Electric resistivity of some soil complexes

Transcription

1 High Temperatures ^ High Pressures, 2002, volume 34, pages 499 ^ 504 DOI: /htjr047 Electric resistivity of some soil complexes M Saad Mostafa, Nasser Afify, Abdel-Fattah Gaber, Mohammed Abu El-Oyoun Physics Department, Faculty of Science, Assiut University, Assiut, Egypt; fax: ; mostafa@aun.eun.eg Received 1 September 2000; in revised form 24 bruary 2002 Abstract. The electric resistivity of some soils was investigated from room temperature to 700 K. The studied soils represent Assiut valley desert soils (S 1 ) and farmland of Assiut University soils (S 2 ). The temperature dependence of the electric resistivity is discussed; it was found to decrease with increasing temperature. The results indicate that the electric resistivity depends on ions and only to a small extent on electrons. Also, the electric resistivity values lie in the range of insulating materials. The results are supported by scanning electron microscope (SEM) analysis and a mechanical analysis. 1 Introduction Soils are commonly sampled with the purpose of measuring their biological, chemical, and physical properties. Soil properties naturally vary in space and time owing to soil formation processes, climate, crop growth, and tillage practices. Variations may also be due to undersampling, sample handling, and analysis. The type of the soil sample may be especially important for soil properties that can undergo rapid transitions and are dependent on local conditions. The main purpose of this work was to study and discuss the electric resistivity of these soils over a wide temperature range. This study is related to the use of these materials as electric insulators in industry because of their cheapness and abundance all over the world. It has been difficult to determine some results at different temperatures with high precision. The present paper is an analysis and selection of data from many experimental results. 2 Materials Sample S 1 was obtained from the Assiut valley desert which lies on the eastern bank of the river Nile, northwest of Assiut City in Egypt. It is generally considered as a great drainage trunk channel that runs southwest from the watershed of the eastern desert to the Nile floor (Said 1981). Sample S 2 was obtained from the farmland of Assiut University. Assiut University campus is located on 600 acres in Assiut City. Assiut lies on the west bank of the Nile, 375 km south of Cairo. The soils at these sites had no visual indications of heterogeneity, such as cracks, colour variations, wormholes, buried crop residues, etc. The calculated densities of the bulk specimens for S 1 and S 2 at normal atmospheric temperature are 1858 and 1892 kg m 3, respectively. These densities agree well with those reported by Ismail (1996) and El-Desoky and Ghallab (1997). Mechanical analysis of the samples was carried out and the results are listed in table 1. For mechanical analysis, the particle size distribution was determined according Table 1. Mechanical analysis of S 1 and S 2 specimens. Sample Sand/wt% Silt/wt% Clay/wt% S S

2 500 M S Mostafa, N Afify, A Gaber, M Abu El-Oyoun Table 2. SEM analysis of S 1 and S 2 specimens. Element Assiut valley soil (S 1 ) Assiut University soil (S 2 ) wt% at% wt% at% Si Ca Al K Cu Zn Ti Total to Day (1965). The scanning electron microscope (SEM) examinations were performed with JOEL JSM 5400 LV, Japan. The SEM is provided with energy dispersive x-ray spectrometry (EDS) to analyse the phases of the specimens. The characteristic x-rays allow quantitative or qualitative analysis of the chemical composition. The values obtained are in agreement with those reported in the literature (Donahue et al 1983; Ismail 1996; El-Desoky 1997; El-Desoky and Ghallab 1997). The major component is clay, which has high electric resistivity. The calcium carbonate contents in the specimens are 35.85% and 8.72% for S 1 and S 2, respectively. The high content of calcium carbonate is probably due to the lime content of the eastern plateau (El-Gibaly et al 1972; Faraghallah 1995). Mechanical analysis shows a large difference in the clay and sand contents of the samples. The electric resistivity of these specimens was derived from x-ray fluorescence (XRF) analysis of soil minerals as oxides (Mostafa 1999, 2000). The SEM analyses are shown in table 2. 3 Experimental The soil samples S 1 and S 2 were air dried, crushed, and passed through a 2 mm sieve to separate gravel, which was not considered. The powdered samples were compressed in a special die under a pressure of 2 tcm 2 to produce disk-shaped specimens 20 mm in diameter and 4 to 6 mm thick. The specimens were treated by heating in a furnace at about 800 K for 4 h in order to expel any water and eliminate oxidation that could occur during the measurements. To ensure good contact, the surfaces of the samples were coated with a thin layer of silver solution. The apparatus used comprised a vacuum system with a vacuum chamber, special holder, associated equipment, and means of measuring the required parameters (Mostafa 1983). A block diagram of the measurement setup is shown in figure 1. The sample 1 is heated by the spiral heater 2. The power is supplied to the heater from an AC voltage stabiliser 3 through a variac transformer 4. The mean temperature is measured by a Chromel ^ Alumel thermocouple 5 (Omega HH 12, USA) attached to the sample. The resistance of the sample is measured with a type 610 C electrometer 6, and the data are used to calculate the electric resistivity of the specimens. Special insulated cables are used for the measurements. 4 Results and discussion The electric resistivity, r, of the samples was measured over the temperature range 300 K to 700 K. Each experimental point represents the average of four values measured at the same temperature. In general, it can be seen that the measured values of electric resistivity decrease with increasing temperature. This decrease may be attributed to the thermal agitation

3 Electric resistivity of some soil complexes Figure 1. Schematic diagram of the measurement setup: 1, sample; 2, spiral heater; 3, voltage stabiliser; 4, variac transformer; 5, Chromel ^ Alumel thermocouple; 6, electrometer. of molecules creating vacancies and deformations. The electric resistivity of these materials is generally determined not only by the properties of the molecules but also by the different modes of intercellular interactions. The temperature dependence of the electric resistivity of S 1 and S 2 specimens is plotted in figure 2. The relation can be represented empirically by the formula: r ˆ r 1 e E=kT 1 r 2e E=kT 2, (1) where E is the activation energy, T 1 and T 2 are the absolute temperatures, k is the Boltzmann constant, and r 1 and r 2 are constants. The first term of equation (1) corresponds to the mechanism of conduction in the low-temperature region, while the second term corresponds to the mechanism of conduction in the high-temperature region. Little work has been carried out on the resistivity of soil at high temperatures for fundamental physics needs. However, there are practical and engineering interests where such properties are relevant. Soils are quite variable and complex in their electric properties. Most of them are insulators, but some are semiconductors. Their electric properties depend largely on their chemical composition. The general theory of electric resistivity of these materials is not adequate. This is because these materials are new to study. In recent S 1 S 2 r=10 8 ohm cm T=K Figure 2. The temperature dependence of the electric resistivity, r, ofs 1 and S 2 specimens.

4 502 M S Mostafa, N Afify, A Gaber, M Abu El-Oyoun ln (r=10 8 ohm cm) S 1 S K=T Figure 3. Plot of ln r versus T 1 for S 1 and S 2 specimens. years the conditions under which electrically insulating materials have to operate in electrical and radio-electronic devices have become much more severe. In a number of cases high working temperatures are required by the functional purpose of the device (electric furnace, high-power electric devices, etc). Therefore further investigation on the application of these materials as insulation is recommended. At high temperatures, equation (1) is reduced to: ln r ˆ ln r 2 E 1. (2) k T 2 Plots of ln r versus 1=T 2 for S 1 and S 2 samples are shown in figure 3. The above formula is similar to that describing quantitatively the electric behaviour of semiconductor materials at high temperatures (Incokuchi and Akamatu 1961). The linear relation indicates that the investigated compounds behave like semiconductors. The calculated activation energies, E, are and ev for S 1 and S 2, respectively. The low values of activation energy of these materials may be attributed to the contribution of ionic conduction. Also, these values are in the range of semiconductor materials and minerals (Rzhevsky and Novik 1971). The electric resistivity seems to be more influenced by soil texture, ie by the electric properties of the soil constituents, than by its structure. Therefore these materials can be used as semiconductors in more applications because they are cheap and economic. Because large-scale oil sampling and related parameter characterisation are time consuming and expensive, alternative methods of investigating spatial variability are desirable (Banton et al 1997). The deviation and scattering of some points in figure 2 may be attributed to the sensitivity of the macrostructure and heterogeneous character of the materials (Touloukian et al 1981; Taylor 1983). Also the discontinuity in the electric resistivity curve with temperature may be attributed to the chemical changes expected in these materials (Mebed 1983). Their electric resistivity behaviour is in accordance with DTA analysis and TGA analysis (Mostafa 1999, 2000). The dispersion of our results may be due to the formation of complexes. Our interpretation of the results were confirmed by SEM analysis and x-ray microanalysis. The SEM analysis for S 1 and S 2 specimens shown in table 2, is in agreement with XRF and XRD analyses (Mostafa 1999, 2000). The x-ray microanalyses obtained by SEM for S 1 and S 2 are shown in figures 4a and 4b, respectively. These figures are in agreement with those obtained from x-ray diffractograms (Mostafa 1999, 2000). From the

5 Electric resistivity of some soil complexes Si 1500 Ca Counts 1000 (a) Al Ca KCa Cu Energy=keV 2000 Si Counts Ca 500 K Ti Ti K Ca Ca Ti (b) Energy=keV Figure 4. SEM micrographs for (a) S 1 and (b) S 2. SEM analysis of these specimens, it is seen that silicon oxide (SiO 2 ) form a large part of their composition, which includes different minerals with high electric resistivity, such as hyalite, chert, and a-quartz. These soils contain also calcium oxides (CaO) with relatively low electric resistivity. The SEM analyses are thus in agreement with the electric resistivity values, which indicated that these specimens are semiconductor materials. However, materials regarded as intermediate between crystalline and amorphous compounds may also be present in a soil deposit (Mostafa and Gaber 1989). Furthermore, the lack of the experimental results in literature on the electric resistivity of these compounds at high temperatures precludes a quantitative comparison. The different types of soils make these comparisons difficult. From the above results it can be concluded that the electric resistivity in the investigated range of temperatures lies between and 10 8 ohm cm. This is the range which characterises semiconductors and dielectric materials.

6 504 M S Mostafa, N Afify, A Gaber, M Abu El-Oyoun 5 Conclusions We conclude that, the electric resistivity of these soil specimens depends on ions and to a small extent on electrons. The temperature dependence of electric resistivity in the range 300 ^ 700 K indicates that they can be regarded as semiconductor materials. The actual values on electric resistivity, r, also lie in the range of semiconductor materials. SEM analysis of these specimens confirmed the results obtained for the electric resistivity. Therefore further investigation into the application of these soils as semiconductor and dielectric materials is recommended. References Banton O, Seguin M K, Cimon M A, 1997 Soil Science Society of America J ^ 1017 Day P R, 1965, in Methods of Soils Analysis Part 1 Ed. C A Black (Madison, WI: American Society of Agronomy) pp 545 ^ 547 Donahue R L, Miller R W, Shickluna J C, 1983 Soils: An Introduction to Soils and Plant Growth (Englewood Cliffs, NJ: Prentice-Hall) El-Desoky M A, 1997, in Proceedings of the First Scientific Conference of Agricultural Science, Assiut University, Assiut, Egypt, 13 ^ 14 December volume 1, pp 483 ^ 496 El-Desoky M A, Ghallab A, 1997, in Proceedings of the First Scientific Conference of Agricultural Science, Assiut University, Assiut, Egypt, 13 ^ 14 December volume 1, pp 432 ^ 448 El-Gibaly M H, Khalifa E M, Kishk M A, 1972 Assiut J. Agric. Sci ^ 450 Faraghallah M E, 1995, MSc thesis, College of Agriculture, Assiut University, Assiut, Egypt Incokuchi H, Akamatu H, 1961, in Solid State Physics volume 12 (New York: Academic Press) Ismail S M, 1996, MSc thesis, Department of Soils and Water, College of Agriculture, Assiut University, Assiut, Egypt Mebed M M, 1983 High Temp. ^ High Press ^ 118 Mostafa M S, 1983, PhD thesis, Assiut University, Assiut, Egypt Mostafa M S, 1999 High Temp. ^ High Press ^ 551 Mostafa M S, 2000 Egypt J. Phys. 31(3) 231 ^ 243 Mostafa M S, Gaber A, 1989 J. Therm. Anal ^ 2321 Rzhevsky V, Novik G, 1971 The Physics of Rocks (Moscow: Mir) Said R, 1981 The Geological Evolution of the River Nile (New York: Springer) Taylor R, 1983 High Temp. ^ High Press ^309 Touloukian Y S, Judd W R, Roy R F, 1981 Physical Properties of Rocks and Minerals (New York: McGraw-Hill) ß 2002 a Pion publication