There are basically three procedures for densifying powder into a transparent ceramic. One of these, referred to as sintering, is essentially chemical in nature, being driven exclusively by the need to minimize surface area (and hence surface energy). This method involves cold pressing of the starting powder to a sufficient “green density”, in the vicinity of 50% of the crystal. The compacted material is then heated in an appropriate atmosphere or vacuum according to a carefully defined schedule to a temperature high enough to allow the material to sinter (fuse) together and the pores to be annihilated by diffusion.
The second technique, hot pressing, involves the mechanical application of ex¬ternal unidirectional pressure to assist in the densification, allowing it to take place at lower temperatures than those used for sintering. Typically the powder is placed in a cylindrical collar between tight-fitting but movable pistons, all of which must be chemically compatible and capable of enduring the requisite tem-peratures. This technique has been used at RMD for producing fully transparent Lu2O3 disks, resulting in the TOC scintillator shown to the left.
Lu2O3:Eu3+ TOC disk (15 mm in diameter, 1 mm thick) produced by hot pressing. (Pink colora¬tion is from Eu luminescence.)
The third approach, termed sinter-HIP, is a two-stage process that provides ad¬vantages not offered by either of the first two. The sintering step is applied not to achieve full density but only to the point where the residual porosity has been sealed off from contact with the external environment. This does not need tem¬peratures as high, nor heating time as long, as in sintering to full density. Then, having been made impermeable, the sintered compact is heated while subjected to high pressure applied isostatically by a non-diffusible inert gas, promoting an¬nihilation of the residual porosity at temperatures substantially lower than would otherwise be needed. Note that this hot isostatic pressing (hence the acronym HIPing) does not involve direct contact with any external mechanical parts, and is applied uniformly and isotropically. Moreover, sinter-HIP has the fewest con¬straints with regard to part size. This process is illustrated schematically to the left.
Illustration of Hot Isostatic Pressing to remove residual porosity and produce a fully dense TOC.
This technique is particularly important for preparation of optically clear samples of materials that melt at very high temperatures (≥ 2000 ℃). For such materials, crystal growth from the melt (using the Czochralski method) can be very difficult and expensive. Ceramic preparation can be carried out at much lower tempera¬ture with simpler processing equipment that is designed to provide a high degree of uniformity. An example of a ceramic scintillator prepared by sinter-HIP is shown to the left.
Lutetium Aluminum Garnet Ceramic Scintillator produced by Sinter – HIP