HM 867 Heating Microscope

Measurements of Glass Transition and Softening Temperature

Two important measurements commonly made on a dilatometer are the determinations of the glass transition temperature, Tg, and the softening point. The figure to the top right shows results of thermal expansion for glass material as measured on TA Instruments True Differential™ Dilatometer. It can be seen here that the sample was heated through the Tg, observed at 471°C, to the softening point, observed at a temperature of 540°C.
The instrument control software includes a unique “automatic softening point detection” function to terminate the test. This feature allows these properties to be measured for unknown materials without concern of damage to the instrument that could result if the sample is melted and flows onto components.

Simulating a Fast Firing Cycle in the Lab with Optical Dilatometry

Fast firing of traditional ceramics has quickly become the preferred processing technology because of the reduction in cost and increased productivity it provides. Because fast firing requires all materials within the ceramic body to obtain optimal stabilization within a few minutes, some raw materials used in slower firing process may not be suitable. Therefore, it is important to understand the suitability of all raw materials in the laboratory before scaling a batch up for production. TA Instruments’ optical dilatometer, ODP 868, includes a furnace capable of rapid heating and cooling, allowing the simulation of thermal profiles of fast firing kilns and eliminating the need for expensive and time-consuming pilot-scale tests. An example is shown in the figure at the top right. Here a green body was heated to 800°C in 4 minutes, then to 1220°C in 4.2 minutes, and held isothermally for five minutes to allow the body to fully sinter. Subsequently, the furnace was fast cooled to 680°C in 30 seconds and then to 100°C in 5.3 minutes. It can be seen in the figure that, at the end of the process, the sample was properly sintered and dimensionally stable.

Pyroplasticity Study of Sintered Bodies with Absolute Fleximeter

The ceramic tile market is trending towards larger formats and faces the challenge of reducing the tile thickness while maintaining planarity. Flatness defects can be induced during pressing, firing, or cooling. During firing, the amount of deformation is dependent on the viscosity of the glassy phases. The viscosity follows an Arrhenius relationship. However, some materials will exhibit a change in the composition of the glassy phase during the firing process which results in an increase of viscosity as the temperature increases. The figure to the right shows results of two different sintered bodies that were tested using the Absolute Fleximetry mode of the ODP 868. It can be seen here that “Sample A” starts bending, and as the temperature continues to increase, the sample can no longer support its own weight and ultimately flexes over 6 mm (i.e. 7.5%). “Sample B” shows dramatically different flexing behavior. For this sample, the softening vitreous phases dissolve other mineral components present in the body. As a result, the viscosity increases with the temperature, and the body tends to flex less. The latter result will produce a tile in a larger format that is easier to process and with better planarity.

Identification of Characteristic Shapes and Temperatures with
Heating Microscopy

Heating microscopy identifies a material’s characteristic shapes and corresponding temperatures, and is a must-have tool for optimizing manufacturing processes in ceramics, metals and alloys. The top right graph shows a comparison of two matt glazes. For Matt Glaze 1 all the five basic characteristic shapes were identified, and the data reveals the sample has a strong surface tension and a poor compatibility with the substrate of the sample holder. In contrast, Matt Glaze 2 not only melts at lower temperature but has a great degree of compatibility with the substrate. After Matt Glaze 2 melts, it spreads evenly over the surface of the substrates making it an ideal candidate for bodies of the same or similar composition of the substrate. The characteristic temperatures were automatically identified by MorphometriXTM, TA Instruments’ image analysis software engine equipped with pattern-matching models for recognition of transformations in materials in real-time with precision superseding the eye of a human operator.

Characterization of Mold Powders for Continuous Casting

Mold powders are added to the top of molten steel in a continuous casting mold.
The powder partially melts, forming a liquid layer next to the molten steel protecting it from reoxidation, absorbing the non-metallic inclusions, lubricating the steel shell as it passes through the mold, and controlling the heat transfer from the solidifying steel shell to the mold. If the mold powder does not readily flow, and completely covers the exposed surface of the molten steel, the thermal insulation it is meant to provide is not sufficient leading to unwanted reoxidation and inefficient removal of non-metallic inclusions. Because the melting behavior of mold powders is dependent on heating rate, it is very important to accurately measure this behavior at actual ballistic heating rates of the process to understand performance during the casting process. TA’s Optical Dilatometer Platform, ODP 868, with flash and rapid heating capabilities is the perfect tool to characterize and understand this behavior. Because the approach is optical, there is no complicated sample preparation as would be the case with traditional push-rod instruments. The figure to the right shows melting points for a mold powder determined at 30°C/min, 80°C/min, and under the most rapid flash conditions. It can be seen that the melting point of the powder decreases with increasing heating rate.

Solid Oxide Fuel Cell Stack Compatibility Study

Solid Oxide Fuel Cells (SOFC) are devices that convert chemical energy into electrical and thermal energy by combining oxygen and hydrogen. The base of a SOFC cell consists of two porous electrode layers separated by a high-density oxygen-conducting electrolyte layer. The thickness of the entire structure is less than 1 mm. The fabrication process consists of forming the anode and cathode layers, deposition of the electrolyte, and co-sintering of the stack. It is important to understand the behavior of the layers during co-sintering to successfully process SOFCs because a mismatch in shrinkage leads to distortion of the final piece. It is virtually impossible to make these measurements on a traditional pushrod dilatometer because the layers are so thin and not mechanically self-supporting.
However, such measurements are possible using TA’s unique optical dilatometer and film sample holder. The graph to the right shows a sintering study of two materials proposed for an SOFC consisting of a 125 μm thick Nickel/Yttria-stabilized Zirconia ceramic metal (Ni- YSZ cermet) anode and 10 μm thick YSZ electrolyte performed by TA Instruments’ optical dilatometer with Thin Film Holder. It can be seen in the graph that the results measured by the optical dilatometer show significant mismatch of the shrinkage during sintering, demonstrating the two materials are incompatible for the co-sintering process.

Ash Flow Temperature of Solid Fuels

Coal, solid recovered fuel, wood and other biomasses are commonly burned to generate heat. During the combustion process, alkaline metals present in these fuels can generate complex eutectic salts and lower the melting point of the ashes. This causes fouling and slagging problems in heat exchangers and superheater tubes, leading to equipment damage and expensive downtime of the power plant. The ash flow temperature (AFT) of a solid fuel gives an indication to what extent ash agglomeration and ash clinkering is likely to occur during gasification. The results of an AFT analysis consist of four temperatures, namely the initial deformation temperature (IDT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT). These temperatures can be measured under either oxidizing or reducing conditions (or both), with the difference between the oxidizing and reducing results often correlating strongly with fluxing agents such as iron. Several standard methods, of which the most common are ASTM 1857, ISO 540 and DIN 51730, define how to prepare and test specimens. All methods require the use of a heating microscope like TA Instruments HM 867. The graph on the right shows a fusibility test performed on a 19 mm tall pyramidal coal sample according to ASTM standard 1857. The temperatures of the characteristic shapes were automatically determined with MorphometriXTM Software.