Study On Negative Thermal Expansion Of Nb₁₄W₃O₄₄ And ZrNb₂WO₁₀

Nb₁₄W₃O₄₄ has attracted much attention due to that its own atomic arrangement has a very regular structure of 4×4 octahedral characteristic.A literature reported that voltage could change when different amounts of lithium sodium atoms embedded in the Nb₁₄W₃O₄₄ bulk structure material.But the thermal performance of the material have not been reported.This is the first discovery that Nb₁₄W₃O₄₄ has a stable negative thermal expansion?NTE?performance in the RT – 433 K temperature range.In this work,the negative thermal expansion characteristics of Nb₁₄W₃O₄₄ are studied and characterize it.1.Nb₁₄W₃O₄₄ was sintered by solid phase sintering,the pellets were sintered with the temperature increased from room temperature to 1523 K temperature for 12 h in the high temperature tubular furnace.After cooling down naturally,the final sample was obtained.The Nb₁₄W₃O₄₄ is tetragonal,The microstructures were observed by scanning electron microscopy,the particles inside the sample is irregularly shaped with the size of 2-8?m.The expansion curve was measured by an expansion tester,and it was found that there was a stable negative expansibility at RT-433 K,after linear fitting,the linear expansion coefficient was calculated to be-7.12×10-6/ K.2.The sintered sample Nb₁₄W₃O₄₄ was subjected to high temperature XRD,the lattice constants and unit cell volumes of the Nb₁₄W₃O₄₄ for XRD pattern of variable temperature by using Powder X software.The calculated lattice points are connected into a line,the a and b axes increase with the increase of the temperature,and the c-axis decreases with the increase of the temperature,the change trend of volume is consistent with the expansion curve,a linear fit for RT-433 K interval and the intrinsic linear expansion coefficient is calculated to be-4.27×10-6 / K.3.Through the expansion curve can be seen,the expansion curve of the sample changes after 433 K and gradually from negative to positive,from the high temperature XRD,Low temperature Raman can inferred that the inflection point around 433 K is the phase change point of the material,and the high temperature XRD analysis shows that the phase transition around 433 K is due to the structural change of the space group?the tetragonal I-4 transformed into a tetragonal P-4?.4.Analysis the negative expansion mechanism of Nb₁₄W₃O₄₄ can be explained by the coupling rotation of the rigid element model.The heat shrinkage mechanism of Nb₁₄W₃O₄₄ can be explained by the coupling rotation of the rigid element model.When the temperature increases,the bridge oxygen atoms occupying the octahedral vertices make lateral vibration and the coupling rotation occurs between the octahedrons.These result in the shortening of the distance between the metal atoms in the octahedral center and the decrease of the total volume,which present negative thermal expansion characteristics on macroscopic manifestations.5.ZrNb₂WO₁₀ was sintered by solid phase sintering,the pellets were sintered with the temperature increased from room temperature to 1523 K temperature for 12 h in the high temperature tubular furnace.The expansion curve was measured by an expansion tester,and it was found that there was a stable negative expansibility at RT-433 K,after linear fitting,the linear expansion coefficient was calculated to be-2.98×10-6 / K.The sample was subjected to room temperature XRD,found that the product is Nb₁₄W₃O₄₄,Nb2Zr6O17 two kinds of material mixture.