Papers by Keyword: Negative Thermal Expansion

Abstract: A negative thermal expansion (NTE) metamaterial is established herein by inspiration from the Sicilian and Manx symbols to form rigid units of the metamaterial. By attaching connecting material of positive thermal expansion to the rigid units, the resulting metamaterial exhibits NTE. Analytical forms for the effective coefficients of thermal expansions (CTE) were established using infinitesimal and finite deformation assumptions for small and large temperature changes, respectively. Results indicate that the negativity of this metamaterial’s thermal expansion is enhanced with the thickness of the connecting material but decreases with the dimensions of the rigid units. The transverse isotropy for this metamaterial’s CTE is useful if thermal expansion compensation is required in two orthogonal directions but zero thermal expansion is required in the remaining orthogonal direction.

Abstract: In this paper, zirconium tungstate ceramic with negative thermal expansion coefficients was prepared from zirconium oxide and tungstic acid by solid phase synthesis and high temperature quenching technique with a sintering temperature of 1200 °C. The phase structure of the material was determined by X ray and the thermal expansion coefficient was measured by dilatometer, while the TG-DTA analysis of the prepared material was also carried out. The results showed that zirconium tungstate with high purity could be obtained by rapid chilled while fired at 1200 °C. The coefficient of thermal expansion at 300 °C was minus 8.5413 × 10^-6K^-1, which is identical with the theoretical value. The thermal expansion coefficient of the material was negative fired lower than 750 °C, while it was positive fired higher than 750 °C, and this indicates that the decomposition temperature of zirconium tungstate is about 750 °C.

Abstract: The ZrW_2-xMo_xO₈ (0≤x≤2) system has a unique property - a negative coefficient of thermal expansion (CTE), that is it shrinks when heated. Such materials solve the problem of undesirable thermal expansion of composites in many areas. In contradistinction to ZrW₂O₈ and ZrMo₂O₈, this system has anomalously low CTE values [1,2] and adverse phase transition is shifted to a lower temperature, unlikely area for operation [3,4]. At the same time, the fundamental question remains about the mechanism of formation of ZrW_2-xMo_xO₈(0≤x≤2). According to [5,6], pure cubic ZrW₂O₈ and ZrMo₂O₈ are obtained by thermal decomposition of the precursor ZrM₂O₇(OH)₂· 2H₂O, where (M = Mo, W). However, the structure of the precursor is extremely sensitive to various changes in the synthesis process. To date, there are a number of works devoted to the hydrothermal synthesis of precursors ZrW_2-xMo_xO₇(OH)₂·2H₂O (0≤x≤2) solid solutions, however, not all formulations were obtained during the hydrothermal reaction. The study of their thermal properties, as well as structural and phase transformations are not fully studied. Within the paper it was first time proposed to obtain full number of nanosized materials by the hydrothermal method and also to study the nature of the thermal expansion of each member of the solid solution. The obtained data will solve many technical problems associated with dimensional nonvariance in the electronic, oil and gas, optical industry, medicine and rocket engineering.

Abstract: Materials and structures are exposed to fluctuating temperature and moisture concentration, which alter their sizes; the capability to adjust the hygrothermal expansion, including into the negative region, permits greater control of hygrothermal stresses and strains. This paper introduces a type of 2D truss microstructure where each unit takes the shape of a kite with an additional reinforcing rod on the kite’s axis of symmetry. The coefficients of thermal and moisture expansions are positive in the direction parallel to the reinforcing rods, but linear negative hygrothermal expansion is obtained in the direction perpendicular to the reinforcing rods. The condition that gives areal negative hygrothermal expansion is also established, wherein this region is demarcated by the zero hygrothermal expansion line, which is attained when the inclined rods are perpendicular to each other.

Abstract: The structural and thermal expansion properties of solid solutions In_2-xFe_xMo₃O₁₂ have been characterized using X-ray diffraction. The lattice parameters decrease linearly with increasing Fe content at room temperature. All compounds exhibit monoclinic structure at room temperature, and transform to orthorhombic phase upon heating. Monoclinic In_2-xFe_xMo₃O₁₂ (x = 0.3, 1.0 and 1.3) possess strong positive thermal expansion, while the linear thermal expansion coefficient of orthorhombic phase varies from zero/near-zero to positive with increasing Fe³⁺ content.

Abstract: Negative thermal expansion (NTE) material Sc₂Mo₃O₁₂ was prepared by solid-state method. The phase structure, morphology and the thermal stability of the obtained products were examined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric-differential thermal analysis (TG-DSC), respectively. To study its negative thermal expansion property, two methods were utilized. One is based on the in-situ XRD results and the thermal expansion coefficient (CTE) was calculated by the cell parameters obtained at different temperatures. Another is based on the thermo-mechanical analysis (TMA) and the CTE was calculated by the length of Sc₂Mo₃O₁₂ bar at different temperatures. The differences between these two methods were also discussed.

Abstract: In this paper, β-eucryptite glass ceramics were synthesized by using solid reaction method. Phase constitution, structure and properties of the material were studied by X-ray diffraction (XRD) and differential thermal analysis (DTA). Furthermore, the effects of heat treatment temperature and preservation time on the thermal expansion coefficient were also analyzed. The results showed that the crystallization temperature of β-eucryptite glass ceramics was in the range of 810-860 °C and the content was more than 90%. With the increase of heat treatment temperature, the material expansion coefficient decreased.

Abstract: The thermal expansion and magnetic properties of Gd₂Fe_16.5Cr_0.5 compound have been investigated by means of X-ray diffraction and magnetization measurements. The Gd₂Fe_16.5Cr_0.5 compound has a rhombohedral Th₂Zn₁₇-type structure. The Curie temperature of Gd₂Fe _16.5 Cr _0.5 compound is about 560K. This value is about 50K higher than that value of the mother compound Gd₂Fe₁₇. A small average thermal expansion coefficient, = 1.65×10-6/K, and a negative average thermal expansion coefficient, = -1.46×10-5/K, were found in Gd₂Fe _16.5 Cr _0.5 compound by X-ray diffraction in the temperature range 294 - 514K, and 514-613K, respectively. There exists anisotropic strong spontaneous magnetostriction in the magnetic state of Gd₂Fe _16.5 Cr _0.5 compound.

Abstract: Raman spectra of the magnetic geometric frustration material – the botallackite-structure α-Cu2(OH)3Cl polycrystalline sample were measured down to liquid N2 temperature. It is found that the hydroxyl stretching bands shift abnormally according to the lattice thermal expansion effect of normal materials, i.e., they redshift with decreasing the sample temperature (negative thermal expansion) using liquid N2 cooling while other bands blueshift. This abnormality was also confirmed by observing the band-shifting caused by local laser heating effect using different laser powers, and can be qualitatively explained by checking the local hydroxyl environment with a trimeric hydrogen bond.

Abstract: Negative thermal expansion material ZrW₂O₈ was synthesized by hydrothermal method. The prepared ZrW₂O₈ and the precursor were characterized by x-ray diffractometer, thermo-gravimetric/differential thermal analyzer, FT-IR spectrometer and scanning electron microscope. The results show that a single-phase ZrW₂O₈ can be synthesized when HCl concentration is as low as 4mol/L. ZrW₂O₈ was obtained through the precursor dehydration and debonding O-H bond. The dehydration reaction includes two stages. There is a sharp endothermic peak in the first stage of the dehydration reaction and the second stage is a slow endothermic process. A new phase appeared when the sintering temperature is above 600 °C. ZrW₂O₈ and the precursor have the same rod-like shape and grow up along a specific direction. The synthesized ZrW₂O₈ particles obtained from 4mol/L HCl appeared large size. It became much finer and the rod with multiple crystal face was easy to form when the concentration of HCl increased to 6~8 mol/L.