Bhuiyan, A H

Chowdhury, F U Z

English

IACS Institutional Repository

Indian J. Phys. 71A (6), 635-642 (1997)I J P  A— an international journalStudy o f  the dc electrical properties o f  B ijoypur  W hite C lay o f  BangladeshF-U-Z Chowdhury* and A H Bhuiyanb ^Department of Physics. Bangladesh Institute of Technology, Chittagong, Chittagong-4349, BangladeshbDepartment of Physics, Bangladesh University of Engineering & Technology. Dhaka-1000. BangladeshReceived J8 June 1997, accepted 7 August 1997Abstract : The current-voltage (1-V) characteristics of washed Bijoypur White Clay (BWC) samples, heat treated at 1273 K were investigated. It is found that Space Charge Limited Conduction (SCLC) mechanism is operative in these materials. The dc conductivity measurements of the different BWC samples prepared at 2000. 3000 and 3500 psi pressures were performed in the temperature range 300 to 673 K The temperature dependence of the dc electrical conductivity shows electrolytic behaviour below 380 K and semiconducting above The temperature dependence of electrical conductivity at the higher temperature region (above 380 K) indicates that the conduction is due to the thermally activated variable range hopping of charge carriers in the localized states.Keywords : DC conductivity, hopping conduction, white clay PACS Nos. : 72.20.Fr, 72.60+g, 72.80.-r1. IntroductionBijoypur White Clay (BWC) found in the Netrokona district of Bangladesh is a white burning clay. BWC is being used in ceramics, insulators and sanitary industries. The chemical analysis of BWC has been reported by several authors [1—3]. Chowdhury and Bhuiyan [4] carried out an investigation on the structural modification of BWC on heat treatment and found that BWC modifies on heat treatment to a possible mullite phase. Sharif and Mian [5] reported preliminary results of the ac and dc electrical studies of graphite powder mixed BWC prepared at temperature 1073—1123 K in air. Rahman and Haque [6] prepared resistors from the mixtures of BWC and graphite powder in the proportion l : 3 by weight and investigated the I-V characteristics up to the applied voltage of about 10 V. It is seen from the different investigations on BWC that it is possible to© 19971ACS636 F-U-Z Chowdhury and A H  Bhuiyanprepare composite type passive electronic components of desirable electrical properties. Before going for useful applications of BWC, it is very much essential to know its electrical behaviour.To the authors knowledge, the detail dc electrical properties of washed and heat treated BWC have not yet been reported. The potentials of BWC for electrical and other technological applications are uncertain without a fundamental understanding of its electrical properties. In this paper, an attempt is made to report the dc electrical properties of as-collected, washed and heat treated BWC.2. Experimental detailsi2.1. Sam ple preparation : \The uncompressed hard and solid BWC was collected after removing th  ^ overburden of 5 m thickness from mine. The as-collected BWC was washed in the laboratory using a process that has been described earlier [7]. The washed clay suspension was dried in an electric oven at 378 K for 24 hours. This dried sample is labelled as washed BWC. The washed BWC was then ground and was passed through a 200 mesh sieve. The BWC was then placed inside a die punch of diameter 0.01 m. The washed BWC pellets were obtained with 2000, 3000 and 3500 psi pressures by using a hydraulic press unit. After removing the sample from the die punch, its upper and lower edges were polished and then a thin coating of conducting silver paste was applied which act as electrode for electrical measurements. Some of the prepared samples were heat treated in air at 773 and 1273 K for 6 hours before the silver paste coating was applied. The samples are designated as :Samples Prepared at pressure (psi)Al 2000As-collected BWC (A) A2 3000A3 3500B/ 2000Washed BWC (B) B 2 3000BJ 3500Washed BWC heat Cl 2000treated at 773 K (C) C2 3000C3 3500Washed BWC heai Dl 2000treated at 1273 K (D) D2 3000D3 35002.2. M easurem ent techn iques:The current and dc resistance was measured by a digital Keithley Electrometer (model 614, Keithley Instrument Inc., Cleveland, Ohio, U.S.A.). A pressure contact specimen holder,Study o f  the dc electrical properties ofBijoypur White Clay etc 637housed in a steel chamber, with an inbuilt heater was employed for these measurements at 303, 373, 423, 473 and 523 K. A transistorized power supply, model 6181C, of Hewlett Packard was used for constant voltage supply across the samples and the dc conductivity was calculated from the measured dc resistance at different measuring temperatures from 303 to 673 K. The I-V characteristic measurements were performed upto 100 V. The temperature was recorded by a Cr-AI thermocouple by measuring the emf with a microvoltmeter (model 197 A of Keithley Instrument Inc., Cleveland, Ohio, U.S.A.). All the measurements were performed in air.3. Results and discussion3 . 1. C u r r e n t - v o l t a g e  ( l - V )  c h a r a c te r i s t i c s  ;I-V characteristic curves of sample D, for measurement temperatures 303, 373, 423, 473 and 523 K are presented in Figure 1. It is seen that current falls first and then increasesL o g ( V ) — -Figure 1(a). Current versus voltage at different temperatures for washed and heat treated at 1273 K BWC samples at 2000 psi.gradually with measurement temperature. This fact will be discussed in the next section. It is observed that the l-V  curves fit to a relation / «= V*. where n is a power factor. It is seen that first three temperature curves (303, 373 and 423 K) for D1 and D2 have three different slopes at low, intermediate and high voltage regions while there are two different slopes in the remaining two high temperature (473 and 525 K) curves. However, there are only two different slopes for all the temperatures in case of D3. The values of n  for all the curves are depicted in Table I.In Table I, it is seen that the low voltage region is ohmic and the intermediate and high voltage region follow a square law and a cube law with trap respectively. It is seen that the ohmic region is not present in the high temperature curves of samples D1 and D2. There are also two slopes over the voltage range employed for all the temperatures in the sample638 F- U-Z Chowdhury and A H BhuiyanD3, which corresponds to the region containing traps. When the transport is slower than the generation of carriers then the conduction is known to be due to Space Charge Limited Conduction (SCLC) [8]. The above results suggest that the SCLC mechanism is operative in these materials. At low voltages, the 1-V characteristics are ohmic which may be related to adsorbed water, water of crystallization etc. In the intermediate and high voltage regions,Figure 1(b). Current versus voltage at different temperatures for washed and heat treated at 1273 K BWC samples at 3500 psi.the SCLC may be due to the higher concentration of the injected free carriers than that of the carriers due to the presence of water in the clay [8]. It is clearly seen that the temperature affects the SCLC behaviour which manifests the presence of traps in the material. Porosity plays an important role in deciding the concentration of the trap centre in a material. The absence of the ohmic region in sample D3 may be due to less porosity (low trapping centre) compared with that in the less pressurized samples D1 and D2 [9], resulting higher concentration of free carriers than that of the carriers related to water in the material even at low voltages.Tabic 1. Values of power factor n for sample D at different measurement temperatures.Pressure in psiVoltagerangen values below 423 Kn values above 423 KLow 0.95 1.462000 Intermediate 2.16 XHigh 3.22 3.22Low 0.70 1.423000 Intermediate 2.30 XHigh 3.01 3.013500 Low 1.46 1.46High 3.23 3.23Study o f she dc electrical properties o f  Bijoypur White Clay etc 6393 .2 . T e m p e r a tu r e  d e p e n d e n c e  o f d c  e le c tr ic a l  c o n d u c t i v i t y :Generally, clay adsorbs water and that affect the electrical properties. To observe the effect of water on dc electrical conductivity o .^, <7dc of sample D3 was measured for successive cycles. It is observed that after the first cycle of measurement, a dc becomes almost stable with lower values and on further heating, there is not appreciable change in the o ^ .  So, second cycle data are presented in this paper.The <7^ of all samples were measured over the temperature range 300 to 673 K. The plots of Ojc against 1/7 are shown for samples A and D in Figure 2. It is found that the room temperature 0dc value is lower for washed samples than that of as-collected BWC and decreases further on heat treatment from 10"M to I O'12 (Q-m)"1. The effect of pressure of pellet preparation on the o .^ is not much significant within the applied pressure range used in this investigation. It is observed that there is an initial increase of a &  up to 310 K in samples A and B and that up to 320 K in samples C and D and then starts decreasing up to 380 K in all the samples and again (7dc increases with increasing temperature.The initial increase in is probably due to the motion of hydroxyl ions and/or other species. The decrease of value above 310 K may be attributed to the compensation effect of ouigassing adsorbed water molecules, thereby decreasing the hydroxy] ion source and/or any other migratory ions. This initial increase and then fall of 0*  is a behaviour of solid electrolyte. Above 380 K, the increase of a*, may be due to the increased movement of adventitious ions and/or electrons within the bulk of the material.It is observed that the curves do not fit fairly to straight lines over the whole temperature range above 380 K for all the samples except for D. In sample D, there is a considerable departure from straight line above 500 K. The activation energies are calculated from the slope of the best fit straight lines of log 0*  -  1/7 plots. The values of Go, obtained by extrapolation of log -  1/7 plots to 1/7 « 0, are of the order of 10“10 to ICH. The activation energies and the pre-exponential factors of all the BWC samples are summarized in Table 2. The observed temperature dependence of conductivity and the activation energies calculated from the Arrhenius plots suggest that thermally activated conduction of charge carrier may be operative in electrical conduction in BWC.Tuhlc 2. Activation energy Er in eV and pte-expottenrial factor qp in (O -m r1 ■Sample Temperature in KPressure 3500 psiPressure 3000 psiPressure2000 psiEr Er Er ObA 3 73 -6 7 3 0.308 2.o x i< r10 0.278 1.4 x  I0 r10 0.238 i.i x i< r10B 373-673 0.255 i,4 x n r 10 0.230 1.0 x I0“ 10 0.227 5.0 x lOr11C 373-673 0.226 2.5 x lO-10 0.211 2.0 x  10^" 0.199 5.0 x lO- "D 373-323 0.225 1.4 x lo r " 0.197 1.3 x lO-11 0.181 1.1 x ICT11523^-673 0.497 i.7 x u r 9 0.411 5.0 x I0 r10 0.375 2.0 x i<r10640 F -U -Z  C how dhury  a n d  A H  B hu iyanT <K )Figure 2(a). DC conductivity against inverse absolute temperature of as- collected BWC sample.T I K ) ------Figure 2(b). DC conductivity against inverse absolute temperature of washed and beat teated a! 1273 K BWC sumple.As the curves illustrated in Figure 2 do not fit to simple straight lines, the sqme data are presented in Figure 3 as log v e r s u s  The plots show fairly straight line behaviour. These results are typical of transport in disordered and amorphous materials [10]. Here, the variable range hopping between localised states leads to a temperatureStudy o f the dc electrical properties o f Bijoypur White Clay etc 641• TD*ox•  2000 psi o 3000pw » 3500 psi<o10■°*• °  *•  o*•  \•o*tol• 6 *•  IO• o \- O -•  o196 20 22 23(K 1 -(b * U° x o•  o"JiI<310•  -  2000 psi o -  3000 psi « -  3600pm•  o *(b)•o*•o  i•  0 x•  O *•©  1 • O XM  20 ■21T'WIK"I* |  ■22 ■23Figure 3(a). Elec meal conduciiviiy versus r 1/4 of Figure 3(b). Electrical conductivity versus T u* ofas-collected BWC sample washed and heal treated (at 1273 K) BWC sample.dependence of Ihc form ff = Oj exp l—(7q /  7’)*^ ], where d  is the dimensionality of transport ( e .R . .  d  = 3 for three dimensional motions, e tc .)  [111. Therefore, thermally activated variable range hopping of charge carriers in localized state is responsible for electrical conduction in these materials.4. ConclusionsFrom n values, il is evident that SCLC with traps occurs in heat treated BWC which is dependent on porosity. The dc conductivity of washed samples is lower than those of as-collected samples. The crdc decreases on heat treatment and its value varies from 10’11 to 1 0 - ' 2  (Q-ni)-1. The initial increase of cr* values below 320 K is due to adsorbed water and/or of hydroxyl ions; and this adsorbed water or hydroxyl ions decrease on increasing temperature above 320 K. As a result, 0* decreases. The further increase of a*  above 380 K is probably due to increased movement of ions/electrons within the material. It is found that thermally activated variable range hopping conduction between the localized states is operative in these materials.71 A (6 M642 F-U-Z Chowdhury and A H BhuiyanAcknowledgmentWe are grateful for the financial supports to the authority of Bangladesh University of Engineering & Technology. One of us (FUZC) also thanks the United Nations Development Programme (UNDP) authority for their financial assistance. He is also indebted to the Bangladesh Institute of Technology, Chittagong authority for giving him study leave during the research work.References[1] M A Biswas and A K Basalt Pak. J. Set. Indus. Res. 4 118 (1961)[2] M Ql-Khuda, M A Biswas and A K Basok Pak. J. Sci. Indus Res. 1 43 (1964) j[3] M A Biswas and S J Ahmed Pak. J. Sci Indus. Res. 1 81 (1964)[4] F-U-Z Chowdhury and A H Bhuiyan Indian J Phys. 7IA 569 (1997)[5] R I Sharif and N H Mian Bangladesh J. Sn. Indus. Res. XVII26 (1982)[6] J Rahman and M Z Haquc Dhaka University Studies B33 179 (1985)[7] F-U-Z Chowdhury MPliil Thesis (Department of Physics, Bangladesh University of Engineering & Technology, Dhaka) (1993)[8] M Lamped Rep Pmy Phys 27 329 (1964)19] A Rose Phys. Rev 97 1538 ( 1955)[10] A K Jonscher J. Vac Tedmol (USA) 8 135 (1971)[11] N F Mott Phil May. (GB) 19 835 (1969)

Study of the Dc Electrical Properties of Bijoypur White Clay of Bangladesh

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Study of the Dc Electrical Properties of Bijoypur White Clay of Bangladesh

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