Laser Sintering Synthesis Of Zrw <sub> 2 </ Sub> O <sub> 8 </ Sub> Ceramics And Their Performance Study

Negative thermal expansive compounds are useful for producing composites with zero or very low thermal expansion coefficient. Zirconium tungstate has an exceptionally large and isotropic negative thermal expansion (NTE) coefficient over a wide range of temperatures (0.3-1050K). So it has been the subject of intense study. Up to now, all the synthesis processes for the production of ZrW₂O₈ are either tedious or environment unfriendly or unsuitable for industrial mass production. Exploring new and rapid synthetic routes are certainly demanded.In this paper, ZrW₂O₈ has successfully been synthesized with a high power CO₂ laser for the first time. It is a rapid process in which tens of grams of a sample can be produced in a few seconds. The samples were analyzed by scanning electron microscopy (SEM), X-ray diffraction and Raman spectroscopy. It was shown thatγphase exists in the sample by laser synthesis, in contrast to the samples synthesized by solid state reaction or other chemial methods. Theγphase formation gives a definite indication that a compressive stress was induced during the solidification process. The strength of the compressive stress is proportional to the cooling rate that is proportional to the temperature difference between the molten pool and the ambient, and inversely proportional to the time for the sample to cool. It is analyzed and confirmed that compressive stress can be effectively reduced by lowering laser scan speed. ZrW₂O₈ samples with cubic structure were produced when the laser scan speed was less than 2mm/s. The samples cooled by water are dominated byα-ZrW₂O₈ and theγphase also is reduced. We also attempted to avoid the appearance ofγphase by doping with Mo. A fraction of MOO₃ was added in the starting materials with the aim to substitute part of W atoms by Mo atoms. As a result, the appearance of the orthorhombic structure is successfully inhibited. This may be due to an increase in the resistance on compressive stress of the material. Raman Spectroscopic study with tempreture reveals that several Raman bands change discontinuously at about 390K, indicating a phase transition fromγto a occurring at this temperature. No obvious changes in the Raman spectra were observed for a toβphase transition. In theβphase, most of the Raman modes give rise to negative Gruneisen parameters, suggesting contribution of the optical phonons to the NTE of the material, at least for theβphase. A recovery of the phase has been observed when the samples were cooled to room temperature. This suggests that cubic structure is only metastable at temperatures above 390 K and at room temperature theγphase is preferred. The SEM observations show that the sample is composed of nanostructures both on the surface and cross section. Nano-threads/rods grew horizontally on the surface region while they align vertically in the cross section. The nano-threads are composed of nano-crystallites.