In recent years, negative thermal expansion (NTE) materials have become of increasing interest. These materials contract upon heating, and have potential for achieving better control of thermal expansion in composite materials. By using an NTE compound as a filler material into these composites, it is possible to offset the positive thermal expansion of other components in the composite. As a result, these NTE materials can find use in a wide range of applications such as optics, polymers, electronics, tooth fillings and any other area where exact positioning of parts over a wide range of temperatures is crucial.
One of the most popular NTE materials is cubic ZrW2O8. Thermodynamically stable zirconium tungstate was first synthesized in the 1950’s through traditional solid state methods. It was only recent that the metastable phases could be achieved through low temperature methods, that involves conversion of a precursor material ZrW2O7(OH)2•2H2O to cubic ZrW2O8. Through hydrothermal synthesis, previous work on ZrW2O7(OH)2•2H2O showed exceptional particle and morphology control with use of alcohols/HCl, which is desirable for optimal composite integration. It was recently discovered that ZrW2O8 particles obtained through this synthesis route had reduced stability in atmosphere. The instability was linked to autohydration that changed the properties of the material resulting in weak positive thermal expansion. Interestingly, nanosized ZrW2O8 obtained hydrothermally in perchlorate/NaCl with the absence of alcohols show very limited autohydration; however this is a non-preferred synthesis route due to high agglomeration levels. Reported autohydration on mixed ZrMoxWx-1O8 solid solutions by Sleight et al. provided a logical defect driven explanation for the cubic ZrW2O8 nanoparticles.
Detailed investigation was performed on cubic ZrW2O8 hydrothermally obtained by the alcohol/HCl synthesis pathway. An understanding of what is causing autohydration was discovered through the used of powder X-ray diffraction, scanning transmission electron microscopy, thermogravimetric analysis and Brunauer-Emmett-Teller surface analysis.