Journal of Applied Sciences and Environmental Management World Bank assisted National Agricultural Research Project (NARP) - University of Port Harcourt ISSN: 1119-8362 Vol. 12, Num. 2, 2008, pp. 121-127

Journal of Applied Sciences and Environmental Management, Vol. 12, No. 2, 2008, pp. 121-127

Size Effects in Zinc-Kaolin Composite Resistors

BABALOLA, O.A.; AKOMOLAFE, T.

Physics Department, University of Ilorin, Nigeria.babalolashadeyinka@yahoo.com. 08052491009
* Corresponding author: Babalola, O.A.

Code Number: ja08039

ABSTRACT

Zinc-kaolin composite resistors have been produced from naturally occurring materials such as zinc and kaolin. The composite resistors were produced with zinc powder content varying from 0, 20, 50, 70 and 90 %(vol.). Composite rods of dimension 65 mm x 6.5 mm x 3.2 mm were produced in a mould using a compaction method with a constant pressure of about (6.35 ±0.02) x 10⁸ N/m². The rods were subjected to varying annealing temperatures ranging between 300°C and 1000°C for a duration of one hour with an electric furnace in increasing steps of 100°C. Results showed that inverse size effect namely, shorter resistors having higher resistance dominate the zinc-kaolin composite resistance at all filler concentrations considered. Inverse effect is highest in resistors annealed at 600°C and lowest for resistors annealed at 300°C. Observation showed that the magnitude of the inverse size effect is a function of the difference between the channel resistance of the rods and the resistance of its end-terminals.

Composite resistors or cermets consists of a thorough mix of an insulating ceramic phase and a suitable electrically conducting phase in powder form which is compressed under a suitable pressure and sintered at a high temperature. The electrical properties exhibited by the composites, which are unattainable in any of the separate constituent phases makes them extremely valuable for use in many applications such as discrete resistors in electric circuits, pressure sensors in piezoresistive transducers, temperature sensors gas sensors e.t.c. (Ruschau et al, 1992 and Ayodele and Akomolafe, 2005). The electrical conductivity of metal-ceramic composites depends mainly on the conducting filler, the insulating matrix, its annealing temperature and the composites temperature. From the filler point of view, the type, size, volume fraction, orientation and aspect ratio of particles in the matrix are important. It is known however that the bulk resistivity of a composite resistor may also be affected by cermet geometry. This is known as size effect; namely the dependence of composite resistor resistance on resistor length. Ideally, for materials which obeys Ohm’s law, resistance should increase linearly with resistor length but Cattaneo et al (1977) have shown that metal migration especially at the resistor termination has been found to influence the resistivity of a resistor near the contacts which would make the otherwise ohmic resistors behave otherwise. Ligabue(1994) and Ruffi (1984) observed that factors which affect the final variation of cermets resistance with length also depend on the nature of materials and firing parameters: interaction of the conducting grains with the insulating matrix, sintering and ripening of the metal-oxide grains and partial crystallisation of glass. These gives rise to size effect, noticed as dependence of cermets resistance on the resistor length. Work done by Akomolafe and Oladipo(1991) on the electrical properties of Fe-clay resistors showed that the resistors exhibited an “inverse effect” although he did not explain why.

When the effective resistance for short resistors is higher than that for longer resistors, the material is said to exhibit “inverse effect”; otherwise it exhibits a “direct effect”. Prudenziati et al (1991), Morten et al (1991) and Hrovat et al (1986) have reported this effects in various works. It is known that size effect in resistive phase is due to one or both of these causes, a change in shape or thickness when the resistor length varies and/or a change in resistivity along the layer associated with a compositional change. This work reports the effect of annealing temperatures on the size effect phenomenon observed in zinc-kaolin composite resistors. We attempt to explain the cause and the variation of this size effect through the use of scanning electron microscopy (SEM), x-ray fluorescence (XRF) of the cermets in conjunction with the variation of composite resistivity. In the cause of this research the effect of metal migration from the terminations was prevented from influencing the results by the use of stainless steel electrodes.

MATERIALS AND METHODS

Commercially available kaolin powder served as the insulating matrix while zinc powder of about 99.5 % purity served as the conducting filler. Two sets of composite resistors were fabricated. In the first set, each resistor was cut into precise lengths of 5 mm to 60 mm in increasing steps of 5 mm. The second set served for the control experiment, each resistor fabricated had a constant length of 65 mm from which varying lengths were tapped along its length by the steel probes, which served as the electrodes. In the first set, composite resistors were fabricated using zinc powder filler contents of 0, 20, 50, and 90 %(vol.) to kaolin powder volume. To each composite mix, a drop or two of sodium silicate was added to

serve as a binder. These resistors were fabricated from the semi-dry powder mixtures using a compaction method in a mould each to a dimension of 65 mm x 6.5 mm x 3.2 mm. All the resistors were fabricated using a constant pressure of about (6.35 ±0.02) x 10⁸ N/m². The resistors were then cut and the terminations were ground flat into varying lengths between 5 mm and 60 mm in increasing steps of 5mm. The composite resistors were then air-dried for about two weeks before being heat-treated at varying annealing temperatures of 300, 600 and 800°C in an electric furnace for a duration of sixty minutes. All the composite resistors were oven-cooled from their respective annealing temperatures to room temperature to prevent internal stresses which can result into the build up of cracks in the cermet rods. Variation of cermets resistance with the lengths of the rods were measured using the two-probe technique with the steel electrodes applied to the ground terminals under a constant pressure. The measured variations of resistance with length were normalized using the relation

as used by Stein et al (1979) and Akomolafe and Oladipo(1991), were Rl is the resistance of a given length of a resistor composition and annealing temperature. R_{Ref(60 mm)} is the resistance of the maximum length of the resistor of the same composition and annealing temperature ( in all cases, we used a length of 60 mm as our maximum or reference length). The plot of the variation of the normalized resistance RN with composite resistor length gives an indication of the of size effect. In the second set of fabricated resistors, each of the zinc-kaolin composite resistors were fabricated as in the first set at a constant pressure of (6.35±0.02) x 10⁸ N/m² using zinc to kaolin composition of 20, 50, 70 and 90 %(vol.) to a dimension of 65 mmx 6.5 mm x 3.2 mm. These composite resistors were also air-dried for about two weeks before being heat-treated at varying annealing temperatures of 300, 600 and 800°C respectively in an electric furnace for a duration of sixty minutes. All the composite resistors were also oven-cooled from their respective annealing temperatures to room temperature. The variations of cermets resistance with the lengths of the rods tapped along its length were also measured using the two-probe technique with the weighted steel electrodes acting as voltage probes. The measured variations of resistance of each cermet rod were also normalized using equation 1 and plotted against increasing probe interval at all filler concentration and annealing temperatures.

RESULTS AND DISCUSSION

The effect of the normalized resistance RN of the first set of cermets resistor with length is shown in Figures 1,2, 3. The result shows that inverse effect, namely shorter resistors having higher resistors dominate the behaviour of the zinc-kaolin cermet resistors at all the filler concentrations observed in agreement with the results obtained by Stein et al (1979) and Akomolafe and Oladipo(1991).

We propose that the inverse effect is most likely the result of depletion of the free zinc filler at the end-terminals of the resistors through oxidation, leaching and chemical reactions with the kaolin matrix composition probably assisted by the furnace gasses ability to diffuse through them. We assume that the depth of oxidation or depletion of the free zinc metal is constant irrespective of the length of the composite resistors so that the resistivity of the end terminals are expected to be much higher than the bulk resistivity of its mid section. As a result of this, shorter resistors would possess higher resistivities than longer resistors i.e. an inverse effect is exhibited. Inverse effect is minimum at an annealing temperature of 300°C probably because the average resistivity of the cermets is high at this temperature and comparable to the resistivity of its end-terminal resistance due to a number of reasons such as incomplete sintering of the zinc-kaolin structure and the non-initiation of coagulation of zinc powder since its melting point is about 419.5°C. We also plotted the variation of composite resistance with annealing temperatures at various zinc filler concentrations an observe that resistivity is highest for composites annealed at 300, 400, 900 and 1000°C and lowest for composites annealed at temperatures between 500 and 800°C as seen in Figure 4, which show the variation of resistance (on a log scale) with length but emphasizing the effects of annealing temperatures.

The low magnitude of the inverse effect at an annealing temperature of 800°C is also likely the result of the averagely high cermets bulk resistivity also comparable to the resistivity of the end terminal resistance. Inverse effect is highest for cermets annealed at about 600°C where the average bulk resistivity of the cermet is lowest. XRF for the 70 %(vol.) zinc-kaolin cermet sample taken from its mid-section and its end-terminals are respectively shown in figures 5a and b respectively for cermets annealed at 700°C. We noticed a higher oxygen content for the end terminal sample as revealed in figure 5b which indicates that oxidation is more advanced than for the samples taken from the mid section.

A marked structural change is also noticed between samples taken from the mid-section (Fig. 6a) and that from the end-terminals (Fig. 6b) from the SEM of the 30 %(vol.) zinc composite annealed also at 700°C. This structural change indicates that the inverse effect observed is as a result of compositional change along the length of the cermet resistors as also observed by Prudenziati et al (1991), Morten et al (1991) and Hrovat et al (1986).

To confirm our suspicion on the origin of the inverse effect, in zinc-kaolin cermet resistors, we plot the variation of cermet resistance with resistor length for the second set of the fabricated composite resistors.

The result show that the variation of resistance with length is linear indication that size effect is absent as can be seen in figure 7.

Figure 8 shows that the variation of cermets resistance annealing temperature is a form of trough where resistivity is high both at low annealing temperatures T_f ≤ 400°C and at high annealing temperatures Tf > 800°C.

This behaviour of the cermets resistance with annealing temperatures is thought to be the cause of the inverse effect observed in zinc-kaolin cermet resistors. The terminations have been made with stainless steel electrodes to prevent the influence of metal/electrode migration as observed by Prudenziati (1977) who used Bi flux on Pt/Au terminations, which was found to diffuse into the composite resistor. Since the electrodes were not fired with the cermets, its deterioration as observed by Kuzel and Josef (1981) also does not affect our results.

Conclusion:

The variation of resistance of a resistor with its length is known as size effect. In some materials the effective resistivity of shorter resistor is larger than that for longer resistors; this is known as inverse effect otherwise on the other hand when longer resistors have higher resistivity than shorter resistors, direct effect is said to be exhibited. We have shown that the method of zinc-kaolin cermet resistor production before annealing affect whether or not “size effect” is exhibited. The inverse effect is most likely the result of oxidation of the end terminals of the resistors. Visual inspection shows that the depth of structural change/oxidation is constant irrespective of the length of the cermet resistors. This causes the shorter resistors to possess higher resistivity than the longer ones.

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