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Sensors and Actuators A 167 (2011) 332–337

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

ignificant increase in humidity sensing characteristics of praseodymium dopedagnesium ferrite

yoti Shaha, Manju Aroraa, L.P. Purohitb, R.K. Kotnalaa,∗

National Physical Laboratory, Council of Scientific and Industrial Research, Dr. K.S. Krishnan Road, New Delhi 110012, IndiaGurukula Kangri University, Haridwar 249404, India

r t i c l e i n f o

rticle history:eceived 24 October 2010eceived in revised form 3 March 2011ccepted 3 March 2011

a b s t r a c t

Magnesium ferrite has been doped by Pr in 0.1 mol% and 0.3 mol% concentration for enhancing humiditysensing properties. The spin density/dangling bond calculated by electron paramagnetic resonance (EPR)increased from 8.15 × 1020 to 15.6 × 1020 for 0.3 mol% Pr doped compared to undoped magnesium ferritesample. The bulk porosity of the samples in this case increases from 8.4% to 34% with Pr doping. Pr

vailable online 10 March 2011

eywords:rgFe2O4

pin densityorosity

doping increased the sensitivity factor Z10%/Z90% from 24 to 113. Impedance gradient |dlog Z/dRH| at low10–30% RH and high 70–90% RH has been determined with respect to spin density and porosity for thesamples. A maximum drift in humidity hysteresis of 22% RH is predominantly reduced to 2% RH by a0.1 mol% Pr doping in magnesium ferrite. Such remarkable improvements identified this material as astrong candidate for humidity sensors.

© 2011 Elsevier B.V. All rights reserved.

umidity hysteresis

. Introduction

Ceramic oxides have been used as humidity sensitive materi-ls due to their porous microstructure [1–4]. In addition to porousicrostructure, defects and vacancies, dangling bonds on oxide sur-

aces are the most active sites for low concentration water vaporissociation. The dissociation of water vapors releases protons foronduction hence resistance of the sensing material decreases. Inrder to enhance surface activity, surface additives or dopants arencorporated in oxides to enhance the humidity sensing perfor-

ance [5–8]. The improvement in humidity sensitivity of ferratesnd aluminates with increased spin density/dangling bonds haseen studied [9]. The increase in hydrogen adsorption measuredy electron spin resonance (ESR) on active sites is also reported10]. Some catalytic ions have been also adopted to enhance theurface charge density [11,12].

Magnesium ferrite is a porous and highly resistive, chemicallynd thermally stable ferrite [13,14]. Researchers explored the phys-cal and electrical properties of MgFe2O4 for humidity sensing

15–17]. In this work Pr doping in nominal amounts 0.1 mol% and.3 mol% in magnesium ferrite to increase its porosity and spinensity/dangling bonds for enhancing humidity sensing propertiesave been studied. A drop in impedance increased to 2–3 orders

∗ Corresponding author. Tel.: +91 11 45608599; fax: +91 11 45609310.E-mail address: rkkotnala@nplindia.org (R.K. Kotnala).

924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2011.03.010

of magnitude (108/109 to 106 �) besides sustaining linearity by Prdoping to magnesium ferrite has been recorded. Humidity hystere-sis drastically decreased by such a nominal Pr doping in magnesiumferrite.

2. Experimental

Analytical grade MgCO3 and Fe2O3 were mixed in a ratio 1:1,and were ground in a mechanical ball mill using zirconium balls.A 0.1% and 0.3 mol% Pr6O11 was mixed in precursor respectivelyto obtain different samples, followed by grinding for 2 h. The threecompositions samples were presintered at a temperature of 850 ◦Cin air for 8 h. The presintered powder was further ground andpellets were pressed in a rectangular 15 mm × 3 mm × 2 mm sizefollowed by sintering at 1000 ◦C in air for 4 h. The details of theelectrical contacts made on samples have been described in earlierwork [17]. For taking the humidity measurements a two pressuremethod based on a standard humidity generator (Thunder Scien-tific 2500 series) was used. The impedance measurements weretaken from 10% to 90% RH in steps of 10% RH at 25 ◦C. The uncer-tainty of measurements of RH generator is within ±0.5% RH. For

measuring best humidity response, one volt a.c. at 1 kHz frequencysignal was applied by Fluke 81 Function Generator. Humidity hys-teresis was plotted by recording the resistance in forward RH cycle(10–90% RH) and reverse RH cycle (90–10% RH). The two pressuremethod humidity generator to measure relative humidity is based

Actuat

o

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Cl

%

ws

3

3

rcawatsdTsXmgvr

Fn

TS

J. Shah et al. / Sensors and

n the following principle:

RH = Pa

Ps× 100 (1)

here Pa = actual pressure of water vapor, Ps = saturated pressuref water vapor.

Humidity generator operates using an on board multifunctionPU in conjunction with other peripheral cards to perform calcu-

ation and control functions.Bulk porosity, P, was calculated using the formula:

P = dx

dexp× 100 (2)

here dx and dexp are the X-ray density and the experimental den-ity of the respective sample.

. Results and discussions

.1. X-ray diffraction

The X-ray diffraction of the sintered pellets was measured atoom temperature within 20–70◦, 2� range using a Bruker AXSonfiguration operating at 40 kV, 40 mA with Cu K� radiation instep angle 0.002◦. All the peaks recorded in the XRD plot coincideith a spinel magnesium ferrite structure, JCPDS Card No. 36-0398,

s shown in Fig. 1. Due to Pr doping, XRD shows shifting of peakowards higher angle side and decrease in intensity of peaks ashown in Fig. 1 (inset). Peak shift attributes to defective sites andecrease in peak intensity corresponds to increase in vacancies [18].he calculated values of the lattice parameter (a) are nearly theame for different Pr concentrations as shown in Table 1. From the

RD analysis Pr ion creating defects and increasing vacancies inagnesium ferrite. The rise in base resistance with Pr doping sug-

ests that it would be segregating at the grain boundaries. Due toariable valance of Pr it may be restricting the oxidation of Fe2+

esponsible for the porosity of the microstructure [19].

706050403020

MgFe2O

4

0.1 mole% Pr

0.3 mole% Pr

2 Theta (deg.)

Inte

nsity (

arb

. units)

[62.8

41]

[57.2

53]

[53.7

46]

[43.2

97]

[35.6

84]

[30.3

26]

36.035.835.635.4

Inte

nsity (

arb

. u

nits)

2 Theta (deg.)

ig. 1. X-ray diffraction of (a) pure magnesium ferrite, (b) 0.1 mol% Pr doped mag-esium ferrite, and (c) 0.3 mol% Pr doped magnesium ferrite.

able 1tructural and physical parameters of Pr doped MgFe2O4 samples.

Sample Lattice constant (Å) Bulk porosity % Grain size distribution Pore

MgFe2O4 8.340 8.4 75 nm to 1 �m 15–40.1 mol% Pr 8.335 24 150 nm to 1.5 �m 45–60.3 mol% Pr 8.335 34 150 nm to 1.5 �m 30–3

ors A 167 (2011) 332–337 333

3.2. Scanning electron microscopy (SEM)

Scanning electron microscopy analysis of surfaces of the sin-tered pellets was carried out in a LEO 404 microscope. The porosityof as prepared magnesium ferrite can be visualized by porous SEMmicrograph. The grain size distribution was calculated by applyinglinear intercept method on SEM micrographs of sample pellets. Thegrains and the pores size distribution of samples prepared werein the range 75 nm to 1 �m and 15 nm to 450 nm respectively asshown in Fig. 2. By doping 0.1 mol% Pr, grains seem to be agglom-erated with the size distribution 150 nm to 1.5 �m. Additionally,the surface open pores are grown in size 45–600 nm. For 0.3 mol%Pr doping the grain size distribution is unaltered but the pore sizedecreased between 30 nm and 300 nm. The bulk porosity is calcu-lated using Eq. (2) which is shown in Table 1. The bulk porosity isincreased by increasing Pr doping. It exhibits Pr doping is favoring amore porous structure. Similar to as reported in our earlier work avery small amount of CeO2 doping at a lower sintering temperatureincreased the porosity of magnesium ferrite [20].

3.3. Electron paramagnetic resonance (EPR)

The EPR measurements on the samples exposed to humiditywere carried out at room temperature using a Varian E-112 EPRspectrometer as shown in Fig. 3. The intensity of EPR plot is pro-portional to the concentration of unpaired spins in the sample.The concentration of unpaired spins is determined by compar-ing the sample intensity to a standard of known unpaired spinsof diphenyl picryl hydrzyl (DPPH) used as a g-marker. It is actu-ally the area under the absorption curve that is a measure of thenumber of ionic moments present in a sample. This area may beestimated from the linewidth times the area under the derivativecurve [21]. So by comparing the area under the derivative of DPPHwith samples, spin density is determined. A dangling bond has oneunpaired electron and is the simplest paramagnetic defect in a ran-dom network. The spin density thus primarily measures the densityof dangling bonds. The chemisorption of water vapors increases thespin/dangling bond density from pure to 0.03 mol% Pr doped mag-nesium ferrite samples as shown in Table 1. As it is also observedby XRD analysis, Pr doping increases defects and vacancies thatare highly active sites for water vapor reaction. The concentrationof spins per gm increased due to more H2O dissociation. Pr dopingmay have increase in the electron accepting sites due to its catalyticbehavior [22]. It is expected that dangling bonds/spin density canbe passivated in connection with dissociation of H2O.

3.4. Humidity response

The electrical impedance of the samples is measured at 10 Hz,100 Hz and 1 kHz for the humidity range 10–90% RH at 25 ◦C. Thebest humidity response was observed at 1 kHz so all humidity mea-surements were carried out at this frequency. The impedance at10% RH is observed 1.43 × 108 �, 3.45 × 108 �, and 1.25 × 109 � for

pure, 0.1 mol%, and 0.3 mol% Pr doped magnesium ferrite samples,respectively. The base impedance of magnesium ferrite is increasedby Pr doping. From 10% to 90% RH range Pr doped samples, log Zdrop increased from 2 to 3 orders (−106 � to 109−106 �). Due tolarge ionic radius of Pr it has low electro negativity, which increased

size distribution Sensitivity factor Sf (Z10%/Z90%) Spin density × 1020 per gram

50 nm 24 8.1500 nm 79 11.0200 nm 113 15.60

334 J. Shah et al. / Sensors and Actuators A 167 (2011) 332–337

F0

tids1a

shtia

70006000500040003000200010000

1800

2000

2200

2400

2600

2800

3000

3200

(c)

(b)

(a)

Inte

nsity (

arb

. u

nits)

Magnetic Field (G)

(a) MgFe2O

4

(b) 0.1 mol% Pr

(c) 0.3 mol% Pr

Fig. 3. Curve between Intensity and Magnetic Field at 9.2 GHz for (a) MgFe2O4, (b)0.1 mol% Pr doped and (c) 0.3 mol% Pr doped samples.

100806040200

6.5

7.0

7.5

8.0

8.5

9.0

1kHz

Lo

g Z

(O

hm

)

MgFe2O

4

0.1 mol% Pr

0.3 mol% Pr

ig. 2. SEM micrographs of (a) magnesium ferrite, (b) 0.1 mol% Pr doping and (c).3 mol% Pr doped samples.

he reaction with high electronegative hydrogen bonded oxygenon (O–H). The humidity response of undoped magnesium ferriteecreases slowly from 30% to 80% RH in comparison to the Pr dopedample as shown in Fig. 4. The slope of log Z becomes steeper in the0–30% RH range with Pr-doped samples, which signifies surfacective sites increased for water vapor dissociation.

On exploring sample to humidity, water vapors initially get dis-ociated in H+ and OH− due to electric field developed on the

ighly active surface and inside the pore, followed by proton migra-ion through hydroxyl groups resulting into conduction [23]. Withncreasing humidity, physisorption of water vapors takes placebove the chemisorbed OH− layers via H-bonding [24]. Water

% RH

Fig. 4. Log Z versus %RH for three compositions at 1 kHz from 10 to 90% RH.

vapors initially start condensing at the grain neck followed by insidethe open pore walls. Groutthus-type proton conduction begins inphysisorbed water layers thus decreasing the impedance of thesample.

A near linear response of log Z for entire humidity range isexhibited by 0.3 mol% Pr doping. The sensitivity factor Z10%/Z90%calculated for undoped and Pr doped samples are 24, 79 and 113,respectively. Spin density and bulk porosity are crucial parametersfor humidity sensing. Both the parameters increased by doping anominal amount of Pr. This has extensively modified the humiditysensitivity of magnesium ferrite.

3.5. Conduction mechanism

There are three types of known mechanisms have been pro-posed for explaining the experimentally proven increase of surfaceconductivity in the presence of water vapor. Two direct mecha-nisms are proposed by Heiland and Kohl [25] and the third, indirect,is suggested by Morrison [26] and by Henrich and Cox [27].

The first mechanism of Heiland and Kohl attributes the role of

electron donor to the “rooted” OH group. The water vapors inter-action mechanism with metal oxides can be described as:

H2Ogas + Mı+ + Oı− ⇔ (Mı+–OH−) + (OH−) + e− (3)

Actuat

wacbelow

2

w

H

Fa

J. Shah et al. / Sensors and

here Mı+ and Oı− are cations (Lewis base) and oxygen ions (Lewiscid) on oxide surface. Hydroxyl ions from water molecules gethemisorbed on metal ions. It results into a high electrostatic fieldy entrapping electrons released in the reaction. Consequently highlectrostatic field formation results into first physisorbed waterayer. Further adsorption of water vapors results in the formationf a hydronium ion (H3O+) and releases the proton to neighboringater molecule as indicated in following reaction:

H2O ⇔ H3O+ + OH− (4)

hich gives further formation of proton to hydration process:

3O+ ⇔ H2O + H+ (5)

2.01.51.00.50.0

0

40

80

120

160

200

240V

on-V

off0.3 mol% Pr

Time (sec)

I (n

A)

10 % RH

50 % RH

90 % RH

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0

50

100

150

200

250

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-Voff

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I (n

A)

Time (sec)

10 % RH

50 % RH

90 % RH

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0

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200

Von

-Voff

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50 % RH

90 % RH

MgFe2O

4

I (n

A)

Time (sec)

a

b

c

ig. 5. (a) Von and Voff response of current for (a) MgFe2O4, (b) 0.1 mol% Pr doped,nd (c) 0.3 mol% Pr doped samples for 10%, 50%, and 90% RH.

ors A 167 (2011) 332–337 335

Thus proton begins to conduct through subsequentlyphysisorbed layers of water molecules. This type of conduc-tion happens in capillary like pores of magnesium ferrite. At higherhumidity values, capillary condensations are also occurring, that isan important factor responsible to contribute water vapor sensingin such compounds [28,29].

For the identification of charge carrier contribution in conduc-tion mechanism, a dc current decay was measured by applying dcvoltage [30–33]. The samples were connected to 1 V potential dif-ference for 10, 50 and 90% RH values. When a steady initial current isreached, voltage turned to 0 V, sample current drops and a low sta-ble final current is attained. During decay in current, time was alsomeasured simultaneously. The initial current falls exponentiallywhen voltage turned to 0 V as shown in Fig. 5(a)–(c). The initial cur-rent for undoped MgFe2O4 at 10% RH was 3 nA and final current was0.18 nA, while at 90% RH the initial current increased to 170 nA andfinal current was 3.3 nA. This trend was also observed in Pr dopedsamples. The exponential fall in initial current is due to ions, whichconfirms the dominating conduction carriers as H3O+ and H+ ions.But the final current does not attain zero at 0 V. Magnitude of elec-tric current was decreased by Pr doping in magnesium ferrite dueto increased resistance. However, the calculated percentage con-tribution of electronic and ionic current from current decay curve

has been in the order of 2–5% and 98–95% respectively for all sam-ples. With increasing relative humidity magnitude of ionic currentincreased largely in Pr doped samples than undoped magnesiumferrite but the percentage current contribution remained same.

0.0300.0280.0260.0240.022

8

9

10

11

12

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5

10

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25

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35

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Poro

sity

%

Spin

Density (

x 1

0 20)

IdLogZ/dRHI

For 10-30 % RH0.3 mol% Pr

0.1 mol% Pr

MgFe2O

4

0.0200.0180.0160.0140.012

8

9

10

11

12

13

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5

10

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25

30

35

For 70-90 % RH

0.3 mol% Pr

0.1 mol% Pr

MgFe2O

4

IdLogZ/dRHI

Bu

lk P

oro

sity

%

Sp

in D

en

sity (

x 1

02

0 )

a

b

Fig. 6. (a) |dlog Z/dRH| versus spin density (×1020) and bulk porosity % for low10–30% RH range at the interval of 10% RH, (b) |dlog Z/dRH| versus spin density(×1020) and bulk porosity % for high 70–90% RH range at the interval of 10% RH.

3 Actuat

Tiwc

flpfldetvo

Fd

36 J. Shah et al. / Sensors and

he steady state electronic conduction is due to the electrons andonic conduction is mainly contributed by H3O+ ions of physisorbed

ater molecules. Thus both ions and electrons are contributing inonduction mechanism.

Fig. 6(a) shows a curve for the humidity gradient |dlog Z/dRH|or change in logarithmic impedance taken on average value forow 10–30% RH of the samples with respect to spin density andorosity %. As the curve shows, the slope of gradient impedanceor spin density is higher than the porosity slope. It implies that atower humidity only monolayer of OH− ions due to water vaporissociation is formed. This reveals, at lower humidity, Pr doping

nhances the electron accepting centers for water vapors dissocia-ion. Similarly, Fig. 6(b) shows a gradient |dlog Z/dRH| taken averagealue for high 70–90% RH for the three samples. The porosity effectn |dlog Z/dRH| has been more prominent than spin density effect

1008060402006.6

6.8

7.0

7.2

7.4

7.6

7.8

8.0

8.2

Humidity

Hystersi

MgFe2O

4

Lo

gZ

(O

hm

)

% RH

Adsorption

Desorption

100806040200

7.0

7.5

8.0

8.5

9.0

9.5

0.3 mol% Pr

LogZ

(O

hm

)

% RH

Adsorption

Desorption

100806040200

6.5

7.0

7.5

8.0

8.5

0.1 mol% Pr

LogZ

(O

hm

)

% RH

Adsorption

Desorption

a

b

c

ig. 7. (a) Adsorption–desorption log Z versus %RH for Mg Ferrite, (b) 0.1 mol% Proping, and (c) 0.3 mol% Pr doping.

ors A 167 (2011) 332–337

on the samples. At high humidity, due to porous microstructure,capillary condensation inside the open pores takes place and elec-trolytic conduction occurs in addition to the protonic conductionin physisorbed layers. It ultimately further drops the impedance ofthe sample. Thus porosity plays dominant role at high humidity.

3.6. Humidity hysteresis

All the samples exhibited hysteresis when adsorption for10–90% RH and desorption form 90% to 10% RH measurements weretaken as shown in Fig. 7(a)–(c). Due to porous microstructure, openpores on the sample, surface act like capillaries so that are highlyactive sites become occupied quickly with increasing water vaporpressure. On the pore walls, coalescence of the two growing lay-ers takes place that exhibit a hysteresis. Various explanations havebeen given in literature for humidity hysteresis [34–36]. A drift inimpedance of 22% RH at 50% relative humidity and a 7% RH drift at80% relative humidity has been observed for undoped magnesiumferrite. A drift within 0–2% RH was measured for 0.1 mol% Pr dopedsample. A 0–5% RH drift has been observed for 0.3 mol% Pr dopedmagnesium ferrite. In comparison to undoped magnesium ferrite,Pr doping reduces the humidity hysteresis for the entire range. Min-imum measured pore size was 15 nm for magnesium ferrite, whichincreased to 45 nm for 0.1 mol% Pr doping and decreased to 30 nmfor 0.3 mol% Pr doping. It exhibits a large humidity hysteresis forpure magnesium ferrite due to smallest 15 nm pores formation.These results revealed that a very small pore size distribution isan obstacle in desorption of water vapors. It is also observed fromFig. 5(a) the decay current is not as steep as for Pr doped sam-ples Fig. 5(b) and (c). It signifies that recovery of conduction carrieris taking longer time in pure magnesium ferrite than the dopedsamples. Further investigation about the role of heat involved inadsorption/desorption process is needed. Humidity response timewas measured between 40 and 70% RH at 25 ◦C for all samples. Theadsorption time was measured within 90 s and desorption time wasmeasured within 120 s as compared to reported 3–5 min [14].

4. Conclusions

In this study, a nominal doping of Pr in magnesium fer-rite remarkable improved its humidity sensing properties. Withpraseodymium doping, both spin density and porosity of mag-nesium ferrite has increased. It resulted into increase in lowerconcentration 10–30% RH water vapor dissociation, hence thelower water vapor sensitivity improved. Increased porosity due todoping has improved its sensitivity towards high humidity 70–90%RH range. Thus, Pr doping in magnesium ferrite has enhancedwide range humidity sensitivity. Humidity hysteresis drasticallydecreased by Pr doping compared to undoped sample, as least areaenclosed in hysteresis is a crucial parameter for precision measure-ment of humidity in sensors. Considering all experimental results,0.1 mol% Pr doping was found to be the favorable choice for humid-ity sensing application.

Acknowledgement

The authors are thankful to Director “National PhysicalLaboratory” New Delhi for providing constant encouragement,motivation, and support to carry out this work. Ms. Jyoti Shah isthankful to CSIR for granting fellowship to pursue research work.

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Biographies

Ms. Jyoti Shah received Ph.D. in humidity sensing properties of magnesium ferritefrom Gurukula Kangri Vishwavidyalaya (Deemed Uni.) Haridwar. Presently she isresearch associate at National Physical Laboratory, New Delhi. Her research interestis in humidity sensing materials, magnetic materials and DMS materials.

Dr. Manju Arora received her Ph.D. on Vibrational spectroscopy of Some Rare EarthCompounds from CCS University, Merrut. Presently she is working as a senior tech-nical officer in EPR group at National Physical Laboratory, New Delhi. Her currentresearch work is on synthesis of nanocrystalline ZnO thin films, nanomagnetic par-ticles and their characterization by EPR, FTIR and FT Raman spectroscopy.

Dr. L.P. Purohit did his Ph.D. on semi conducting materials from Delhi University.Currently he is working as a reader in Gurukula Kangri Vishwavidyalaya (DeemedUni.) Haridwar. His current research interest is on bulk and thin films of semicon-

ducting materials and oxide magnetic materials.

Dr. R.K. Kotnala obtained Ph.D. degree in solar cell from Indian Institute of Technol-ogy, Delhi in 1982. At present he is working as a senior scientist in National PhysicalLaboratory, New Delhi, India. His current field of interest is magnetic materials,standards, sensor materials, and spintronic materials.