Effect of isovalent substitution on phase transition and negative thermal expansion of In2−xScxW3O12 ceramics

Solid solutions of In_2−xSc_xW₃O₁₂ (0≤x≤2) were successfully synthesized using the solid state reaction method. Effects of substituted scandium content on the phase composition, microstructure, phase transition temperatures and thermal expansion behaviors of the resulting In_2−xSc_xW₃O₁₂ (0≤x≤2) samples were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and thermal mechanical analyzer (TMA). Results indicate that the obtained In₂W₃O₁₂ ceramic undergoes a structure phase transition from monoclinic to orthorhombic at 248 °C. This phase transition temperature of In₂W₃O₁₂ can be easily shifted to a lower temperature by partly substituting the In³⁺ with Sc³⁺. When the x value increased from 0 to 1, the phase transition temperatures of In_2−xSc_xW₃O₁₂ (0≤x≤2) samples decreased from 248 to 47 °C. All the In_2−xSc_xW₃O₁₂ (0≤x≤2) ceramics show fine negative thermal expansion below their corresponding phase transition temperatures. The negative thermal expansion coefficients of the In_2−xSc_xW₃O₁₂ (0≤x≤2) ceramics change in the range from −1.08×10⁻⁶ °C⁻¹ to −7.13×10⁻⁶ °C⁻¹.     

Phase transition and negative thermal expansion properties in isovalently substituted In2−xScx(MoO4)3 ceramics

Sc-substituted In_2−xSc_x(MoO₄)₃ (0 ≤ x ≤ 2) ceramics were successfully synthesized by the solid state reaction method with the goal of tuning the phase transition temperature. Effects of Sc³⁺ substitution on the phase composition, microstructure, phase transition temperature and thermal expansion behavior of the In_2−xSc_x(MoO₄)₃ (0 ≤ x ≤ 2) ceramics were investigated using XRD, FESEM, EDX, XPS and TMA, respectively. The results indicate that all samples are single phase. The relative densities of the In_2−xSc_x(MoO₄)₃ ceramics increased gradually with increasing Sc³⁺ content. Investigations on thermal expansion properties of the In_2−xSc_x(MoO₄)₃ ceramics reveal that the temperature-induced phase transition from monoclinic to orthorhombic symmetry is strongly correlated to the Sc³⁺ content. The obtained In₂(MoO₄)₃ ceramics undergoes a structural phase transition from monoclinic to orthorhombic around 355.3 °C. The phase transition temperature can be significantly shifted from 355.3 °C (x = 0) to 31.9 °C (x = 1.5) by partially replacing In³⁺ cations with less electronegative Sc³⁺ cations. All In_2−xSc_x(MoO₄)₃ (0 ≤ x ≤ 2) ceramics exhibit strong negative thermal expansion above the phase transition temperature. In_0.5Sc_1.5(MoO₄)₃ exhibits linear NTE over the 100–700 °C temperature range with a linear coefficient of thermal expansion of −3.99 × 10⁻⁶ °C ⁻¹.     

Effects of HCl concentration on the growth and negative thermal expansion property of the ZrW2O8 nanorods

A kind of negative thermal expansion ZrW₂O₈ nanorods were synthesized using a hydrothermal method, followed with a post-annealing at 570 °C for 2 h. Effects of HCl concentration on the microstructure, morphology and negative thermal expansion property in resulting ZrW₂O₈ powders were investigated by X-ray diffraction (XRD) and transmission electron microscope (TEM). Results indicate that the formation of the precursor ZrW₂O₇(OH)₂(H₂O)₂ significantly depends on the HCl concentration, and the precursors ZrW₂O₇(OH)₂(H₂O)₂ can form in the 2-8 mol/L HCl solution. With increasing the concentration of the HCl solutions from 2 to 8 mol/L, the rod-like ZrW₂O₈ particles become more homogeneous, and the average dimension change from 10 μm × 0.5 μm to 700 nm × 50 nm. All the ZrW₂O₈ powders obtained in different conditions exhibit negative thermal expansion property, and the average negative thermal expansion coefficients from 15 °C to 600 °C decrease gradually with the increasing HCl concentration.     

Sc- and W-substitution and tuning of phase transition in the Al2Mo3O12

Al₂Mo₃O₁₂ is a typical negative thermal expansion (NTE) material, whose thermal expansion behavior depends on its crystal phase. The thermal shock caused by temperature-induced phase transition limits its wide application. The two series of Al_{2. x}Sc_xMo₃O₁₂ (0 ≤ x ≤ 1) and Al₂Mo_3-xW_xO₁₂ (0 ≤ x ≤ 2.5) solid solutions with controllable phase transition temperature were synthesized via single cation substitution at the A or B position. The problem of thermal shock caused by the change of temperature is effectively solved in the synthesized Al_1.6Sc_0.4Mo₃O₁₂ and Al₂Mo_0.5W_2.5O₁₂, showing stable NTE performance above room temperature, and the coefficients of thermal expansion of which are ?2.19 × 10^?6 °C^?1 in 100–550 °C and ?4.25 × 10^?6 °C^?1 in 85–500 °C, respectively. A-site cation substitution is a more effective way to tune the thermal expansion properties of Al₂Mo₃O₁₂, which is attributed to the fact that the bond strength of A-O is weaker than that of B–O in the compound.     

Giant negative thermal expansion in Zn2-xCuxP2O7 ceramics via microstructure effect

As a novel thermophysical behavior, negative thermal expansion (NTE) has been studied in many materials. However, rare materials have realized giant NTE, and the methods to improve NTE are lacking. Herein, a giant NTE has been achieved in Zn_2-xCu_xP₂O₇ ceramics via microstructure effect. In the Zn_1.96Cu_0.04P₂O₇ ceramic body, the linear contraction measured by dilatometry reaches to 0.9% (3ΔL/L = 2.7%) when heated from ?30 °C to 125 °C, while the intrinsic crystallographic volume contraction derived by X-ray diffraction is only 1.68%. The remarkable NTE enhancement in the ceramic sample is attributed to the microstructure effect. An apparent shrinkage of the voids has been observed by in-situ atomic force microscope (AFM). The voids with large size in the ceramic body is the key factor to enhance NTE. This is the first time to observe direct experimental evidence by AFM for microstructure effect. Microstructure effect is an effective method to produce giant NTE.     

Tuning the phase transition temperature of Cr2(MoO4)3 by A-site substitution of scandium

A series of Cr_2-xSc_x(MoO₄)₃ solid solutions with tunable monoclinic-to-orthorhombic phase transition temperature have been synthesized via solid-state reaction. X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) results show that all synthesized Cr_2-xSc_x(MoO₄)₃ (0?< x?≤?1.4) solid solutions are single phased with no impurities identified, which reveals that Cr³⁺ has been substituted by Sc³⁺ in Cr₂(MoO₄)₃. Monoclinic to orthorhombic phase transition temperature of Cr_2-xSc_x(MoO₄)₃ can be effectively tuned from 372?°C to room temperature as the substituted Sc³⁺-content (x) varies from 0 to 1.4. The synthesized Cr_0.6Sc_1.4(MoO₄)₃ crystalizes in an orthorhombic structure at room temperature, exhibiting anisotropic negative thermal expansion throughout the testing temperature range. The coefficient of thermal expansion measured by thermal mechanical analyzer (TMA) for Cr_0.6Sc_1.4(MoO₄)₃ is ?11.17?×?10^?6 °C^?1 in the testing temperature range of 30–600?°C. Moreover, the crystallization, micromorphology, density and coefficient of thermal expansion of Cr_0.6Sc_1.4(MoO₄)₃ are obviously sensitive to the twice sintering temperature, whereas none of such sensitivity is found for the phase transition temperature.     

Negative thermal expansion and shift in phase transition temperature in Mo-substituted ZrW2O8 thin films prepared by pulsed laser deposition

Mo-substituted ZrW₂O₈ (ZrW_1.1Mo_0.9O₈) thin films have been deposited on quartz substrates by the pulsed laser deposition (PLD) method. The effects of oxygen pressure, substrate temperature and annealing temperature on the morphologies and phase compositions of the ZrW_1.1Mo_0.9O₈ thin films were systematically investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM). The negative thermal expansion and shift in phase transition temperature in cubic ZrW_1.1Mo_0.9O₈ thin films were characterized using high temperature X-ray diffraction. The results indicate that as-deposited ZrW_1.1Mo_0.9O₈ thin films show amorphous phases. Crystallized cubic ZrW_1.1Mo_0.9O₈ thin films were prepared by heating at 1050 °C for 7 min. The growth of the ZrW_1.1Mo_0.9O₈ thin films was strongly influenced by the substrate temperature and oxygen pressure. The ZrW_1.1Mo_0.9O₈ thin film deposited at 500 °C with an oxygen pressure of 10 Pa was smooth and compact, and its thickness was about 720 nm. The high temperature X-ray diffraction analyses demonstrated that the cubic ZrW_1.1Mo_0.9O₈ thin film exhibited strong negative thermal expansion and its thermal expansion coefficient was calculated to be −8.65×10⁻⁶ K⁻¹ from 100 °C to 600 °C. The substitution of Mo in ZrW₂O₈ thin film leads to a remarkable decrease in phase transition temperature, with the α to β structure phase transition occurring below 100 °C. However, with increased testing temperature, the substitution results in part of the cubic ZrW_1.1Mo_0.9O₈ thin film gradually changing into a trigonal phase.     

复合氧化物材料的负热膨胀机理

介绍了相转变、桥氧原子的横向热振动、刚性多面体的旋转耦合、固体内压转变、相界面弯曲、阳离子迁移等六种模式的负热膨胀机理。并对其应用前景和发展趋势进行了预测     

ZrW2O8负膨胀陶瓷材料进展

叙述了各向同性负膨胀ZrW2O8陶瓷材料的制备、结构、负膨胀机理和应用.     

In_2W_3O_(12)陶瓷的相变特性及其负热膨胀性能(英文)

以分析纯In2O3和WO3为原料,采用固相反应法制备In2W3O12陶瓷。利用X射线衍射仪、场发射扫描电子显微镜、热重分析仪、差示扫描量热仪和热机械分析仪对样品的物相组成、微观结构、相变和热膨胀特性进行了表征。结果表明:在900℃烧结6h可制备出纯的单斜相In2W3O12陶瓷,In2W3O12陶瓷断面晶粒均匀,平均尺寸为4～6μm。In2W3O12陶瓷在253.34℃发生单斜相到斜方相的相转变,单斜相的In2W3O12陶瓷显示正热膨胀,在27～249℃,其平均热膨胀系数为16.51×10-6℃-1,斜方相的In2W3O12陶瓷显示负热膨胀,在273～700℃,其平均热膨胀系数为-3.00×10-6℃-1。     

燃烧法合成高纯度负热膨胀材料ZrW2O8粉体

采用燃烧法在较低温度下成功合成了各向同性的负热膨胀材料ZrW2O8粉体.用X射线衍射、扫描电镜、红外光谱综合分析和研究了反应过程中炉温、硼酸和尿素含量、W6 与Zr4 的摩尔比对合成ZrW2O8纯度的影响.结果表明:燃烧法可以合成高纯度、粒径为0.5μm的ZrW2O8粉体.燃烧法合成高纯ZrW2O8的最佳条件是:炉温为500℃,硼酸的摩尔分数为10%,(NH2)2CO与(NH4)5H5[H2(WO4)6]·H2O ZrOCl2·8H2O的质量比为2∶1,(NH4)5H5[H2(WO4)6]·H2O与ZrOCl2·8H2O的摩尔比为1∶3.2.所合成的ZrW2O8在50～700℃之间的线膨胀系数a=-5.08×10-6/℃,其线膨胀系数与温度的关系符合方程dL/L0=-1.4×10-2-4.5×10-4T(50℃≤T≤700℃).     

水热法制备负热膨胀性ZrW2O8粉体

用水热法并经570 ℃热处理6 h制备了ZrW2O8粉体,对水热法制备的前驱体进行了热重-差热分析.用X射线粉末衍射、扫描电子显微镜对ZrW2O8粉体的微观结构及形貌进行表征,结果表明:ZrW2O8粉体为单一α-ZrW2O8相,粉体颗粒为规则的长方体棒状,尺寸约为1.2μm×1.2μm×10μm.原位X射线粉末衍射分析表明:所得ZrW2O8粉体具有很好的负热膨胀特性,从室温到500 ℃,其热膨胀系数为-6.30×10-6 ℃-1;在150～175 ℃温度范围内发生了α-ZrW2O8向β-ZrW2O8相的转变.     

Anisotropic field induced phase transitions and negative electrocaloric effect in rhombohedral Mn doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

The electrocaloric effect (ECE) of Mn doped Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT:Mn) single crystals with particular emphasis on the impact of crystallographic orientations and phase transitions were investigated systematically. Orientation-dependent phase transitions have been demonstrated by the dielectric and strain behaviors. Intriguingly, the negative ECE of –0.02?°C and –0.002?°C were obtained firstly in [001]-oriented PIN-PMN-PT:Mn crystals near the rhombohedral→tetragonal phase transformation and in [011]-oriented crystals near the rhombohedral→orthorhombic phase transformation, respectively. However, only the positive ECE was found in [111]-oriented crystals near the tetragonal→rhombohedral phase transition. Additionally, the maximum ECE temperature changes calculated in [001]-, [011]- and [111]-oriented crystals were 0.33?°C, 0.46?°C and 0.38?°C, respectively. Our results suggest that the negative ECE is attributed to electric field-induced phase transitions, whose critical field decreases with the increase of temperature. The phase transition-mediated coexistence of positive and negative effects in the relaxor-ferroelectric single crystals is beneficial to enhance the efficiency of the solid-state cooling devices.