AC conductivity of C130 type electroporcelain as a function of firing temperature and compaction pressure

Dependence of the AC conductivity of the C-130 type electroporcelain on the compaction pressure was measured. The samples were manufactured from spray-dried mixtures using isostatic compaction in vacuum at 5 different pressures (70 MPa to 110 MPa, 10 MPa step size). AC conductivity measurements were performed at 25 different frequencies ranging from 50 Hz to 3 MHz at room temperature on samples with dimensions 10 × 10 × 3 mm3. In addition to the compaction pressure, the effect of different firing temperatures was studied (1100°C and 1200°C). The highest values of AC conductivity were observed in the case of the raw samples, due to an amount of physically and structurally bound water. Conductivity relaxation was observed after firing at 1200°C in all samples at 2.5 MHz. A small effect of the compaction pressure was observed for the raw samples and samples fired at 1200°C.


Introduction
Ceramic materials are widely used in numerous fields of everyday applications.One of these fields is the distribution of electricity, which requires insulation materials.Ceramic materials are good insulators.The preparation process, as well as the source materials, are cost effective, therefore making them ideal for these applications.The technology of preparation of ceramic materials is of special importance, as it affects the final properties of the manufactured ceramic parts.Thus, the preparation method (such as compacting, extrusion, etc.) is usually chosen considering the final application of the prepared part [1][2][3].Compacting is a very widely used preparation process, as it allows the preparation of ceramic materials in various shapes and sizes.The C-130 type high strength alumina porcelain is one of the most widely used materials for preparation of high voltage ceramic insulators, which must cope with weather fluctuations for several decades.Numerous authors have studied the effect of the compaction pressure on the mechanical, microstructural, and other physical parameters (bulk density, shrinkage upon firing or water absorption) [4][5][6][7][8][9].An increase in the mullite crystals length was reported with an increasing compaction pressure.Nonetheless, no effect of the compaction pressure on the mechanical properties (Vickers hardness, flexural strength, and Young's modulus) was observed.During thermal treatment, the mass loss and the shrinkage decreased with the compression pressure of the samples.However, only a minor effect of the compaction pressure on the bulk density was reported.As reported in [10], the effect of the firing temperature was more pronounced than the effect of the compaction pressure.As the firing temperature increased, the porosity of the samples decreased from 28% at 1100°C to

Materials and methods
The electroporcelain raw material was supplied by the PPC Čab company and was prepared according to the standard IEC 60672-3 [11] using feldspar (26 wt.%), chamotte (20 wt.%), clay (20 wt.%), and alumina (34%).The aforementioned initial materials comply with the standard industrial recipe for the C-130 type electroporcelain.After wet mixing of the initial materials, the spray drying procedure was used to obtain dry granules with particle size ~4 µm.The dried powder was then used for the preparation of the raw electroporcelain bodies using isostatic compaction under vacuum at pressures ranging from 70 MPa to 110 MPa (with a step of 10 MPa).Samples for the AC conductivity measurements (10 × 10 × 3 mm 3 ) were then cut from the as-prepared samples.AC conductivity measurements were performed using the Tegam 3550 LCR analyser at 25 different frequencies ranging from 50 Hz to 3 MHz.The conductivity was calculated from the measured values of parallel equivalent resistance (Rp) according to the equation.
where h is the sample thickness and S is the contact area between sample and electrode.The samples were placed between two platinum electrodes (13 × 13 mm 2 ).A thin graphite layer was coated to the samples surface prior to the measurements to ensure a good contact between sample and electrode.
Measurements were taken on samples in raw state, fired at 1100°C, and after firing at 1200°C.In all cases, a set of 2 samples was measured and then the average value was taken.If the two samples exhibited significantly different results, a third measurement was performed.All samples were dried after the application of the graphite layer at 120°C for 60 min.The measurements were then performed at room temperature after free cooling of the samples.

Results and discussion
Electrical conductivity of ceramic materials is mainly determined by the concentration of free, or weakly bound ions, which are capable of carrying the electrical current.Therefore, it is expected, that in raw state the electrical conductivity will achieve higher values, compared to that after firing, due to a content of physically and chemically bound water.From a point of view of AC conductivity, water molecules act as dipoles and, up to a certain frequency, will follow the changes of the external electric field.On the other hand, after the removal of the physically and structurally bound water a decrease in the conductivity is expected.Firstly, the AC conductivity of the raw samples was examined (Figure 1a).In the low frequency region (up to ~2 kHz) the dependence of the AC conductivity on the frequency exhibited a small gradual increase for all samples, being the most significant for the sample prepared with compaction pressure of 70 MPa.However, by increasing the frequency a more pronounced increase in the conductivity was observed.At low frequencies, the long-rage hopping (DC-like conductivity) of the charge carriers dominates.The number of these species is limited to the weakly bound ions, and thus the conductivity is low.However, with an increase in the frequency, the short-range interactions become important.On the other hand, at very high frequencies, the dipoles became unable of following the changes of the external field, and thus they no longer participate in the conduction mechanism.In the present case, the maximum frequency was still low enough for the ions to follow the changes of the field.Despite the different compaction pressures applied, no differences in the frequency dependence of the AC conductivity of the samples were observed.A similar behaviour was observed after firing at 1100°C (Figure 1b).Nonetheless, the slow gradual increase in the AC conductivity of the samples with an increasing frequency was prolonged and ended at ~200 kHz.After the samples were heat treated at 1200°C (Figure 1c), the nature curve of the dependence of the AC conductivity of the frequency was changed.The exponential growth of the AC conductivity with an increasing frequency started above ~200 kHz.The conductivity then manifested a steep increase up to 2.5 MHz.At higher frequencies, the conductivity decreased suggesting, than a relaxation mechanism was taking place.In all cases, no difference between the individual samples was observed.To reveal the dominant conduction mechanism, Jonscher's law was employed [12] () =  0 + where σDC is the frequency independent part of the conductivity (DC-like), A is the pre-exponential factor, ω is the angular frequency, and s is the frequency exponent.Table 1 summarizes the fit parameters.The frequency exponent s varied in a narrow range between 0.5 and 0.8 suggesting, that ion hopping was the dominant conduction mechanism in all samples [13][14][15][16][17].The frequency independent part of the conductivity steadily decreased with an increasing firing temperature, reaching the highest values for the raw samples.The AC conductivity of the raw samples increased with increasing compaction pressure up to 90 MPa.This increase was followed with a decrease in the AC conductivity as the compaction pressure increased to 100 MPa and 110 MPa.After firing at 1100°C, no dependence of the AC conductivity on the compaction pressure was observed.For the samples fired at 1200°C an initial increase in the AC conductivity with the compaction pressure was observed (up to 80 MPa), which was followed with a decrease, after which no dependence on the compaction pressure was shown.Table 1.Fit parameters of the frequency dependence of the AC conductivity of the samples.Comparison of the AC conductivity of the raw sample and the fired samples (Figure 2a and 2b) revealed that the conductivity decreased with an increasing firing temperature.This decrease was the most evident in the high-frequency part.In raw state, the samples contained an amount of water molecules, which increased the conductivity.After performing the heat treatment, these molecules were removed from the samples what in turn led to a decrease in the AC conductivity.The aforementioned clearly proves, that while the effect of the compaction pressure is negligible, the effect of the firing temperature is significant.During heat treatment, the samples mineral composition has changed.In raw state, the samples contained 26 wt.% of amorphous phase, which increased to 56 wt.% and 62 wt.% after a firing at 1100°C and 1200°C, respectively [10].

Conclusions
AC conductivity of the C130 type electroporcelain was studied as a function of firing temperature (raw, 1100°C, and 1200°C) and compaction pressure at 25 different frequencies ranging from 50 Hz to 3.5 MHz.The experimental samples were prepared using isostatic compaction at 5 different pressures ranging from 70 MPa to 110 MPa with a step of 10 MPa.It was found, that -The AC conductivity of the raw samples increased with increasing compaction pressure up to 90 MPa, which was followed with a decrease as the compaction pressure further increased up to 110 MPa, -After firing at 1100°C, no dependence of the AC conductivity on the compaction pressure was observed, -after firing at 1200°C an initial increase in the AC conductivity with the compaction pressure was observed (up to 80 MPa), which was followed with a decrease to a constant value (no dependence on the compaction pressure), -Jonscher's power law was employed to reveal the dominant conduction mechanism.As the frequency exponent remained below 1, ion hopping was identified as the dominant conduction mechanism, -the firing temperature had a significant effect on the AC conductivity of the samples, mostly at high frequencies, -after a firing at 1200°C a relaxation of the AC conductivity was observed at 2.5 MHz.

Figure 1 .
Figure 1.AC conductivity of C-130 electroporcelain samples prepared at different pressures in raw state (a), after a firing at 1100° (b), and after a firing at 1200°C (c).

Figure 2 .
Figure 2. AC conductivity of samples prepared at 70 MPa and 110 MPa compaction pressure after different heat treatments.4.ConclusionsAC conductivity of the C130 type electroporcelain was studied as a function of firing temperature (raw, 1100°C, and 1200°C) and compaction pressure at 25 different frequencies ranging from 50 Hz to 3.5 MHz.The experimental samples were prepared using isostatic compaction at 5 different pressures ranging from 70 MPa to 110 MPa with a step of 10 MPa.It was found, that -The AC conductivity of the raw samples increased with increasing compaction pressure up to 90 MPa, which was followed with a decrease as the compaction pressure further increased up to 110 MPa, -After firing at 1100°C, no dependence of the AC conductivity on the compaction pressure was observed, -after firing at 1200°C an initial increase in the AC conductivity with the compaction pressure was observed (up to 80 MPa), which was followed with a decrease to a constant value (no dependence on the compaction pressure),