Pressure-assisted flash-sintering is a modern technology of materials prosessing under the application of an electrical field and pressure. Flash sintering is expected to be much more advantageous and cheaper compared to other technologies for densification of ceramics: First, the equipment often requires only intermediate temperature furnaces (with operation temperatures often less than ~1100 oC), which are used for preheating specimen to an onset temperature (at which Joule heating overcomes heat loss by radiation and thermal conduction and ﬂash sintering is triggered), and those furnaces are much cheaper than typical high-temperature furnaces used in conventional high temperature (>~1600oC) sintering. Secondly, the FS process is much faster (often within 1 min), which saves tremendously in both time and energy consumption. One additional benefit of flash sintering is the possibility to use different sintering atmospheres without the need for significant changes in system design, which enables greater flexibility in optimizing processing parameters for desired materials microstructure and properties. Another advantage of flash sintering is possibility to process materials in not equilibrium conditions. Rapid heating and short sintering time permits formation of thermodynamically metastable materials using this technology.
The concept of pressure-assisted flash sintering (PAFS) is presented in Fig. In such configuration, the material is located between two electrodes made from a refractory yet electrically conductive material (e.g., graphite or molybdenum). Note that, different from conventional SPS, the current passes directly through the sample in PAFS and not through the graphite die as in SPS. Based on this, a PAFS system with a single pair of electrodes was constructed by the Andriy Durygin and Zhe Cheng at the Center for the Study of Matter at Extreme Conditions.
Concept of pressure-assisted flash sintering (PAFS)
In this setup, a pneumatic cylinder is used to apply a constant compressive force upon the sample assembly, which results in an average pressure of ~35MPa upon the 3.1 mm diameter samples. The applied pressure helps to maintain a stable electrical conduction path during the flash sintering process and prompts the densification of the sintered materials. A digital indicator with a micro-controller-based computer interface is mounted on the piston side of the pneumatic press to monitor displacement, which provides continuous reading about the change in sample dimension (in this case, thickness) during flash sintering. PAFS experiments could be carried out at ambient temperature (i.e., without preheating of the sample) or with preheating. DC power supply is used to create an electric field across the sample. For sample temperature, though difficult to measure directly, it could be estimated with reasonable confidence through (indirect) measurement combined with finite element modeling
Schematic diagram of the flash sintering setup containing a pneumatic cylinder, a digital indicator, and the sample assembly
Drawing of the flash sintering sample assembly.
A representative heat distribution profile for the sample assembly during the reaction flash sintering was obtained via finite element simulation.
“Pressureless” flash sintering
The conventional “pressureless” flash sintering is carried out as illustrated in Figure. A vertical tube furnace is used to bring sample to the critical onset temperature. Sample is monitored by video camera coupled with fiber spectrometer. Such a configuration allows simultaneous observation of the sample and processes occurring during sintering for example specimen densification (change of linear dimensions) as well as temperature measurement. Temperature determination of selected fragment of sample is be based on analysis of thermal radiation spectra emitted by the sample and captured by spectrometer. The system permits FS of materials in controlled atmosphere or vacuum. Experiments on this setup are providing us baseline data for pressure-asisted flash sintering for comparison purpose and for obtaining electrical properties of the materials.
- Belisario, S. Mondal, I. Khakpour, A. F. Hernandez, A. Durygin, and Z. Cheng, “Synthesis and flash sintering of (Hf1-xZrx)B2 solid solution powders,” J. Eur. Ceram. Soc., Dec. 2020, doi: 10.1016/j.jeurceramsoc.2020.12.015.
- S. Mondal, A. Durygin, V. Drozd, J. Belisario, and Z. Cheng, “Multicomponent bulk metal nitride (Nb 1/3 Ta 1/3 Ti 1/3 )N 1− δ synthesis via reaction flash sintering and characterizations,” J. Am. Ceram. Soc., vol. 103, no. 9, pp. 4876–4893, Sep. 2020, doi: 10.1111/jace.17226.