Igor Štubňa

Permeability of the electroceramics to gas and its dependence on the firing temperature

Abstract Permeability and porosity of electroceramic material (50% kaolin, 25% quartz, 25% feldspar) were measured after firing at temperatures of 20–1300 °C. Experimentally it was found that the permeability (measured by means of the penetration of the pressed air through the sample), caused by dehydroxylation, increased by 37%. The porosity (measured by the pycnometric method) increased by 6%, which is in agreement with geometric calculations. The value of the permeability is explained by creating the micropores in metakaolinite.

Mechanical Properties of Kaolin during Heating

Flexural strength (MOR) and Young’s modulus (YM) of Sedlec kaolin were measured using the three point-bending method and modulated force thermomechanical analysis (mf-TMA). Thermal analyses DTA, thermogravimetry and thermodilatometry (TDA). An escape of the physically bound water (20 – 250 °C) strengthens the sample and YM and MOR increase their values significantly. MOR and YM lower their values as dehydroxylation starts at 400 °C. Both quantities, MOR and YM, pass through minimum in the dehydroxylation region (400 – 650 °C). Their next increase is probably caused by the van der Waals forces acting between metakaolinite crystals and by the starting of the solid-state sintering. YM steeply increases above 950 °C as a consequence of the solid-state sintering. A Weibull’s modulus passes through the sharp maximum at the interval 300 – 400 °C.

Young's modulus and mechanical strength of porcelain at the firing in the cooling stage

Abstract Youngs modulus and mechanical strength of alumina-enriched porcelain (30% kaolin and clay, 25% Al 2 O 3 , 30% feldspar, 15% chamotte; 19% and 0·8% total amount of quartz and cristobalite impurities respectively) were measured in the cooling stage of firing (5°C/min). It was confirmed that β → α transformation of quartz is a source of the microcracks and the same influence on the Youngs modulus and strength had the β → α transformation of cristobalite: the sharp minimums of Youngs modulus at 570 and 280°C were discovered. Mechanical strength increased transienlty after β → α changes.

Uncertainty Of The Measurement Of DC Conductivity Of Eramics At Elevated Temperatures

Abstract The electrical DC conductivity is measured at room and elevated temperatures on green ceramic samples prepared from kaolin. The arrangement of the sample, with two platinum wire electrodes inserted in the kaolin prism that was used is suitable for measurements of temperature dependences of the DC conductivity from 20 °C to 1100 °C in the air. The uncertainty analysis taking into account thermal expansion of the sample, homogeneity of the temperature field, measurement regime, corrosion of the electrodes, and overlapping of the electrodes is done for 1000 °C. Uncertainties connected with current and voltage measurements and uncertainties connected with the instruments that were used are also considered. The sum of all the partial uncertainties gives an expanded uncertainty of the conductivity measurement. The uncertainty varies with temperature and reaches the value of ∼ 6.5% at 1000 °C.

Thermoanalytical investigation of ancient pottery

Ceramic potsherds excavated in Biskupice (Banovce County, Slovakia) dated to the Hallstatt culture (8th to 6th centuries BC) were investigated by TGA, thermodilatometry (TDA) and XRD analysis. It was found that the samples consisted of illite/muscovite, feldspar and quartz. Their thermal behavior was typical for ceramics buried in soil for a long time: the mass loss (∼3 mass%) due to escaping the physically bound water (from room temperature to 300 °C) was followed by a gradual mass loss from dehydroxylation (∼3 mass%) as a consequence of the former rehydroxylation. Above the temperature 800 °C a rapid shrinkage of samples was observed in TDA curves. As follows from these results, maximal firing temperatures did not exceed 800 °C.

Mechanical properties of alumina porcelain during heating

The mechanical strength and Young’s modulus of green alumina porcelain (50 wt. % of kaolin, 25 wt. % of Al2O3, and 25 wt. % of feldspar) were measured during heating up to 900 °C and 1100 °C, respectively. To this end, we used the three point-bending method and modulated force thermomechanical analysis (mf-TMA). The loss liberation - of the physically bound water (20 – 250 °C) strengthens the sample and Young’s modulus increases its values significantly. The dehydroxylation that takes place in the range of 400 – 650 °C causes a slight decrease in Young’s modulus. On the other hand, the mechanical strength slightly increases in this temperature range, although it has a sudden drop at 420 °C. Beyond the dehydroxylation range, above 650 °C, both Young’s modulus and mechanical strength increase. Above 950 °C, a sharp increase of Young’s modulus is caused by the solid-state sintering and the new structure created by the high-temperature reactions in metakaolinite.

The Firing Temperature of Romanesque Brick from Pác

Abstract A pavement brick taken from a Romanesque part of the church in Pác, in the Trnava County, Slovakia, was investigated by x-ray diffraction analysis (XRD) and thermal analyses as differential thermal analysis (DTA), thermogravimetry (TG) and thermodilatometry (TD). It was found that the brick contained dehydroxylated illitic clay, calcite and quartz. As revealed, dehydroxylation was completely finished and no redehydroxylation was observed. Partial decomposition of calcite was also found. The estimated firing temperature is between 600 °C and 700 °C.

Young’s modulus of alumina ceramics during isothermal heating

The Young’s modulus is studied in dehydroxylated alumina ceramic samples in the solid-state sintering region. The initial composition is 35.5 wt.% of kaolin, 35 wt.% of Al2O3, and 29.5 wt.% of feldspar. Since solid-state sintering and dehydroxylation run simultaneously, the samples are preheated at 490 °C for 40 h to reach a complete transformation of kaolinite into metakaolinite. Afterwards, the samples are studied by the modulated force thermomechanical analysis in the isothermal regime at the temperatures 500, 600, 700, 800, 900, 1000, and 1050 °C when no liquid phase is created. The Young’s modulus is measured for 8 h. For temperatures 500, 600, 700, and 800 °C only a small increase (2 – 8 %) of the Young’s modulus is observed, and after 4 h the samples became stabilized. On the other hand, for temperatures higher than 1000 °C the increase of the Young’s modulus continues during the whole time period and reaches over 90 %.The Young’s modulus is studied in dehydroxylated alumina ceramic samples in the solid-state sintering region. The initial composition is 35.5 wt.% of kaolin, 35 wt.% of Al2O3, and 29.5 wt.% of feldspar. Since solid-state sintering and dehydroxylation run simultaneously, the samples are preheated at 490 °C for 40 h to reach a complete transformation of kaolinite into metakaolinite. Afterwards, the samples are studied by the modulated force thermomechanical analysis in the isothermal regime at the temperatures 500, 600, 700, 800, 900, 1000, and 1050 °C when no liquid phase is created. The Young’s modulus is measured for 8 h. For temperatures 500, 600, 700, and 800 °C only a small increase (2 – 8 %) of the Young’s modulus is observed, and after 4 h the samples became stabilized. On the other hand, for temperatures higher than 1000 °C the increase of the Young’s modulus continues during the whole time period and reaches over 90 %.

Influence of milling on physical properties of illite

Raw illitic clay (80 % illite, 4 % montmorillonite, 4 % orthoclase, and 12 % quartz) was milled in a planetary ball mill for 0, 60, 120, and 180 min in air. From milled clay, samples for XRD, granulometry, thermogravimetry, DTA, and dilatometry measurements were prepared. It was found that 1) the phase composition is almost unchanged by milling; 2) the distribution of grain sizes shows two distinct maxima; the ratio of their areas depends on the milling time – longer milling leads to larger agglomerates; 3) the dehydroxylation of illite is a two-step process: the first step is shifted to lower temperatures with the milling time; the temperature of the second step decreases only slightly with the milling time; 4) the mass loss during both steps of the dehydroxylation slightly decreases with the milling time; 5) the onset temperature of sintering sharply decreases at the longest milling time.

Building ceramics with an addition of pulverized combustion fly ash from the thermal power plant Nováky

Pulverized combustion fly ash (PFA) from the Power plant Novaky (Slovakia) is analyzed for its potential use in the production of building ceramics. Three materials are used to prepare the mixtures: illite-rich clay (IRC), PFA and IRC fired at 1000 °C (called grog). The mixtures contain 60 % of IRC and 40 % of a non-plastic compound (grog or PFA). A various amount of the grog is replaced by PFA and the effect of this substitution is studied. Thermal analyses (TGA, DTA, thermodilatometry, and dynamical thermomechanical analysis) are used to analyze the processes occurring during firing. The flexural strength and thermal conductivity are determined at room temperature after firing in the temperature interval from 800 to 1100 °C. The results show that an addition of PFA slightly decreases the flexural strength. The thermal conductivity and porosity are practically unaffected by the presence of PFA. Thus, PFA from the Power plant Novaky is a convenient non-plastic component for manufacturing building ceramics.