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Technical Paper
LABORATORY TECHNICIAN (PART 2)
Jan DuPlessis Theron
Welcome back to the story of the “Jumping Jacks”! For readers who have forgotten or have missed it, in the March 2017 issue of the Refractories Engineer I shared a strange experience of discs made from AZS fused cast material jumping out of a high alumina “crucible” during a firing to 1500°C.
The challenge, for those who dearly love and live for refractories, was to try and figure out how this could happen.
Figure 1 should provide the clue for those who momentarily forgot the properties of their materials and would most likely find no interest in reading further.
Those who have not dealt with the intricate properties of refractory materials in the past, the explanation may be interesting and could well provoke other thoughts about composite materials where zirconia is added as a means of improving thermal shock resistance (a thought that crossed my mind as I am writing this).
However, let’s get back to the business of samples feeling the heat and leaving the hot seat, metaphorically speaking.
During this test a glass phase exudes out of the fused cast material as every glass maker would know. This happens at high temperatures, most likely anything above 1100°C depending on the composition of the glassy phase as this will determine at what temperature it will melt. During the initial heat-up in the laboratory furnace, the ASZ discs containing anything from 32% to 47%, or higher in some cases, will move relative to the alumina support it’s resting on due to the different expansion rates. Refer to the red line and green line in Figure 1. However, it will slide relatively freely over the alumina surface as any liquids that will form during the heat-up will either
Figure 2: Glassy phase exude evident on the top of the sample
not be exuded at this stage or, if so, it will only become more fluid as the temperature increases. Differential expansion between the two materials will therefore be possible by either “dry friction” or lubricated surfaces, both allowing free movement.
Figure 1: Reversible Thermal expansion curves for zirconia and alumina. The zirconia curves may vary slightly depending on the amount of stabilisation additives, but the graphs depict very much an average between test results for a number of ASZ materials tested at Lucideon and literature values.
July 2017 Issue ENGINEER THE REFRACTORIES
The test is done at temperatures of 1450°C or higher to ensure operating conditions inside the glass furnace are simulated. It also ensures that the glassy phase has the best chance to exude out of the bulk of the material. Interestingly enough, as is evident from the image shown in Figure 2, glass exudation predominantly occurs on the top side of the discs in the test (Refer also to Figure 4 of the March 2017 issue). Contrary to the expectation that the glassy liquid will run down to the bottom of the sample and accumulate between the sample and the holder due to the effect of gravity, liquid phases are forced out of the material by a number of other mechanisms. One is the shrinkage of the zirconia at around 1100-1200°C as it goes through a phase transformation from monoclinic to tetragonal during heat-up. This will force liquids from the structure: theoretically in all directions. Another reason for exudation is the oxidation of partially reduced species such as elemental carbon and sulphur dissolved in the silicious glassy phase. The CO2 and SO2 that form, being gasses, will rise upwards, thereby pushing the liquid to the top more than the bottom. Exudation is also thought to be caused by the re-boil of
A DAY IN THE LIFE OF A
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