| Budget Amount *help |
¥3,000,000 (Direct Cost: ¥3,000,000)
Fiscal Year 2005: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 2004: ¥2,200,000 (Direct Cost: ¥2,200,000)
|
|---|
| Research Abstract |
Most of materials usually show positive thermal expansion, but ZrW_2O_8 is one of very interesting materials having an isotropic negative thermal expansion over a wide temperature range from 0.3 to 1050K. Our group found firstly in the world from heat capacity measurement that phase transitions of ZrW_2O_8 and HfW_2O_8 at around room temperature were the order-disorder-type. The objective in our study is to control phase transition temperature of negative thermal expansion materials ZrW_2O_8 by forming disordered nano space around the substituted ions such as Sc, Y, In (without f electrons) and Tm, Yb, Lu (with f electrons) having three valence electrons as well as Mg, Zn etc. with two valence electrons.
Zr_<1-x>M_xW_2O_<8-y> (M=Sc, Y, In, Gd, Tm, Yb, Lu, Mg, Zn etc., x=0-0.2) samples were synthesized by solid state reaction and precursor methods. We succeeded to prepare Zr_<1-x>M_xW_2O_<8-y> substituted for Zr sites by Sc, Y, In ions (without f electrons) or Tm, Yb, Lu ions (with f ele
… More
ctrons) having three valence electrons, but could not for the substitution of Mg, Zn etc. with two valence electrons.
Integrated peak intensity ratio, S_<310>/S_<210>, where S_<310> and S_<210> are the integrated peak intensities of super-lattice and main lattices, respectively, was calculated from two intensities of X-ray diffraction profiles as a function of temperature, and phase transition temperature was determined as the disappearance temperature of S_<310>/S_<210>. We also found very interesting experimental results that the transition temperature of ZrW_2O_8 was decreased by a maximum of 80K with the substitution of only 4% trivalent Sc content. The synchrotron radiation experiment revealed the evidence of the strong correlation between the phase transition temperature and the number of WO_4 units with the ordered orientation. A model was proposed to interpret the drastic compositional dependence on the transition temperature, by considering the existence of a local region including the WO_4 pairs with the orientation disorder in Zr_<1-x>M_xW_2O_<8-y>. The local region has the crystal structure of the high-temperature phase in a non-equilibrium state, and spreads out around the substituted trivalent cations. We had useful information to be able to control the phase transition temperature by introducing disordered nano space. Less
|
