Relative humidity effects on the surface electrical properties of resistive plate chambermelaminic laminates uncoated and coated with polymerized linseed oil filmAndrea Bearzotti and Lucrezia Palummo Citation: Journal of Applied Physics 102, 064911 (2007); doi: 10.1063/1.2783992 View online: http://dx.doi.org/10.1063/1.2783992 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/102/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Temperature and humidity effects on superhydrophobicity of nanocomposite coatings Appl. Phys. Lett. 100, 053112 (2012); 10.1063/1.3680567 Monitoring the Effect of Relative Humidity During Curing on Dielectric Properties of Composites at MicrowaveFrequencies AIP Conf. Proc. 820, 469 (2006); 10.1063/1.2184565 Investigation of the humidity effect on the electrical properties of single-walled carbon nanotube transistors Appl. Phys. Lett. 87, 093101 (2005); 10.1063/1.2032594 Humidity effect on electrical performance of organic thin-film transistors Appl. Phys. Lett. 86, 042105 (2005); 10.1063/1.1852708 Effects of lamination on soft magnetic properties of FeN films on sloping surfaces J. Appl. Phys. 81, 4507 (1997); 10.1063/1.364932

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Relative humidity effects on the surface electrical properties of resistiveplate chamber melaminic laminates uncoated and coated withpolymerized linseed oil film

Andrea Bearzottia�

IMM-CNR, “Area di Ricerca di Roma Tor Vergata,” Via del Fosso del Cavaliere 100, 00133 Rome, Italy

Lucrezia Palummob�

Physic Department, Rome University, “Tor Vergata,” Via della Ricerca Scientifica 1, 00133 Rome, Italy

�Received 11 April 2007; accepted 2 August 2007; published online 27 September 2007�

Relative humidity is an important quantity to control in many manufacturing environments such assemiconductor industry. Humidity and moisture can affect many electronic devices, generallyrendering their operation worse. In this study we present results showing that in some specificapplications, humidity can improve the performance of an electronic device. Resistive platechambers are used as trigger detectors of the muon system in LHC �large hadron collider�experiments ATLAS �a toroidal LHC apparatus�, CMS �compact muon solenoid� and ALICE �alarge ion collider experiment� and as detector in cosmic rays experiment ARGO �astrophysicalradiation with ground-based observatory�. These detectors are made of phenolic-melaminic laminateelectrodes, coated with a polymerized linseed oil film delimiting the gaseous sensitive volume. Theloss of some of the detector capability can be progressive in time and due to the intrinsic limits ofthe detector materials. One of these effects is due to an increase of the total plate resistance, that iscorrelated to ion migration and relativity humidity phenomena. Our purpose is to understand therelative humidity �RH� influence on the conduction mechanisms on the electrodes surface. Resultsof amperometric measurements on laminate samples kept at a fixed temperature of 22 °C, cyclingRH between 10% and 90% are here presented. © 2007 American Institute of Physics.�DOI: 10.1063/1.2783992�

I. INTRODUCTION

In particle physics, resistive plate chamber1–5 �RPCs� areplanar gaseous detectors made of two parallel electrodeplates, which limit a gaseous gap of 2 mm. When the gas isionized, an electrical discharge is generated by the uniformfield of about 5 kV/mm acting inside the gap. The electrodeplates are made of thermoset polymer laminates of 2 mmthick, with a phenolic core sandwiched between twomelaminic superficial layers. In order to reduce the detectornoise, the electrode plate surface exposed to the gas is coatedwith a polymerized linseed oil layer of about 1 �m thick.Phenolic and melaminic laminates exhibit a moderate bulkelectrical conductivity that is sufficient to feed the electricaldischarges in the gas.

For the large hadron collider �LHC� applications, theseelectrodes are required to conduct a stable current that cannotexceed 100 �A m−2 during about 10 years of operation. Thepresence of linseed oil on these electrodes helps to reduce theelectrical noise produced by the current leakage due to sur-face imperfections. A gradual resistivity increase for longworking times has been revealed for such large current den-sities. This phenomenon6 was observed in 1993. This agingphenomenon7 can be reduced or eliminated when the plates

operate in an appropriate and controlled humidified environ-ment �relative humidity �RH� �40%�. This paper reports astudy on the interaction of the electrode surfaces with watervapor.8 For this purpose, we monitored the surface conduc-tivity of a number of small size phenolic-melaminic lami-nates, both uncoated and coated with oil, which were keptunder well controlled RH and temperature conditions. More-over, the aging effects of linseed oil on RPC was investigatedafter long-lasting operative conditions. The decomposition ofthe gas mixtures under electrical discharge produces a preva-lence of hydrofluorocarbon components, with a significantfluorine ions F− concentration,9 which can be one of the pos-sible causes of the inner surface damaging.

All these investigations gave information on the waterad/absorption mechanisms10 of uncoated phenolic-melamincplates and coated with linseed oil film and on the effects thata RH increase can produce on the surface resistivity.

II. EXPERIMENTAL

We have taken pieces of RPC detectors in factory beforethey were assembled. The phenolic-melaminic sample with-out linseed oil is called BAK, the phenolic-melaminicsample where linseed oil was deposited by deep coating, atRH and temperature controlled, is called OIL. While thesample called OILF is been cut by a small RPC �10�10 cm2� after the chamber has worked with 10 kV appliedand with a mixing streams of tetrafluoroethane C2H2F4

a�Tel.: �39 06 49934537. FAX: �39 06 49934066. Electronic mail:andrea.bearzotti@imm.cnr.it

b�Tel.: �39 06 2023644. FAX: �39 06 72594027. Electronic mail:lucrezia.palummo@roma2.infn.it

JOURNAL OF APPLIED PHYSICS 102, 064911 �2007�

0021-8979/2007/102�6�/064911/4/$23.00 © 2007 American Institute of Physics102, 064911-1

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�90%� and isobuthane C4H10 �10%� feed inside the gap andirradiated with radioactive source. The oil used was theGALLOIL, industry supplied.11

Tests were carried out using a cylindrical stainless steelcell of 40 cm3 in volume flushed with nitrogen �Fig. 1�. Bothtemperature and RH were measured using reference sensorslocated inside the cell. Samples of 1�1 cm2 were intro-duced in the cell. The surface conductivity was measuredinside an area delimited by two thin electrode fingers of10 mm long and 4 mm spaced. The electrodes were obtainedby thermal evaporation under vacuum of chromium througha metallic mask to geometrically define the output contacts.RH inside the cell was controlled with a two channel MKSmass flow meters, mixing streams of dry and wet N2 withappropriate flow ratios. Mass flow meters operated at a con-stant total flow rate of 200 SCCM �SCCM denotes cubiccentimeter per minute at STP�. The relative humidity wasmeasured with a commercial sensor.12

The electrical response of the resistive-type samples toRH variations was measured. The resistance values weremeasured using the standard two probe method recording thecurrent versus applied voltage �I /V�, maintaining fixed therelative humidity. The current response toward RH variationswas recorded with a constant applied voltage �5 V, T=22 °C�.

Energy dispersive x-ray spectroscopy �EDS� analysiswas performed to characterize the changes of materialsphysical-chemical properties as a result of degradation.13

III. RESULTS AND DISCUSSION

The measurements with EDS technique �Fig. 2� showedthree different spectra for samples BAK, OIL and OILF. Inthe case of Bakelite, with two melaminic outer layers of theelectrode plates, the peaks of carbon, oxygen, and nitrogenwere observed, according with its chemical formula C3H4N5.For oil coated melaminic laminates, the absence of the nitro-gen peak suggested a good covering by linseed oil, which ismade of three acid: oleic C12H34O2, linoleic C12H32O2, andlinolenic C12H30O2, respectively, with one, two, and threedouble bonds in the hydrocarbon chain. For the OILFsample, the EDS spectrum clearly showed fluorine and nitro-gen peaks. The appearance of the fluorine peak can be re-lated to an interaction between the gas mixtures, containing

mainly HFC components and the surface of linseed oil. Thetie carbon fluorine formation is presumed. On the other handthe nitrogen peak arises evident and this could be attributedto the removal of a linseed oil part or to new componentformation.

The fluorine peak presence in the EDS analysis pushedus to analyze the behavior of the samples covered with fluo-rine charged linseed oil toward relative humidity variations.

Figure 3 shows the variation of conductivity to cyclicRH changes for the tested samples, where current was re-corded during repeated RH variations lasting for more than24 h. The humidity was cycled between 10% and 90% RH,with steps of 2.5% RH every 300 s, corresponding to a fullcycle lasting about 7 h. The devices were polarized with anapplied voltage of 5 V. The response curves �Fig. 3� showeda large change in current intensity, pointing out a significantinteraction of the polymeric surface with the water vapor. Acurrent intensity increase with RH was observed for all thetested samples. Stronger effects were registered for laminatescoated with linseed oil film �OIL�, as shown in Fig. 3. Theslope of current intensity recorded versus time during RHincreasing of the oil coated sample with fluorine �OILF� is

FIG. 1. Experimental setup.

FIG. 2. EDS spectrum of three samples: Bakelite �BAK�, bakelite coatedwith linseed oil �OIL�, and with fluorine linseed oil �OILF�.

FIG. 3. Cyclic electrical response toward RH variations of the three samples�5 V, time step ts=300 s� vs time, the dynamic for three sample Imax/ Imin

�BAK� =60, Imax/ Imin �OIL� =35, and Imax/ Imin �OILF� =5.

064911-2 A. Bearzotti and L. Palummo J. Appl. Phys. 102, 064911 �2007�

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similar to oil coated sample, while during the RH decreasingthe slope is less steep. The variation range of conductivityresults smaller compared to the sample without fluorine.

However, a different behavior for the oil with fluorinewas observed. The maximum value of current intensity wasthe same as for the melaminic laminates. This could be at-tributed to a reduction of active sites for water absorption.

In Fig. 4 for all the three samples, two time steps weretested for the I-RH curves: ts=300 s corresponding to a fullcycle lasting about 7 h, and ts=600 s lasting about 14 h. It isworth noting that in the measurements performed with a timestep of 600 s, a larger current intensity was observed, to-gether with a smaller hysteresis than for the measurementsperformed with a step of 300 s. These findings can be as-cribed to the fact that with a longer time step, the materialsare able to load a larger amount of water vapor during ad-sorption, and this corresponds to a larger current intensitycompared to the same value observed with the time step of300 s. On the other hand, during the desorption process thelonger time steps allowed a better release of the chargedwater reducing consequently the hysteresis.

Measurements performed at high relative humidity val-ues showed that the materials, and, in particular, the OILsample, were able to absorb a large amount of water �Fig. 5�.

A current intensity increase was observed also after that therelative humidity has reached a stable value close to 100%RH. This variation ranging from 1.5�10−10 to 2.5�10−8 Ashowed that the linseed oil absorbed the water vapor, in-creasing the charge carriers. The variation is of two orders ofmagnitude and the conductivity of the OIL samples wasabout a factor of 100 larger with respect to the BAK andOILF samples.

Samples showed a different responses to RH variations,suggesting a different ad/absorption mechanism. The materi-als exhibited good reversibility properties. Moreover, con-secutive measurements showed the absence of memory ef-fects caused by irreversible or temporary modifications onthis scale of time.

On-off cycles �i.e., nominally 100%–0% RH� were per-formed on anode and cathode of RPC electrode plates to putin evidence possible differences in their behavior toward wa-ter adsorption. Figure 6 shows the diagrams of four OILFsamples, two taken by anode �symbol �� plates and two bycathode �symbol �� plates. The shape of the electrical re-sponses and their absolute values are quite similar, demon-

FIG. 4. Cyclic electrical response toward RH variationsof the three samples with two different time steps ts

=300 s and ts=600 s �5 V�.

FIG. 5. Electrical response of the three samples vs time exposed to highconcentration of relative humidity �5 V�.

FIG. 6. Cyclic electrical response toward RH variations of different cathodeand anode samples; the samples are maintained at wet condition for 10 000 sand at dry condition for 4000 s �5 V, time step ts=300 s�.

064911-3 A. Bearzotti and L. Palummo J. Appl. Phys. 102, 064911 �2007�

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strating that no particular water effects were recorded on theelectrodes that were used with different polarizations on theRPC.

IV. CONCLUSIONS

This work is relevant in order to understand the ad/absorption mechanisms when the RPCs operate in controlledhumidified environments �RH �40%�.

The presence of fluorine on the RPC inner surface platesconfirmed the breaking of tetrafluoroethane molecule duringoperative conditions.

The interaction with water vapor of phenolic-melaminiclaminate with oil coated and uncoated surfaces is stronger forthe laminate coated with linseed oil than natural melaminicsurface, and this is reflected in a faster response to RHchanges and in higher surface conductivity values. Moreover,also the oil coated sample, with fluorine on his surface, ex-hibits a different behavior of current intensity toward RHcompared to the other samples. This suggest that the fluorinepresence changes the property of ad/absorption in function ofthe humidity.

For long exposure to high values of RH, the oil coatedsample is able to collect large amount of moisture. Althoughthe linseed oil has hydrorepellent properties, it can, however,adsorb moisture. No significant differences were found be-tween anode and cathode samples.

Investigations concerning the formation of fluorine onthe internal surface and its interaction with the linseed oil arein progress. One critical point is to determine whether thereare different percentages of fluorine on the two electrodes. Itis in writing course a detailed paper on the electrical proper-ties and humidity ad/absorption mechanisms of the linseedoil that is used to cover the phenolic-melaminic laminates.

All investigations here reported are useful to understand

and optimize complexes devices, such as RPC, that are usedin several particle physics experiments that will operate formany years.

ACKNOWLEDGMENTS

The authors are grateful Professor Rinaldo Santonicoand all members of RPC Group of INFN Roma Tor Vergata,in particular, to Dr. G. Aielli, both for supplied materials andfor many useful discussions and information about the prob-lem related to humidity in the RPCs. Moreover to Dr. Al-berto Rainer and Professor Enrico Traversa of the Depart-ment of Chemical Science and Tecnology, University ofRoma Tor Vergata, for support with EDS measurements, andMr. Marco Maiani, Mr. Luciano Frenguelli, and Mr. AntonioLampasona of the IMM-CNR Roma for technical support.

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064911-4 A. Bearzotti and L. Palummo J. Appl. Phys. 102, 064911 �2007�

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