Negative thermal expansion (NTE) materials belong to a group of solids that perform contractions in volume under heating conditions, instead of thermal expansion observed in most other solids. Due to this special negative thermal property, NTE materials are gradually drawing research interest for their potential application as fillers in composites used to control thermal stress' generating from mismatch of thermal expansivity and to govern the overall thermal expansion behavior.

Zirconium tungstate (ZrW2O8) is a ceramic material that exhibits strong, isotropic NTE over a wide temperature range from 0.3 to 1050 K. The formation and transition of three phases ZrW2O8 can be achieved under certain temperatures and pressures. ZrW2O8 nanoparticles can be synthesized at lower temperatures by hydrothermal reaction followed by a subsequent heat treatment by converting the precursor ZrW2O7(OH)2*2H2O to ZrW2O8. However, nano-sized ZrW2O8 was reported to display a hydration phenomena when exposed to ambient external conditions. Through the careful selection of experimental parameters such as initial reactant types, acids types and concentrations, as well as reaction time and temperature, the effect of reaction conditions on morphologies and crystallite sizes of ZrW2O8 nanoparticles was characterized using scanning electron microscopy and powder X-ray diffraction. Three types of ZrW2O8 nanoparticles with distinct morphologies and crystallite size-scales were chosen for characterization and comparison. Thermal and physical properties, as well as the hydration problem, were compared using various techniques, including isothermal X-ray diffraction, Brunauer-Emmett-Teller (BET) surface area analyzer and thermogravimetric analysis. It was found that the synthesized ZrW2O8 nanoparticles present more negative CTE values than bulk-scaled in á-phase, and the CTE is dependent on the crystallite sizes of nanoparticles. All three types of nanoparticles underwent different extents of hydration problem originated from moisture adsorption, which is determined by pores structure formed by agglomeration of nanoparticles. It was observed that ZrW2O8 nanoparticles with smaller dimensional sizes tended to form large pores and suffered from most severe hydrations.