Reactive Processing in Ceramic‐Based Systems

Abstract

Reactive hot pressing is discussed as a processing method to form ceramic‐based materials. Fundamental aspects of thermodynamics such as favorable Gibbs'‐free energy changes, phase equilibria, and adiabatic temperature are presented as criteria for determining whether the desired reactions can be used to form dense materials. Two case studies are presented as examples to describe control of microstructure and properties. The fabrication of Al2O3–Nb and ZrB2 are discussed with respect to the three thermodynamic criteria as well as the microstructure and properties of the materials that are produced.

Pressureless reactive sintering of ZrB2 ceramic

“…Second, previous work on reactive hot pressing of ZrB 2 concluded that the B diffuses into the Zr phase to form the ceramic. 15,16 Although the detailed reaction mechanism for formation of ZrB 2 from the elements is not known, the hexagonal ZrB 2 unit cell is smaller than that of Zr (Zr: a = 3.232 Å, c = 5.147 Å from ICDD 05-0665; ZrB 2 : a = 3.169 Å, c = 3.530 Å from ICDD 34-0423). The pores created from reaction in reactive hot pressing are closed by the applied pressure.…”

“…Reactive processing (RP) possesses advantages, such as lower processing temperatures, improvement in the cleanness of the grain boundaries and the usage of more readily available and less expensive precursors. 15 This processing route requires thermodynamically favorable reactions. The Gibbs free energy of reaction between Zr and B to form ZrB 2 is −306 kJ/mol at 727 • C and −280 kJ/mol at 1727 • C, 3 confirming the possibility of using RP for the formation of bulk ZrB 2 .…”

“…Research targeting the control of grain size during reactive hot pressing by controlling the particle size distribution of the precursors is currently under way. 15 The objective of this work was to study for the first time, the processing of ZrB 2 ceramic using pressureless reactive sintering of Zr and B elemental powders in order to investigate if this processing route could overcome the known difficulties of directly pressureless sintering ZrB 2 powders. Conventional sintering cycles were used with ZrB 2 powders to compare the reactive and non-reactive pressureless sintering processes.…”

Fabrication of ZrB2 ceramics by reactive hot pressing of ZrB and B

“…The difficulty in densifying the reactive Zr+2B powder system was partly related to large density deviation of the dense Zr+2B compact (4.88 g/cm 3 ) from that of the targeted dense ZrB 2 ceramic (6.103 g/cm 3 , JCPDS 06‐0610), in addition to highly covalent bond of ZrB 2 . In the solid‐state reaction of Zr+2B=ZrB 2 , large density difference of Zr+2B from ZrB 2 left holes duplicating the B particles after B was absorbed by Zr. The holes remained as residual pores (~26 vol%) and was hard to eliminate.…”

Reactive hot-pressing of ZrB2-ZrC-SiC ceramics via direct addition of SiC

“…[9] Previous works have indicated that ZrB 2 -based ceramics, containing a second phase such as SiC [7,[10][11][12] and ZrC [10], exhibit improved densification behavior and material properties, compared to the corresponding monolithic ceramics. Besides, some other efforts have been made to solve this issue, like effective densification approaches: hot pressing (HP) [4,[13][14][15][16], pressureless sintering (PLS) [7,14,17,18], spark plasma sintering (SPS) [14,[19][20][21][22], reactive processing [23][24][25] and so on. However, more simplified processing is aspired.…”