctuating. Clearly, if they are going to cover a big share of

the energy production, there is the need to store the energy they produce for the

time periods when their electricity production is not able to match the demand.

Hydrogen could be the missing piece in the puzzle. Hydrogen is defined

as an energy carrier and not as an energy source. It is not available in nature

but has to be anthropogenically produced in order to be technically available.

Though, it is still very promising: it can be produced independently from fossil

fuels and does not emit GHGs when utilized, both if burned in internal combus-

tion engines (ICE) and if used in fuel cells.

Hydrogen can be considered a renewable fuel only only if it is directly produced

from solar light or indirectly through electricity generated by a renewable source,

for example wind power or concentrating solar plants. This way hydrogen be-

comes also an energy storage media, hence representing a solution for the discon-

tinuous generation that characterizes the renewable energy production technolo-

gies.

Electricity from a renewable energy sources can produce hydrogen through elec-

trolysis of water. The state of the art in this field is based, on alkaline electrol-

ysers, characterized by an efficiency of approximately 82% [4]. There is also a

lot of promising research in the direction of high-temperature electrolysis, based

on Solid Oxide Fuel Cells (SOFC), in the attempt to enhance the efficiency of

1.3 Hydrogen Storage 5

the conversion. the production from electrolysis, makes hydrogen “recyclable”:

in fact, in the production process, water is split and oxygen is liberated in the

atmosphere, then, in the conversion to energy, hydrogen recombines with oxygen,

becoming water again. This has been called the hydrogen cycle [4].

Hydrogen is characterized by a high chemical energy per mass, 141 MJ kg−1,

compared to liquid hydrocarbons 47 MJ kg−1 [8], making it very interesting for

applications in the transport sector. Yet, at room temperature and atmospheric

pressure, 1 kg of hydrogen occupies a volume of 11 m3 kg−1 [4], which makes its

usage quite problematic. In order to guarantee a driving range comparable to

gasoline, it must be found a way to store hydrogen efficiently.

1.3 Hydrogen Storage

There are different technical methods to store hydrogen: pressure cylinders, liq-

uid hydrogen, physisorbed hydrogen, ammonia, and solid state hydrogen storage.

All of them try to increase as much as possible the gravimetric and the volumetric

hydrogen density.

The most common storage systems, to date, are the high-pressure gas cylin-

ders, with a maximum pressure of 20 MPa. The development of new lightweight

composite cylinders, capable of supporting 80 MPa, has brought their hydrogen

volumetric density to 36 kg m−3, half as much as in the liquid form. Regarding

this technology, it must be considered that the compression of hydrogen requires

energy (reaching 80 MPa requires roughly 6-7% of the energy stored per kg) and

high pressure vessels threaten the security of on-board applications and densely

populated areas [9].

Liquid hydrogen is stored in cryogenic tanks at -250◦C, requiring a lot of energy

for the liquefaction process (more than 30% of the energy stored per kg); more-

over the continuous boiling-off of the hydrogen is hardly avoidable, hampering

the possible applications of this system [4].

Recent development in nanoscale engineering yielded a series of high-surface-area

materials such as nanoporous scaffolds, carbon nanotubes, etc. able to absorb gas

molecules on their surface by the so called van der Walls interactions. Owing to

these weak interactions, physisorption is observed only at very low temperatures,

less than -170◦C, with also low amounts of adsorbed hydrogen (around 2-4wt%)

[4, 9].

6 Introduction

Ammonia, besides being a potential fuel for the transport sector, is a very in-

teresting hydrogen carrier, due its high gravimetric hydrogen density 17.6wt%.

There exist serious concerns about its toxicity, which, actually, falls if it is used in

the form of metal ammines, like Mg(NH3)6Cl2. Also the current methane-based

production, based on the Haber-Bosch process, is another issue to be solved for

reaching the fossil fuel independency, together with the energy required to liber-

ate the hydrogen from the NH3 molecule. Nevertheless, the encouraging research

on the fundamentals of the process and the seek for optimal catalysts make it a

promising solution2 [10, 11].

Solid state hydrogen storage is represented by metal hydrides and complex hy-

drides. Metal hydrides are also called interstitial hydrides, since the hydrogen

atoms, upon reaction with the metal, intermetallic compound or alloy, occupy

the interstitial sites of the lattice structure. Unfortunately, the metal hydrides

working around ambient temperature and atmospheric pressure are based on

transition metals, hence their gravimetric hydrogen density is limited to less

than 2wt%. Exploring the properties of lightweight metal hydrides is still a chal-

lenge [4]. Complex hydrides, instead, offer high gravimetric hydrogen density.

Categories of compounds such as the alanates (M +x[AlH−4 ]x) and the borohy-

drides (M +x[BH−4 ]x), are characterized by the highest H2 weight percentage, e.g.

LiAlH4 with 10.5wt% and LiBH4 with 18.4wt%. However, they are know to de-

compose and release the stored hydrogen only at elevated temperatures, and the

reversibility of the process is frequently hampered by low kinetics and requires

high pressures. Therefore a material engineering effort is needed to meliorate

their hydrogen storage properties towards the achievement of: high gravimetric

and volumetric hydrogen density, fast desorption and absorption kinetics at rela-

tively low temperatures, high reversibility in terms of number of ab-/desorption

cycles, low cost, purity of the released hydrogen and safety. The criteria to be

fulfilled by an on-board system3 using such materials has been set by the U.S.

Department of Energy (DOE), and revised in 2009. The values can be seen in

table 1.1 on the next page.

Finding or producing such a material is definitely arduous, indeed a lot of efforts

has been spent in this quest, but when a compound satisfies one of the criteria,

frequently is far from achieving the others, as mentioned for the borohydrides.

Anyway, this is a good reason to keep on with the research efforts, since some

2There would be a lot to say about ammonia and metal ammines, but it would be out of thescope in this work.

3Note that a system requirement is inherently much stringent than for the pure material,since it comprise all the devices apt to the functioning.

1.3 Hydrogen Storage 7

2015 Ultimate Full Fleet

ρm 5.5wt% or 1.8 kWh kg−1 7.5wt% or 2.5 kWh kg−1

ρv 40 g L−1 or 1.3 kWh L−1 70 g L−1 or 2.3 g L−1

fueling rate 1.5 kgH2 min−1 2 kgH2 min

−1

cycles 1500 1500

costs $2 kWh−1 or $67 kg−1H2—

max Td 85◦C 95-105◦C

Table 1.1: DOE targets for hydrogen storage systems for 2015 and for the fullfleet (end target) for the gravimetric hydrogen density ρm, volumetrichydrogen density ρv, the fuelling rate, the number of cycles, the costsand the maximum delivery temperature for an on-board hydrogenstorage system in a light-duty vehicle. Data taken from [12].

of the requirements are already satisfied and all that is needed is to tailor the

others.

This work is focused on LiBH4 and the engineering of its hydrogen storage prop-

erties.

8 Introduction

Chapter 2

Background

Among the borohydrides, LiBH4 is one of the most promising alkali metal boro-

hydrides thanks to its high gravimetric density (18.4 wt%). The unfavorable ther-

modynamic properties of this salt, however, hinder its direct use in the transport

sector. Therefore it is necessary to engineer its characteristics in order to make

it usable.

2.1 Lithium borohydride: stability and thermodynam-

ics

LiBH4 is a salt-like, hydroscopic, crystalline material with a reported melting

point of 275 ◦C [13] or 278 ◦C [14] and densities of 0.681 or 0.66 g · cm−3 at25 ◦C. At 0 ◦C its vapor pressure is much less than 10−5 mbar and the salt

neither decomposes nor sublimes. Its enthalpy of formation and entropy has

been measured and its heat capacity cp was determined from 15 to 303 K [15].

At 298.16 K the values are: ∆fH = −194.44 kJ · mol−1, S0 = 75.91 J · K−1 ·mol−1, cp = 82.60 J · K−1 · mol−1.

First-principle calculations on the electronic structure of LiBH4 [16] show that

10 Background

a boron forms sp3 hybrids and covalent bonds with four surrounding H atoms,

located on the vertexes of a tetrahedron, to form [BH−4 ] anions. The charge is

compensated by the Li+ cations. The electronic structure points out that the

charge transfer from the Li+ to the [BH−4 ] plays a key role in the stability of

LiBH4, therefore defines its decomposition temperature.

The thermal decomposition of LiBH4 has been intensively investigated, e.g. in[17,

14, 18, 19]. Upon incremental heating, besides minor releases corresponding to the

polymorphic transformation at 108 ◦C and the melting at 277 ◦C [20], the decom-

position starts at temperatures above 380 ◦C desorbing hydrogen up to around

500 ◦C (the final temperature might depend on the experimental conditions, e.g.

the heating rate, the sample preparation, etc.). The complete dehydrogenation

reaction can be sketched as in equation 2.1, where the first step liberates 13.77

wt% (referred to LiBH4 molar mass) of the hydrogen, in the temperature range

mentioned above. The decomposition of LiH liberates the remaining 4.59 wt.%,

but it occurs only at temperatures above 727 ◦C [21].

LiBH4 −−→ LiH + B + 32 H2 −−→ Li + B + 2 H2 (2.1)

The change in enthalpy (∆H) and entropy (∆S) for the first step of equation 2.1

were found to be 74 kJ · mol−1H2 and 115 J · K−1 · mol−1H2 , from the pressure-

concentration-temperature (pcT) isotherm measurements, in the temperature

range of 410 - 520 ◦C [19], and the application of Van’t Hoff equation 2.2.

ln

(peqp0eq

)=

∆H

RT− ∆S

R(2.2)

where peq is the equilibrium pressure, p0eq is the reference pressure and R is the

gas constant.

The dehydrogenation of LiBH4 is reversible, provided that no diborane (B2H6) is

released1. LiBH4 can be formed from the end products, LiH and B, when exposed

to hydrogen at high temperature and pressure, for example at 600 ◦C under 35

MPa of H2 for 12 h of H2 [22]. The sluggish mechanism of the formation of LiBH4from LiH and B is still unclear and various paths have been proposed [21], but the

most trustworthy is based on the direct recombination of the dehydrogenated LiH

and B. It requires the breaking of the boron lattice and the subsequent diffusion

1Some evidence of diborane release during LiBH4 decomposition has been reported [21]. Ifsome boron atoms leave the compound, the process loose its reversibility over the cycles.

2.2 Destabilization 11

of the two species towards each other to form an intermediate state. Among

the different candidates, Li2B12H12 has been pointed out as the most probable

intermediate, because of its stability [23, 24]. This intermediate is then filled up

with hydrogen.

Clearly, LiBH4 is too stable for dehydrogenation/hydrogenation cycles on on-

board applications and its thermodynamics must be tailored. Approaching the

problem from the Gibbs free energy point of view, the favorableness of the reac-

tion is largely defined by the enthalpy change. In fact, in equation 2.3

∆G = ∆H − T∆S (2.3)

the entropy change ∆S mainly comes from the production of the gaseous hydro-

gen2 (i.e. S0H2= 130 J K−1 mol−1H2

). ∆H is calculated as the difference between

the products and the starting material. Thus, two main approaches can be used

to reduce ∆H: the first is to destabilize the starting material, the second is to

stabilize the products of the dehydrogenation3. In this work, the first approach

is adopted.

2.2 Destabilization

The destabilization of the reactants in the thermal desorption starts from the

assumption that, as already specifically mentioned for LiBH4, the charge transfer

between the cation and the borohydride group is a key element in the thermody-

namic stability of the material. It has been identified and tested [27, 28] that the

enthalpy of formation4 ∆fH, hence the desorption temperature, is inversely pro-

portional to the Pauling electronegativity, χp of the atom M in MBH4. The higher

χp of the atom is, the less the bond with the BH4 group is of ionic character. As

the nature of the bond becomes more covalent, the compound is less stable. In

this framework, with LiBH4 as starting material, four different “sub-strategies”

2This is true for intermetallic hydrides, but for complex hydrides the situation is somewhatmore intricate since it depends on the compound. Thus, on the basis of theoretical calculations[25], it can be estimated that 100 ≤ ∆S ≤ 130 J · K−1 · mol−1H2 .

3It has been reported that the mixture LiBH4 + MgH has a 30◦C lower desorption temper-

ature due to the exothermic formation of MgB2. It has also been referred that the mixture hasan enhancing effect on the rehydrogenation kinetics [26].

4The formation enthalpy gives an indication on the energy required for the decomposition∆decompH thanks to the relation ∆decomp = ∆fHproducts − ∆fHstarting material.

12 Background

can be outlined:

1. Partial substitution of the Li atoms by some more electronegative metals,

as theoretically proposed by Miwa et al. in [29] and experimentally verified

by Au et al. [30].

2. Complete substitution of the Li atoms by metals with the desired χp. Start-

ing from LiBH4 it is in principle possible to obtain the suitable M(BH4)n

through metathesis reactions such as LiBH4 + MCln −−→ M(BH4)n + nLiCl[27, 31]. This is, however, limited by the difficulty to adjust the electroneg-

ativity because of the discrete value of χp for a single cation. In fact, as

reported by Nakamori et al. [28], when χp ≥ 1.5 the M(BH4)n becomestoo unstable and considerable release of diborane occurs.

3. Synthesis of multication borohydrides, which would allow to tune pre-

cisely to the desired thermodynamic properties [32, 33]. In the case of

lithium borohydride it would be LixMy(BH4)n. Some of these multications

borohydrides have already been sinthesized, including LiSc(BH4)4 [34] and

LiZn2(BH4)5 [35], which display onsets of hydrogen desorption respectively

around 177 ◦C and 127 ◦C5. These so called mixed borohydrides can be

obtained by high energy ball milling of LiBH4 together with the desired

metal halides.

4. Synthesis of mixed-anion and mixed-cation borohydride, mixing LiBH4 with

the desired metal halide, similarly to what has been reported in literature

for KZn(BH4)Cl2 [36], produced via the addition reaction

KBH4+ZnCl2 −−→ KZn(BH4)Cl2 which is found to have a significantly lowerdecomposition temperature than KBH4 (110-130

◦C instead of 500◦C).

2.3 Aims of the Project

In this work the mixture investigated is CdCl2 + LiBH4. From this mixture, in

the literature, it is reported the formation of the following products: Cd(BH4)2,

Li2Cd(BH4)4, ClCd(BH4) and LiCdCl(BH4)2 [37]. Among these compounds, the

considerably large Cd electronegativity χp = 1.7, according to the theoretical pre-

dictions, would make Cd(BH4)2 probably too unstable. Nevertheless, it would

5In case of LiZn2(BH4)5 the release of B2H6 has been detected in the decomposition process.

2.3 Aims of the Project 13

be worth to investigate its hydrogen storage properties, also in light of the not

precise information available in the literature [10].

Li2Cd(BH4)4 could open the possibility to tune properly the thermodynamic

properties of the material, if it turns out that it is possible to adjust the ratio

between the cations in the compound.

As a variation of this option, it must be considered also the possibility of forma-

tion of a Li2Cd(BH4)xCl4−x phase, imaginable through an anion substitution in

the Li2CdCl4 phase with the borohydride groups from LiBH4. The resulting com-

pound would be really interesting for solid state ionic conduction applications, as

it is going to be discussed in section 4.2.2 on page 52.

The main goals of this work can be divided in two parts:

• Mechanically synthesize and characterize the mixture LiBH4 + CdCl2 us-ing x-ray diffraction. In particular, a good understanding of the reactions

taking place in the ball mill should be obtained.

• Study the ab-/desorption properties of the materials using a Sieverts equip-ment and mass spectrometry measurements. It is interesting to check if

the desorption temperatures are close to the working temperatures of the

PEMFC for onboard applications and if the materials can be reversibly

rehydrogenated.

14 Background

Chapter 3

Experimental

CdCl2 (no. 202908, ≥ 99.99%), LiBH4 (no. 686026, hydrogen storage grade≥ 90%) and LiCl (no. 62480, ≥ 98.0% ) were purchased from Sigma Aldrich(Fluka and Riedel Co.). The mixture LiBH4 + CdCl2 is mechanically milled to

promote solid state reaction between the compounds. In some experiments also

LiCl is used. X-ray diffraction is useful to characterize the mixture after milling

and to investigate the dehydrogenation and rehydrogenation of the compounds.

Volumetric Sieverts equipment and mass spectrometry are used to evaluate the

hydrogen storage properties of the mixture: hydrogen weight capacity, desorption

temperature and reversibility.

3.1 Ball Milling

The mixture LiBH4 + CdCl2 was milled in a planetary ball mill. The process is

sketched in figure 3.1 on the next page. The study of the process goes under the

discipline of mechanochemistry: solid state chemical reactions are promoted via

mechanical activation [38]. In general, in any form of mechanical activation, the

result is the formation of a stress field in the solid materials processed. Subsequent

relaxation of this stress field may proceed through different paths: formation of

16 Experimental

new activated surface, evolution of heat, creation of different kinds of defects,

initiation of a chemical reaction1 and so on. Which way(s) this relaxation take

will define the evolution of the process. During the mechanical treatment of

mixtures of solids using planetary ball milling, two main aspects of the solid

state reactions are predominant: the activation of the individual components

and the occurence of the interaction between the compounds. The mechanical

impulse may affect different factors: the change in the contact surfaces between

the two reactants; the conditions at the contact between the phases, as the local

increase in temperature and pressure, the local share deformations, the contact

fusion, etc.; the constant removal of the products from the reaction zone. The

role of these factors depends on the reactant properties and the conditions of

the mechanical activation. The latter factor, the energy put into the process, is

tunable by adjusting the revolutions per minutes of the ball mill, the number of

spheres and the duration of the process.

Figure 3.1: Schematical iconography of the ball milling process. Into the vial,the powder is placed together with some inert material spheres.

The first step of this work was to optimize the ball milling conditions in order

to get the maximum yield of conversion of the reactants without compromising

the final hydrogen storage properties. Part of this task was possible via the

monitoring of the pressure and temperature into the vial.

3.1.1 Experimental Procedure for the Synthesis

A Fritsch Pulverisette 6 (figure 3.2 on the facing page), was used to rotate a

stainless steel vial. The vial used has an inner volume of 250 ml and the rotational

speed applied in this work are 400 and 650 rpm, depending on the objective. In

1As it has been reported [39], the breaking of bonds in the crystalline lattice of solids leads toa decrease in the activation energy with respect to the activation energy of the non-disorderedsolid.

3.1 Ball Milling 17

all the cases, a 2 g sample was processed. The number of tungsten carbide spheres

was 20 in the case of 400 rpm and 25 in the case of 650 rpm, resulting in ball-to-

powder weight ratio (Bpr) of 158/2 and 197/2, respectively. The balls take up a

volume of 10.4− 13 ml in the vial. The milling time was adjusted depending onthe aim of the experiment.

(a)

(c)

(b)

Figure 3.2: (a) The Fritsch Pulverisette ball mill. (b) The vial with the speciallid for the temperature and pressure measurements. The data aresent to a computer through a wireless connection. (c) The vial withthe special lid endowed with the two valves for the analysis of thegasses released during the milling.

18 Experimental

3.1.2 Pressure and Temperature during the Ball Milling

A special lid was used together with the stainless steel vial, allowing for data

collection of the pressure and the temperature inside the vial during the milling.

The data are transfered via a wireless connection to a computer and recorded

as a function of time. The data provide indications on the reaction taking place

during the milling process. As mentioned above, the mechanical impulse during

the milling increases the probability for a chemical reaction to occur. The reaction

can be either exothermic or endothermic, thus affecting the temperature inside

the vial. If the reaction involves the formation of gases, the pressure in the vial

will increase. From the simultaneous measurement of T and p, it is possible

to estimate the amount of gas released and consequently get an idea about the

chemical processes going on during the milling.

Nevertheless, the rotation itself and the motion of the balls into the milling vial

of the spheres during the milling affect the temperature and the pressure inside

the vial. Indeed, the balls hit the walls of the vial and, in addition, exert a

dynamical pressure on the atmosphere inside the vial, as depicted in figure 3.3.

The latter phenomenon is noticeable at the beginning of the milling, when the

Figure 3.3: Detail of the ball milling process: the balls hit the walls of the vialwith a consequent increase of the temperature and “push” the at-mosphere inside the vial.

pressure signal steeply increases (see figure 3.4b on the facing page). This effect

is difficult to quantify. The former effect, conversely, can be quantitatively taken

into account considering the ideal gas equation 3.1

ptV = ntRTt (3.1)

3.1 Ball Milling 19

where n is the number of moles, R is the ideal gas constant and V is the inner

volume of the vial.

The pressure change ∆pt,thermal due to the temperature increase ∆Tt in the vial

can be subtracted from the measured pressure pt in order to account only for

the pressure changes due to the chemical reactions ∆nt. This correction can be

calculated revising equation 3.1:

pt,corrected = pt −∆pt,thermal = pt −n0R(Tt − T0)

V(3.2)

where n0 is the initial number of moles, calculated as

n0 =p0V

RT0(3.3)

inserting this relation into equation 3.2

pt,corrected = pt − p0Tt − T0T0

(3.4)

An example of the pressure thermal-correction can be seen in figure 3.4a and 3.4b

0 10 20 30 40 50 60 70Time [min]

28

29

30

31

32

33

tem

pera

ture

[°C]

LiBH4

(a) The temperature increase is less relevantthan in other harsher conditions appliedto different samples.

0 10 20 30 40 50 60 70Time [min]

99

100

101

102

pres

sure

[kPa

]

LiBH4 measured LiBH4 corrected

(b) The pt,corrected is almost flat, indicatingthat there is no gas released during theprocess.

Figure 3.4: Temperature, measured and corrected pressure for the ball millingof LiBH4. 400 rpm, 20 spheres, 70 min.

Another special lid was used, depicted in figure 3.2 on page 17. This lid is

equipped with two valves, kept closed during the milling and opened afterwards

to connect the vial to a mass spectrometer. The gas species eventually released

during the milling are analyzed.

20 Experimental

3.1.3 Estimation of the Gas Release

The compounds used in this work are LiBH4, CdCl2 and LiCl. Therefore the

eventual gasses released would comprise mainly H2 but, possibly, B2H6 as well,

as it has been demonstrated in the chemistry of boranes [40]. Some other chlorine

containing gas species could also be released. Even so, it was verified that the

main gas component liberated during the milling was H2 (see section 4.1.3 on

page 40 for the LiBH4 + CdCl2 mixture). Therefore, recalling the ideal gas

equation (eq. 3.1 on page 18) and the results from the previous discussion, the

hydrogen release during the milling can be estimated as follows:

nt,released = nt − n0 =V

R

(pt,corrected

Tt− p0T0

)(3.5)

where nt,released is the number of hydrogen moles released at time t, calculated

as the difference between the number of moles present at that moment and the

initial number of moles. The difference between the final and the initial point

of the milling gives the total number of moles released nreleased. The volume V

used in this calculation accounts also for the volume occupied by the balls and

the powder inserted at the beginning of the process. The hydrogen release, in

terms of the weight of LiBH4 in the sample – this will be the common reference

in this work to quantify the hydrogen storage capacity of a material – can be

calculated as:

H2wt% =nreleased ·MH2

mLiBH4· 100 (3.6)

where MH2 is the molecular mass of hydrogen gas and mLiBH4 is the total mass

of lithium borohydride inserted into the vial.

3.2 Powder Diffraction

The samples obtained by ball milling were characterized by x-ray powder diffrac-

tion (XRD). The Bragg-Brentano geometry, the most commonly used for powder

diffractometers [41], was adopted in this work. In this configuration, the source

and the detector are moved of an angle θ, while the sample is kept horizontal.

The sample is used in reflection mode: the x-ray beam is scattered off the sample

and the scattered intensity is recorded by the detector. The angle θ is varied

in a predefined range, resulting in a diffraction pattern: a file containing the

3.2 Powder Diffraction 21

intensities as a function of the angle.

3.2.1 Lattice structure and Bragg’s Law

An XRD experiment is based on the interaction between matter and electromag-

netic waves. The sample is a crystalline powder containing a very large amount

of small crystals, known as crystallites, oriented randomly to one another. The

crystals are made up by atoms, which are arranged in a regular array, a lattice.

In this 3D framework, it is possible to identify different sets of parallel planes

containing the lattice points, as can be seen in figure 3.5.

(f)

(d)(c)

(e)

(b)(a)

Figure 3.5: Miller indices of some lattice planes: (a,b)(100); (c,d)(010);(e,f)(110). Reproduced (modified) from [42]

A set of planes is identified by a triplet (hkl) called Miller indices; the planes iden-

tified by the same Miller triplet are equally spaced with a distance dhkl. These

planes represent all the possible scattering planes.

When the wavelength of the radiation is similar to that of the plane spacing

in the lattice, the diffraction gives rise to interference and thus to a set of well

defined beams. More precisely, a beam of radiation impinging upon a set of

planes will give constructive interferences if the geometry of the situation fulfills

specific conditions, defined by Bragg’s Law [42]:

nλ = 2dhkl · sin θhkl (3.7)

where n is an integer, λ is the wavelength of the radiation. This law states that,

for having constructive interference, the path lengths of the interfering beams

22 Experimental

must differ by an integer number of wavelengths [43], as illustrated in figure 3.6.

Each set of planes, belonging to the crystal structure of a phase, will give rise

!

!

2!

dhkl

!

X-ray source

X-ray detector

Figure 3.6: Bragg’s condition: two parallel monochromatic x-rays are incidentto the planes at an angle θhkl. For the refracted rays to emerge asa single beam of detectable intensity, they must be in phase afterthe scattering. This constructive interference takes place only if thepath lengths of the interfering beams differ by an integer numberof wavelengths λ, i.e. ρ in the picture must be fulfill the equation2ρ = nλ. And trigonometry tell us that 2ρ = 2dhkl · sin θhkl.

to a peak in the refraction when Bragg’s Law is satisfied2. The intensity of each

peak will depend on the scattering factor, thus the electron density, of the atoms

filling the corresponding set of planes [42].

It can be concluded that, thanks to the intensities and their positions in a diffrac-

tion pattern, it is possible to get a fingerprint of the crystalline phases present in

a sample.

3.2.2 The Diffraction Pattern

Only in theory, a peak in a diffraction pattern can be thought as a delta-shape

function, corresponding to a precise scattering plane. In reality, the observed

intensity y(θi) at a certain angle (θi) in a diffraction pattern is the sum of the

contributions from all the neighboring reflections hj(θi), plus the background

2In equation 3.7 on the previous page, n is the order of the diffraction. Note that the secondorder diffraction from (hkl) is indistinguishable from the first order diffraction from (2h2k2l)[43].

3.2 Powder Diffraction 23

b(θi) [44]:

y(θi) =∑j

hj(θi) + b(θi) (3.8)

The background is attributable to some noise in the measurement, plus the pos-

sible amorphous phases hidden in the sample.

An observed diffraction peak profile h(θ) is the convolution of the contribution

f(θ) from the sample with an instrumental function g(θ)

h(θ) = f(θ) ∗ g(θ) (3.9)

The instrument contribution comes from several functions due to the geometry

of the instrument used (size of the source, beam divergence, slit widths, etc.)

convoluted with the distribution of wavelength in the incident beam. The latter,

in fact, is not truly monochromatic, thus Bragg’s condition will be satisfied in a

domain around the theoretical angle. However, the optimum configuration will

always be a compromise between resolution and adequate intensity.

The sample contribution is mainly related to two main factors: the size of the

domains over which diffraction is coherent, i.e. the thickness of the crystallites,

and the distortion of the lattice, known as strain effect. When the crystallites are

in the order of 1 µm or smaller, or they are strained, the Bragg peak width may

increase substantially. This effect is due to the lack of long (enough) range order

and it can be seen comparing figure 3.7a and 3.7b on the following page where

different crystallite sizes have been used to simulate the diffraction pattern of the

two compounds LiBH4 and CdCl2. Another important information contained in

the two pictures is the much lower scattering factors of Li, B and H compared to

Cd and Cl, which make LiBH4 more difficult to detect when mixed with CdCl2.

In order to extract useful information from a diffraction pattern, i.e. phase com-

position, cell parameters, occupancies, etc., it is necessary to de-convolute the

functions contributing to the peak function. Among the different methods avail-

able, one of the most widespread is to assume that g(θ) and f(θ) are analytical

functions, called Voigtian. These functions are the convolution of one or more

Laurentzian and Gaussian functions whose breadth can be modeled [45]. It is,

therefore, possible to make a mathematical fit (a refinement) of the measured

pattern adjusting the parameters defining the mentioned functions. Such a fit

was developed by Hugo Rietveld [46]. This is a least square method that requires

two models at the start of the refinement: a structural model with preliminary

24 Experimental

10 20 30 40 50 60

2theta [°]

0

200

400

600

800

inte

nsity LiBH4

10 20 30 40 50 60

2theta [°]

0

20000

40000

60000

80000

100000

120000

inte

nsity CdCl2

10 20 30 40 50 60

2theta [°]

0

50

100

150

200

250

300

350

inte

nsity LiBH4

10 20 30 40 50 60

2theta [°]

0

10000

20000

30000

40000in

tensity CdCl2

(a) Big size particles.

10 20 30 40 50 60

2theta [°]

0

200

400

600

800

inte

nsity LiBH4

10 20 30 40 50 60

2theta [°]

0

20000

40000

60000

80000

100000

120000

inte

nsity CdCl2

10 20 30 40 50 60

2theta [°]

0

50

100

150

200

250

300

350

inte

nsity LiBH4

10 20 30 40 50 60

2theta [°]

0

10000

20000

30000

40000

inte

nsity CdCl2

(b) Small size particles.

Figure 3.7: Simulation of the XRD patterns for LiBH4 and CdCl2 with differentcrystallite sizes. Note that for smaller particles the peak shape getsbroader. Note also the different values of the intensities, because ofthe different scattering factors of the atoms in the two compounds.

positions of the atoms and a non structural model describing the Bragg peaks in

terms of analytical functions. Both have to be considered in the fitting. There

exists also another method that does not require the structural model of the com-

pound: the Le Bail method extracts the peak intensities without the knowledge

of the atomic position in the structure [47]. The former is more convenient if the

exact structure and composition of the phases in the sample need to be known,

the latter is useful in preliminary analysis regarding the relative abundance of

phases in a sample. Both methods can nowadays be performed in a variety of

softwares, some of which are free, like the one used in this work: Rietica [48].

3.2.3 Experimental Procedure

The collection of the synthesized samples from the ball mill vial was done into a

glove box filled with argon. For the XRD characterization, the sample was placed

on a plastic sample holder. In order to avoid contact with air or moisture, the

3.3 The Volumetric Sieverts Equipment 25

sample was sealed with a poly-ethylene film, fixed on top of the sample holder

with a rubber ring. The plastic film gives rise to a broad peak around 21.5◦.

The characterization of the synthesized samples by powder x-ray diffraction was

carried out using one diffractometer in the Bragg-Brentano geometry: a Bruker

D8 (voltage 40 kV and current 40 mA) with a Cu Kα radiation and a wavelength

of 1.542 Å. A Lynx-Eye detector was used. The measurements were done in the

range of 10◦≤ 2θ ≤ 65◦, with a step size of 0.04◦ and an exposure of 4 sec foreach step, except for some fast scans3 where the parameters were 0.04◦/0.4sec .

The analysis of the data was conducted primarily using the software Eva, which,

based on the PDF database, developed by the International Center for Diffraction

Data (ICDD), helped in the identification of the phases. When it was possible,

Rietvield Refinements were performed. The latter, in fact, in some cases gave

some really questionable results. This is due to the nature of the XRD patterns:

many peaks were the results of the contribution of different phases and some

peaks had to be “shaded” as they corresponded to phases for which there were

no structural data. These characteristics made the refinements quite difficult

and hardly reliable. When the Rietveld refinement was not really helpful, also

the Le Bail method was used. If the XRD data has been collected under the

same conditions (step size and exposure time), the integrated intensities from

different patterns can be compared to deduct information regarding the relative

abundances of the present phases.

3.3 The Volumetric Sieverts Equipment

The hydrogen ab-/desorption properties of the samples were studied in a Sieverts

apparatus. This technique is based on the fact that upon heating, the borohy-

drides release hydrogen, as already mentioned in chapter 2 on page 9 [17, 14].

If this is performed in a closed system, such as the Sieverts, the pressure will

consequently rise, according to the change in temperature and number of moles.

This process can be investigated via the temperature and pressure data recorded

throughout the experiment as a function of time.

3Since the plastic film was not unlimitedly air tight, when it was considered that the samplewas containing a minimal amount of air sensitive phase, which was the main target of theinvestigation, a quick scan was required.

26 Experimental


A picture of the Sieverts equipment at the Material Research Division at Risø-

DTU can be seen on figure 3.8 and a schematic of the apparatus on figure 3.9

on the facing page. The sample is inserted into the reactor unit under inert

Ar atmosphere. The reactor unit is mounted into the furnace and connected to

the Sieverts, keeping all the valves closed. The usual procedure for a desorption

experiment can be outlined as follows:

• The pipe and the low pressure (LP) tank are purged using the vacuumpump. The valves T3, R2 and obviously T1 need to be opened for the

purpose. When the conditions for the experiment are different from the

previous one, the line to the bottles of He and H2 are purged.

• After purging, the Sieverts is filled with He up to the desired pressure (∼1 bar). The shutter valve and R1 are opportunely operated to reach the

target.

• Another purging and refilling of the system are performed to ensure thepurity of the gas in the system.

• Once the furnace and the LabView software are set with the desired heat-ing ramp (◦C/min), maximum temperature and dwelling time, T4 is opened

and the experiment (together with the recording) can start.

The maximum temperature point was defined according to the experiment, in

most cases it was 500 ◦C but other values were also used. The heating ramp was

in most cases 1 ◦C/min, except for some cases which required a slower rate (0.5◦C). The dwelling time was usually set to 20 minutes, besides those cases that

required a constant temperature for a prolonged time.

The procedure described above is valid for desorption experiments. When the

absorption properties of the samples have to be tested, the valve R2 is closed

while R3 is opened and the high pressure (HP) tank is used, filled with H2 up to

the desired pressure.


Figure 3.8: A picture of the Sieverts equipment.

3.3 Desorption

Figure 3.5: Top: A photograph of the Sieverts apparatus when an experiment isrunning. Bottom: A schematic of the Sieverts apparatus. The red circles denote thetwo kinds of valves used in the system, two-way valves (!) and rotary valves (×).

37

Figure 3.9: A schematic of the Sieverts equipment. The valves correspond tothe red circles: two-way valves (>) and rotary (×). Taken from[49].

28 Experimental

3.3.2 Calculation of the Gas Desorption

The temperature and pressure data recorded during the experiment can be used

to calculate the amount of gas release. The volumes of the components of the

apparatus are listed in table 3.1. For a dehydrogenation, the volumes to be con-

sidered are those of the low pressure (LP) tank, the pipes and the reactor. At

the beginning of a desorption experiment, the only gas in the system is He at

relatively low pressure (∼1 bar). Since the pressure increases during the experi-ments are slight (limited number of moles of hydrogen in the sample), the ideal

gas law can be used. Thus, the number of moles in the system, at each recorded

point, can be calculated as:

nt =ptVreacRTreac

+ptVLPRTroom

+ptVpipeRTroom

(3.10)

where it is assumed that the tank and the pipe, not affected by the temperature of

the reactor, are at room temperature, troom. The total amount of moles released

at the end of an experiment is simply the difference between the final and the

initial values:

nreleased = nend − n0 (3.11)

Assuming that hydrogen is the only gas released during the heating, the weight

percentage, relative to the mass of LiBH4 present in the sample, can be calculated

as:

H2wt% =nreleased ·MH2

mLiBH4· 100 (3.12)

This value together with the value released during the milling should sum up

to 13.77%, the theoretical H2 content in LiBH4 releasable up to 500◦C. Lower

values can be considered as an indication that diborane has been released instead

of hydrogen.

Component Symbol Volume [m3]

LP tank VLP 4.799 · 10−6HP tank VHP 326.8 · 10−6Pipes Vpipe 5.09 · 10−5Reactor Vreac 7.08 · 10−6

Table 3.1: The volumes of the Sieverts apparatus components.


3.3.3 Calibration of the System

During the project, a calibration of the volumes of the Sieverts system was per-

formed, in order to be sure about the reliability of the obtained ab-/desorption

results. For this purpose, three cycles of isothermal ab-/desorption of hydrogen

from the LaNi5 interstitial metal hydride were performed. The set conditions and

the results of the third cycle can be seen in figure 3.10a and 3.10b. The compar-

ison between the two values for the hydrogenation, where the HP tank was used,

and the dehydrogenation, where the LP tank was used, should reveal the exis-

tence of an error in the volumes used as input values in the calculations. Indeed,

80 82 84 86 88Temperature [°C]

20

40

60

80

100

H2 a

bsor

ptio

n [%

]

LaNi5 absorption

(a) Absorption at 80◦C. The exothermic effectcan be noticed. The final value is 102%.

92 94 96 98 100Temperature [°C]

0

20

40

60

80

100

H2 a

bsor

ptio

n [%

]

LaNi5 desorption

(b) Desorption at 100 ◦C. The endothermiceffect can be noticed. The final value is97%.

Figure 3.10: The percentages of ab-/desoprtion corresponding to the nominalcomposition LaNi5H6.

considering the third cycle, the values for the absorption and the desorption are

different, whereas the process was supposed to give the same result in both di-

rections. Thus, a linear system was set, with two equations: one corresponding

to the absorption and one to the desorption. The only difference between them

was the volume of the tank used (VHP for absorption and VLP for desoprtion).

Into both equations, there were two variables: a correction to the volumes and

the number of moles ab-/desorbed, as it can be seen in equation 3.13ndes =

ptVreacRTreac

+pt(VLP + Vpipe + ∆Vdes)

RTroom− n0,des

nabs =ptVreacRTreac

+pt(VHP + Vpipe + ∆Vabs)

RTroom− n0,abs

(3.13)

30 Experimental

It is assumed that ndes = nabs and ∆Vdes = ∆Vabs. These assumptions are to get

an idea of the possible error in the volumes of the system.

The result is a correction of 2.18 ·10−5 m3, which is roughly half of the volume ofthe pipe. If referred to the two processes, in the case of hydrogenation the error

consists of 5.7% of the utilized volume, in the case of dehydrogenation the error

consists of 0.45% of the volume.

This calibration turned out to be useful when dehydrogenation measurements of

pure LiBH4 were performed and used as reference. In the first of these exper-

iments, LiBH4 was used without any pre-treatment. At 500◦C the amount of

gas released was far from reaching the theoretical 13.77% expected from such

an experiment. The calibration of the Sieverts system lead to the repetition of

the experiment, but with a previous ball milling treatment of the LiBH4, at 400

rpm for 70 minutes. The thermal desorption, after this treatment, gave a result

of 12%, much closer to the theoretical value. This is probably connected to the

temperature ramp rate set for the furnace: 1 ◦C/min was too fast for the as

purchased materials to complete the thermal reaction.

3.4 Mass Spectrometry

The analysis of the gasses formed in the milling vial and of the gasses released

during the thermal decomposition of the compounds was performed using a mass

spectrometer.

This technique is based on the application of Newton’s Second Law and the

Lorentz Force. Combined, they say that a charged particle, moving in a given

electromagnetic field, will run a specific trajectory according to its mass. There-

fore, this technique can be used, as in this work, to identify and quantify the

different species in a gas mixture with the help of electromagnetic fields, once the

particles have been ionized. In fact, mass spectrometer are able to scan through

a selected range of mz and determine, from calibration measurements, the amount

of each one. The fundamental processes of a mass spectrometry experiment are:

ionization, separation and detection [50].

Ionization The first step in the mass spectrometry analysis of compounds is

the production of gas phase ions of the compound. This removal or addition of

3.4 Mass Spectrometry 31

an electron or proton can be done in various ways, but the most common for gas

analytes, and the one used in this project, is the electron ionization, formerly

called electron impact.

This source consists of a heated filament giving off electrons. The electrons are

accelerated towards an anode and collide with the gaseous species injected into

the source. If the electron wavelength is sufficiently small, the energy transfer

causes an electron to be expelled from the molecule. On average, an ion is pro-

duced for every 1000 molecules entering the source. The energy of the beam can

be tuned in order to singly ionize the species, avoiding multiple ionization, not

suitable for mz selection. The cations are then accelerated by a voltage imposed

by different slits into a finely focused beam directed to the analyzer where they

are separated.

Besides the single ionization, also another phenomenon is likely to occur in the

ionization chamber: fragmentation of the molecules. The energy of the electron

beam can be enough to break one or more bonds of the ionized molecule, which

will also be detected according to its mz . Luckily, molecules frequently display a

characteristic fragmentation pattern for the energy used for the ionization.

Negative ionization can also occur, but under conventional electron ionization

conditions it is negligible and almost inexistent when compared with the forma-

tion of cations.

Separation Once the gas phase ions have been produced, they need to be sep-

arated according to their mass to charge ratio mz . The separation is done by the

so called analyzer, which applies an electric or a magnetic field to deflect the

trajectory of the charged particles coming from the ionization. If the ions are

all singly ionized, the deflection depends only on the different masses. Lighter

species get deflected more than heavier species. Several types of analyzer have

been developed. They differentiate in the way they use the electric or magnetic

field to achieve the separation: allowing only certain mz ratio to go through in a

given time or allowing the simultaneous transmission of all the ions. In this work,

a quadrupole mass analyzer (belonging to the first category mentioned before)

is adopted. It uses four perfectly parallel rods of circular (ideally hyperbolic)

section. An electric field oscillating at radio frequency is superposed on a con-

stant electric field to selectively destabilize the trajectory of the cations passing

through. A positive ion entering the space between the rods will be drawn to-

wards a negative rod. If the potential changes sign before the cation discharges

on the rod, the ion will change direction. The frequency of oscillation is used to

32 Experimental

select which mz is stabilized and hence reach the detector.

Detection The ions that have passed through the mass analyzer are then de-

tected and transformed into a usable signal by the detector. Detectors generate

an electric current from the incident ions. This can be done by a Faraday cup

detector, where an ion hitting its surface is discharged by an electron. Hence a

current proportional to the number of ions detected is generated and the am-

plified by a conventional electronic amplifier. If a large sensitivity is required, a

secondary electron multiplier (SEM) is used. This device multiplies the intensity

of the signal by a cascade effect driven by accelerating plates called dynode. An

ion striking the conversion dynode (the first one, converting the ions into elec-

trons) causes the emission of several secondary electrons. The amount of emitted

electrons is related to the nature (the mass) of the striking ion. A SEM is usu-

ally endowed with a series of dynodes, held at decreasing negative potential, in

order to create a cascade of electrons, hence amplifying the signal up to 6 or-

ders of magnitude. A variant of the SEM is called C-SEM, where C stands for

continuous. In this kind of detectors, the cascade is not generated by a series

of dynodes, but rather by the interior of a tube coated with a conductive layer

with a low working function. A high voltage is applied to the layer in order to

obtain a uniform voltage gradient throughout the length of the tube. The SEM

are restricted at operating at pressures of less than 10−5 mbar. In this work, the

C-SEM detector was used in most of the experiments.


The Mass Spectrometer (MS) used in this work is a Pfeiffer OmniStarTMGSD

320 O1, with range of 1-100 amu, endowed with a tungsten filament for the

ionization and a QMA 200 M quadrupole analyzer. The capillary tube connected

to the instrument was constantly heated to prevent condensation of water. The

quadrupole was operated with a dwell time of 100 ms for each mz and the C-SEM

detector with a voltage limited to 1000 V, in order to avoid any saturation. The

data recording, the filament and the detector were controlled via the software

Quadera R©.

To analyze the gasses that may form during the milling process, the MS investi-

gation comprised the more abundant ionized masses (cut off = 10% abundance)

3.4 Mass Spectrometry 33

of the main components of the air, like N2,O2,CO2,H2O (this was helpful for

the detection of any leak in the connections and also in the interpretation of the

result itself, as it is explained later) plus the possible gasses released during the

synthesis reactions: H2,B2H6 plus Cl2,HCl. The MS was connected to the ball

mill vial through one of the two valves of the special lid of figure 3.2 on page 17

and the inner valve of the MS was opened. Once the capillary was sufficiently

purged by the pump of the MS (the pressure indicator of the MS was lower than

10−6 mbar), the valve on the lid was opened. The gasses inside the ball mill vial,

in the pressure range of 1 to 1.35 bar, were pumped and analyzed by the MS.

For the analysis of the gasses produced during the thermal desorption of the

compounds, the MS investigation comprised the same species mentioned above,

but focusing on H2 and B2H6. The reactor unit, loaded with a small amount

of sample powder (0.03 g), was mounted into the furnace and connected via the

capillary to the MS. The amount of borohydrides was kept small in order to

avoid an excessive increase of the pressure during dehydrogenation, preventing

any saturation of the SEM. The capillary connection was opportunely purged

before starting the experiment. The furnace was set with a moderate temperature

ramp rate of 0.5 ◦C/min, so that the detection of the desorbed gasses could be

matched to the exact temperature.

3.4.2 Data Analysis

The output from a MS experiment was a file with the values of ionic current

measured for each investigated mass and the pressure in the MS at each time.

The value of a single specie was simply the sum of the corresponding masses. A

semi-quantitative analysis of the relative abundance of the species was conducted.

It is based on the assumption of the good calibration of the detector for the

different mass to charge ratios. The idea is that the concentration of a specie is

proportional to the correspondent current integrated in time. To perform this

integration, every signal was scaled on its background value, that is its value at the

beginning of the experiment. This way, comparing the integrals, it was possible

to get an insight about the relative abundance of the investigated species.

34 Experimental

Chapter 4

Results and Discussion

In this chapter, the investigation regarding the optimal ball milling procedure,

the characterization of the samples produced and the examination of the ab-

/desorption properties of the mixture LiBH4 + CdCl2 are discussed on the basis

of the results collected in the laboratory.

4.1 Synthesis Optimization

4.1.1 8 Hours Ball Milling

Lithium borohydride and cadmium chloride were ball milled in 1:1 ratio for 8

hours at 650 rpm with a Bpr of 197:2 1. The temperature and pressure evolution

can be seen in figure 4.1a and 4.1b on the following page. The temperature rises

all along the process, but not steadily. It is difficult to explain the temperature

behavior of such a long experiment: the first hours increase is likely to be due

to the balls hitting the walls. The leveling off after the second hour, might be

related to the approach of the equilibrium temperature with the temperature in

the room. The experiment ended in the evening, therefore the last hours might

1Note that, in this work, at 650 rpm the ball-to-powder ratio (Bpr) is always 197:2.

36 Results and Discussion

be also affected by the decreasing temperature in the room. It is difficult to

get clear information regarding the eventual reactions in the vial. Nevertheless,

the most interesting result is seen in figure 4.1b, where the pressure, corrected

for the thermal expansion, as explained in section 3.1.2 on page 18, displays a

singular pattern. As indicated in the graph, the pressure increases rapidly in

the first minutes, probably due to the combined effect of the motion of the balls

and the fast activation of some releasing gas reaction. Afterward, the pressure

evolution presents two different trends: one steadily but moderately increasing

up to approximately 4 hours and a steeper one for the last hours. If the pressure

increase is assumed to be caused by the release of hydrogen, as it will be discussed

in section 4.1.3 on page 40, this means that the mixture was severely loosing its

hydrogen content. Indeed, a thermal desorption experiment of 312 mg of the

sample milled for 8 hours gave no gas release.

0 2 4 6 8Time [h]

25

30

35

40

Tem

pera

ture

[°C]

(a) Temperature evolution.

0 2 4 6 8Time [h]

100

120

140

160

180

pres

sure

[kPa

]

(b) Pressure evolution.

Figure 4.1: 8 hours ball milling, 650 rpm, 197:2 Bpr. The thermal expan-sion has been subtracted to the pressure data (see section 3.1.2 onpage 18)

Hence, the synthesis reaction was investigated up to 4 hours of ball milling.

4.1.2 4 Hours Ball Milling with Steps

To follow the evolution of the reaction during the milling, the process was per-

formed in steps with a total duration of 4 hours. The milling was interrupted

after the first 30 minutes, then after 1 hour, 2 hours, 3 hours and finally 4 hours.

At every step, a small amount of the powder, just enough for an XRD character-

ization, was taken out of the milling vial. The samples were always handled in

the glove box. The results of the XRD characterization can be seen in figure 4.2.

4.1 Synthesis Optimization 37

After the first step, some CdCl2 is still detectable but it is difficult to identify the

presence of LiBH4. This might suggest that all the borohydride reacted whereas

the CdCl2 did not completely. Yet, this is difficult to say with certainty, because

of the different scattering factors in the borohydride and in the chloride salt, as

discussed in section 3.2 on page 20. In addition, LiCl, Li2CdCl4 and pure Cd are

also found in the sample.

Besides the “plastic peak” (21.5◦), there are others peaks not identified. One

around 36.5◦ is due to some impurity present in the CdCl2 (see section A.1 on

page 77 for the CdCl2 XRD pattern). Then there is one peak at around 58.5◦,

one at around 54◦ and another at around 43.5◦. Furthermore, there is one peak at

37.2◦ that might be misinterpreted as CdCl2: actually it can not be only CdCl2,

if compared with the intensities of the others peaks from CdCl2. Thus, the peaks

that could originate from a new phase, product of the milling, are: 58.5◦, 54◦,

43.5◦ and 37.2◦.

BM 650 25

01-085-1266 (C) - Cadmium Chloride - CdCl2 - Y: 10.81 % - d x by: 1. - WL: 1.5406 - Rhombo.R.axes - a 6.230

01-082-0906 (C) - Lithium Cadmium Chloride - Li2CdCl4 - Y: 12.87 % - d x by: 1. - WL: 1.5406 - Cubic - a 10.63

00-027-0287 (N) - Lithium Boron Hydride - LiBH4 - Y: 8.48 % - d x by: 1. - WL: 1.5406 - Orthorhombic - a 6.8000

00-004-0664 (*) - Lithium Chloride - LiCl - Y: 8.54 % - d x by: 1. - WL: 1.5406 - Cubic - a 5.13960 - b 5.13960 - c

00-005-0674 (*) - Cadmium, syn - Cd - Y: 8.13 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 2.97930 - b 2.97930 -

Operations: Y Scale Add 3750 | Y Scale Add -833 | Y Scale Add -792 | Y Scale Add 3750 | Y Scale Add 1000 |

LiBH4+CdCl2BM4H - File: LiBH4+CdCl2BM4H.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 74.996 ° - Ste

Operations: Y Scale Add 3750 | Y Scale Add 7500 | Y Scale Add 6104 | Y Scale Add 1000 | Y Scale Add 3000 |


Operations: Y Scale Add 3750 | Y Scale Add 5833 | Y Scale Add -3500 | Y Scale Add 8801 | Y Scale Add 917 |


Operations: Y Scale Add 3750 | Y Scale Add 792 | Y Scale Add -833 | Y Scale Add 3333 | Y Scale Add 3583 | Y


Operations: Y Scale Add 3750 | Y Scale Add 1000 | Y Scale Add 1000 | Y Scale Mul 1.208 | Background 0.174,

LiBH4+CdCl2BM0.5H - File: LiBH4+CdCl2BM30min.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 65.006 ° -

Lin

(C

ou

nts

)

0

10000

20000

30000

40000

50000

60000

2-Theta - Scale

11 20 30 40 50 60

BM 650 25

01-085-1266 (C) - Cadmium Chloride - CdCl2 - Y: 10.81 % - d x by: 1. - WL: 1.5406 - Rhombo.R.axes - a 6.230

01-082-0906 (C) - Lithium Cadmium Chloride - Li2CdCl4 - Y: 12.87 % - d x by: 1. - WL: 1.5406 - Cubic - a 10.63

00-027-0287 (N) - Lithium Boron Hydride - LiBH4 - Y: 8.48 % - d x by: 1. - WL: 1.5406 - Orthorhombic - a 6.8000

00-004-0664 (*) - Lithium Chloride - LiCl - Y: 8.54 % - d x by: 1. - WL: 1.5406 - Cubic - a 5.13960 - b 5.13960 - c

00-005-0674 (*) - Cadmium, syn - Cd - Y: 8.13 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 2.97930 - b 2.97930 -

Operations: Y Scale Add 3750 | Y Scale Add -833 | Y Scale Add -792 | Y Scale Add 3750 | Y Scale Add 1000 |


Operations: Y Scale Add 3750 | Y Scale Add 7500 | Y Scale Add 6104 | Y Scale Add 1000 | Y Scale Add 3000 |


Operations: Y Scale Add 3750 | Y Scale Add 5833 | Y Scale Add -3500 | Y Scale Add 8801 | Y Scale Add 917 |


Operations: Y Scale Add 3750 | Y Scale Add 792 | Y Scale Add -833 | Y Scale Add 3333 | Y Scale Add 3583 | Y


Operations: Y Scale Add 3750 | Y Scale Add 1000 | Y Scale Add 1000 | Y Scale Mul 1.208 | Background 0.174,

LiBH4+CdCl2BM0.5H - File: LiBH4+CdCl2BM30min.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 65.006 ° -

Lin

(C

ou

nts

)

0

10000

20000

30000

40000

50000

60000

2-Theta - Scale

11 20 30 40 50 60

CdLiClLiBH4Li2CdCl4CdCl2new phase

4h

3h

2h

1h

30min

Figure 4.2: XRD patterns for the ball milled sample with “steps”. It is possibleto follow the increase of the Cd content and the decomposition ofthe new phase with the increasing ball milling time (figure enlargedin section A.2 on page 80).

Comparing the XRD patterns taken at the different steps, starting from the

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