Thermodilatometry of Quartz Porcelain Mixture Under Tensile Loading

Specimens composed of 50 wt.% kaolin, 25 wt.% quartz, and 25 wt.% feldspar were subjected to constant tensile stress during linear heating at 5 °C/min rate to measure their tensile deformation behavior. Deformation occurs at temperatures above 300 °C and depends on the tensile stress: higher external tensile stresses led to a more pronounced expansion of the sample during the dehydroxylation region. Consequently, at temperatures above 1100 °C, sintering of the samples in the liquid state and intense contraction occurred. The dilatation behaviour of the samples was governed by the combined effect of opposing forces. One of these forces originated from external loading and caused the elongation of the specimen. The other force resulted from surface tension, which resulted in the shortening of the specimen. At temperatures above 1100 °C, a distinct V-shaped minimum appeared on the dilatometric plot, indicating a balance between these forces. As the temperature increased, the viscosity of the glassy phase decreased, weakening the surface forces. Eventually, the external force prevailed, causing further elongation of the samples.


Introduction
Quartz porcelain, a crucial component in the ceramic industry, undergoes significant physical and phase changes when subjected to high-temperature firing.These changes include releasing physically bound water, dehydroxylation, transformation of α-quartz into β-quartz, high-temperature reactions in kaolinite, and sintering.The resulting alterations in microstructure and structure have a direct impact on mass, dimensions, and mechanical and electrical properties [1][2][3][4].
A specific mixture composed of 50 wt.%kaolinite, 25 wt.% quartz, and 25 wt.% feldspar have a broad use in the ceramic industry, e.g., in the production of electrical insulators.An unfired highvoltage station post insulator is put on the bottom of the kiln cart to be subjected to temperatures between 1300 °C and 1350 °C.The firing process initiates crucial reactions within the insulator body as it heats up.At approximately 950 °C, the kaolinite undergoes high-temperature reactions and sintering, leading to a significant contraction of the insulator body.This contraction is a critical aspect of the firing process and influences the mechanical properties of the insulator.As the insulator undergoes contraction, its upper boom comes into mechanical contact with the beams of the kiln cart framework, with the insulator hanging on it (Figure 1).This mechanical contact is essential for stabilizing the insulator during the firing process.The weight of the insulator (the part with sheds and the bottom boom) applies tensile stress on the highest part of a core, and the stress continues to increase as the firing temperature rises.Throughout the firing process, the tensile stress can experience a significant change of approximately 18%, with values exceeding 60 kPa at the highest firing temperature.
To understand the processes occurring during firing, an apparatus tailored to imitate the firing conditions experienced by the post insulators had been designed and constructed.The paper aims to more accurately study the physical and phase changes that occur in the insulator during the firing process.
Understanding the complex physical and phase changes that quartz porcelain undergoes during firing is crucial for producing high-quality ceramic insulators.By examining the effects of hightemperature reactions, sintering, and mechanical stress, the manufacturing process could be improved to design more efficient insulators.The new apparatus provides a valuable tool for studying and optimizing the firing process, leading to advancements in ceramic materials and electrical insulation technology.

Materials and methods
Green samples were made from 50 wt.%kaolinite, 25 wt.% quartz, 25 wt.% feldspar, and water.The chemical composition of input materials is in Table 1.The Ø30 mm cylindrical blank for small low-voltage porcelain insulators was made using the vacuum extruder.After air drying, the blank contented ~1 wt.% of the physically bound water.Then the body of the shape of doubled T (pictured in Figure 2) was cut from the blank.The central part of the body was a prism with dimensions 6 × 6 × 30 mm and served as a sample for loading thermodilatometry.Two booms were in its upper and lower end.The basic diagram of the measuring unit is shown in Figure 3.The upper boom, which serves as the support of the body, is in the measuring unit.The position of this boom, transferred with the help of the alumina rod, is registered with a gauge.The lower boom served for the alumina rods transferring the tensile forces from the weights on the prism.The second role of these rods is transferring the position of the low boom to the gauge.The sensitivity of the gauges was 0.01 mm.The prism (sample) length change was measured based on differential principle, similar to a push-rod dilatometer.The relative expansion of the sample is where  0 is the length of the prism at room temperature,   and   are the readings of the gauges.The term   / 0 is the relative expansion of alumina rods.Here, the influence of the compressive strength was taken into account using the relationship between Young's modulus of alumina ceramics (2 % porosity) and temperature:  = 380 − 0.051 where  is Young's modulus in GPa, and  is the temperature in °C [5].Relative contraction of the alumina rods is negligible at the compressive stress of 60 kPa in the whole used temperature range.This was also confirmed by comparing the dilatometric results obtained with a common dilatometry and a loading dilatometry in which the alumina rod (as the sample) was under compressive stress at 60 kPa.The dilatometric curves, measured on the dilatometer described in [6], differed less than 0.05 %.
Three tensile loadings (37 kPa, 50 kPa, and 60 kPa) were used for the loading dilatometry.Every analysis was conducted at the temperature rate of 5 °C/min up to 1250 °C.

Results and discussion
The results of the analyses are shown in Figure 4.The dilatometric lines up to the start of dehydroxylation of kaolinite are almost identical for all loadings (37 kPa, 50 kPa, and 60 kPa).Dehydroxylation influences the dimensional (and volume) changes -the sample contracts as known from a common dilatometry without external loading due to the change in the structure of kaolinite, which losses its OH -groups from the octahedral sheet [1][2][3].Although α-quartz, which is a part of the sample, expands during heating from room temperature to 573 °C (transformation α-quartz → β-quartz) and then β-quartz contracts slightly, and feldspar expands from room temperature up to 1100 °C (start of the melting of feldspar); the contraction is dominant when the transformation kaolinite → meta-kaolinite takes place.The influence of tensile stress is evident.This stress supports the elongation of the sample; therefore, the greater the tensile stress, the more significant the expansion of the sample.Dilatometric lines split up as dehydroxylation begins.This behavior can be caused by weakening the kaolinite/meta-kaolinite structure, which was also confirmed via different properties measured during linear heating and at room temperature after heating in the dehydroxylation region [7][8][9].Two concurrent forces act in this temperature region: The first is the external tensile force, and the second is the force originating from the intrinsic structure changes in kaolinite.This second force, which has a tendency to shorten the sample, becomes more significant than the external tensile force at temperatures over 600 °C, so the contraction is registered up to ~800 °C.
When dehydroxylation is over at ~800 °C, the expansion continues to ~950 °C when metakaolinite turns into Al-Si spinel and mullite.These changes are accompanied by the contraction of the sample.At temperatures over 1100 °C, liquid state sintering and intensive contraction take place.The dilatometric behavior is determined by the sum of forces contrary to each other.One of them originates from the external load -it elongates the sample.The second is the force from a surface tension -shortening the sample.
The V-shape minima are visible in Figure 4.They represent the points of the equilibrium between these forces.When the forces originated from the internal changes (natural tendency to reach the minimum of the surface energy during sintering) prevail, the sample contacts up to the noticed point.The biggest external stress (60 kPa) leads to the lowest contraction (0.23 %), and the lowest external stress (37 kPa) leads to a contraction of 1.18 %.No stress acts in the sample at points of the lowest values of contraction, which are the equilibrium points.As the temperature increases, a viscosity of the glassy phase decreases and the surface forces weaken, the external force prevails, and the sample elongates.

Conclusions
Green samples (6 × 6 × 30 mm) made from 50 wt.%kaolinite, 25 wt.% quartz, 25 wt.% feldspar, and water were investigated using loading dilatometry with different external tensile stress.The dilatometric lines are identical up to dehydroxylation for all loadings (37 kPa, 50 kPa, and 60 kPa).The sample contracts during dehydroxylation, but dilatometric lines are separated according to the external stress: the bigger the external tensile stress, the bigger the expansion of the sample.Two concurrent forces act in this temperature region: the external tensile force and the force originating from the intrinsic structure changes in kaolinite.This second force becomes bigger at temperatures over 600 °C, so the contraction is registered up to ~800 °C.
When dehydroxylation is over at ~800 °C, the expansion continues to ~950 °C when metakaolinite turns into Al-Si spinel and mullite.These changes are accompanied by the contraction of the sample.At temperatures over 1100 °C, feldspar begins to melt, and liquid-state sintering occurs, leading to an intensive contraction.The dilatometric behavior is determined by the sum of the forces contrary to each other.First originates from the external load and elongates the sample.The second is the force of the surface tension at the sintering, which shortens the sample.
The V-shape minima, which are the points of equilibrium between the forces described above, are visible in dilatometric figures above 1100 °C.When the forces originating from the natural tendency to reach the minimum surface energy during sintering prevails, the sample contracts up to the maximum contraction point.As the temperature is further increased, the surface forces weaken, and the external forces dominate, leading to the elongation of the sample.

Figure 2 .
Figure 2. Schematic sketch of the sample.

Table 1 .
Chemical composition of raw input materials (in wt.%).