Photoluminescence and thermal expansion of zirconium vanadate

The optical property and phase transition of ZrV₂O₇ were investigated by temperature-dependent X-ray diffraction, photoluminescence (PL), and Raman spectra. It was found that the material exhibited PL emission in the visible region. The temperature-dependent PL emission showed that the peak energy for emission (at red wavelength) presented a red shift from 10 to 80 K, and then a blue shift with the increase in temperature. The PL emission bands of ZrV₂O₇ shifted with change in temperature relating to its phase transition (80-100 K), which could be confirmed by temperature-dependent Raman spectra.     

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     

Near‐zero thermal expansion transparent lithium aluminosilicate glass‐ceramic by microwave hybrid heat treatment

The material of choice for space applications which demand very high dimensional stability is lithium aluminosilicate (LAS) based Ultra Low thermal Expansion Glass‐Ceramic (ULEGC). Generally, the controlled crystallization process recommended for the processing of transparent ULEGC involves a long soaking duration to achieve the required crystal number density. This paper brings out the process optimization procedure adopted for realizing transparent and nanocrystalline ULEGC from conventionally processed LAS glass using microwave‐assisted (hybrid) crystallization. The experimental strategy involves two stages (i) identification of the optimum crystallization temperature (T_c) under a microwave field (ii) optimization of a microwave‐assisted crystallization process to achieve near zero Coefficient of Thermal Expansion (CTE).. Optimum heat‐treatment schedules for nucleation and crystallization under a microwave environment were found to be 720°C/24 hours and 775°C/0.3 hours, respectively. The optimized heat‐treatment condition revealed the efficacy of the microwave hybrid heating, by producing nanocrystalline (35‐50 nm) and transparent (>82%) ULEGC having a thermal expansion of ?0.03 × 10^?6 K (0°C to 50°C).     

Anomalous thermal expansion behavior in Er1-xCaxMnO3

Perovskite Er_1-xCa_xMnO₃ (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5) was synthesized using a solid-state method. Thermal expansion behavior was tested using a thermal dilatometer and high-temperature X-ray diffraction (XRD). The experimental results indicated the doping contents of Ca (x) in the Er_1-xCa_xMnO₃ have a dramatic effect on their thermal expansion behavior. The samples of Er_1-xCa_xMnO₃ (x = 0.1,0.2 and 0.25) exhibit positive thermal expansion (PTE) characteristics while Er_0.7Ca_0.3MnO₃ (x = 0.3) exhibits a negative thermal expansion (NTE) property with a thermal expansion coefficient of −3.1 × 10⁻⁶ K⁻¹ in room temperature (RT) −750 K. In addition, Er_0.6Ca_0.4MnO₃ (x = 0.4) exhibits NTE properties only at RT–500 K, and Er_0.5Ca_0.5MnO₃ (x = 0.5) exhibits PTE properties at RT–750 K. The thermal shrinkage mechanism is the Jahn–Teller effect of the Mn³⁺ ions and the double exchange of Mn³⁺–O–Mn⁴⁺ in Er_0.7Ca_0.3MnO₃. This phenomenon causes Mn–O octahedral distortion and oxygen vacancy, causing Er_0.7Ca_0.3MnO₃ to become anisotropic. This feature results in the elastic deformation of Er_0.7Ca_0.3MnO₃ during heating, which consumes the void and displays NTE at macro level.     

Expanding negative thermal expansion range of ZrMnMo3O12 to cover room temperature by introducing V5+

Solid solutions of Zr_1+xMn_1-xMo_3-2xV_2xO₁₂ (0 ≤ x ≤ 0.5) are developed with reduced phase transition temperature (from 362 to 160 K) by introducing V⁵⁺ into ZrMnMo₃O₁₂. Zr_1+xMn_1-xMo_3-2xV_2xO₁₂ adopt monoclinic (P2₁/a) and orthorhombic (Pbcn) structure at room temperature (RT) for x ≤ 0.1 and x ≥ 0.2, respectively. The formation of bond V–O induces a larger average effective negative charge on oxygen to enhance the repulsive force between them and then strengthens the bond of Mo–O, which reduces the phase transition temperature due to the reduction in effective electronegativity and expands negative thermal expansion (NTE) range covering RT. NTE property in a wide temperature range (from 160 to 673 K) for Zr_1.5Mn_0.5Mo₂VO₁₂ is realized, implying great potential for applications. The NTE property of the materials is induced by low-frequency phonons.     

超低膨胀锂铝硅透明微晶玻璃的研究与应用现状

环境温度的变化,会引起材料的膨胀与收缩,在材料中产生热应力,会影响材料使用。零膨胀锂铝硅透明微晶玻璃卓越的热学性能,优良的光学和机械性能使之成为不可或缺的综合材料。本文主要介绍了零膨胀LAS系透明微晶玻璃的主要形态和结构特征,热学、光学性能以及应用现状,并提出了研究中存在的问题及今后发展的方向。     

Low thermal expansion ZBLAN-based glass ceramics containing CaZrF6 crystals

Fluoride glass is considered as an excellent optical material owing to its ultralow phonon energy (578 cm⁻¹). However, its high thermal expansion coefficient (15.87 × 10⁻⁶/K) and deformability severely limit applications under service environments with high temperature, high humidity, and high-power irradiation. To overcome these limitations, we developed a ZBLAN-based fluoride glass ceramics (CZBLAN GCs) contained the negative thermal expansion material-CaZrF₆ crystals by phase-separation engineering. The ability to control the precipitation of the desired crystal phase was achieved by adequate compositional design for phase separation, yielding improved or new properties in the development of fluoride GCs. With the successful precipitation of very uniformly dispersed single CaZrF₆ crystals in the glass phase (up to ∼40% crystallinity), a significantly lower thermal expansion coefficient (3.66 × 10⁻⁶/K) in the fluoride system was achieved. Moreover, enhanced fluorescence properties of Eu-doped CZBLAN GCs were observed compared with those of Eu-doped ZBLAN glass owing to the lower phonon energy in GCs.     

High-pressure structural stability,equation of state,and thermal expansion behavior of cubic HfO2

The structural stability, equation of state, and thermal expansion behavior of nanocrystalline cubic HfO₂, an ultra-high-temperature ceramic, have been investigated using X-ray diffraction at extreme conditions of pressures and temperatures. High-pressure studies show that the cubic structure is stable up to 26.2 GPa, while the high-temperature studies show the stability of the cubic structure up to 600°C. The Rietveld structure refinement of the high-pressure data reveals the progressive transition of secondary monoclinic phase to the cubic phase at higher pressures. The phase progression is accompanied by incompressibility along the b axis and a large compressibility along the c axis of the monoclinic structure. The second-order Birch-Murnaghan equation of state fit to the unit cell volume data yielded a bulk modulus of 242(16) GPa for the cubic structure. A linear thermal expansion value of α_a(c) = 8.80(15) × 10⁻⁶°C⁻¹ and a volume thermal expansion value of α_v = 26.5(4) × 10⁻⁶°C⁻¹ have been determined from the in situ high-temperature X-ray diffraction studies. The results are discussed by comparing with the high-pressure and high-temperature behavior of isostructural ZrO₂. To the best of our knowledge, this is the first experimental report on the structural stability of cubic HfO₂ at high pressures.     

Graphite thermal expansion relationship for different temperature ranges

For practical reasons, the coefficient of thermal expansion (CTE) has to be measured over a particular temperature range, for example 20-120 °C, 20-400 °C or 500-700 °C. However, in many cases, engineers or scientists involved in the assessment of graphite components such as nuclear reactor moderator bricks, electrodes or moulds require CTE over temperature ranges other than that of the original measurement. This paper compares three different techniques used to convert CTE from one temperature range to another. The method used by the UK nuclear industry is compared with techniques proposed by two international companies. There was close agreement between two of the methods. However there was some divergence in the case of the third method. This may be related to the type of graphite (fine-grain) for which the third method was developed.     

b-direction anomalous thermal expansion behavior in SmCrO3

The ceramic composites of Sm_1-xBi_xCrO₃ (x = 0, 0.1, 0.2, 0.3) have been prepared by solid state reaction. X-ray diffraction patterns demonstrate that Bi-doped polycrystalline samples match with standard pattern of SmCrO₃. After Bi doping, the lattice constants in three directions increase slightly. The thermal expansion properties were studied by X-ray powder diffraction over the temperature range from 100 to 400 K. The results showed that the compound SmCrO₃ exhibits positive thermal expansion behavior in the a and c directions in the whole measurement temperature range. The anomaly is that in the b direction the lattice exhibits positive thermal expansion behavior at low temperatures, while above 200 K it exhibits near zero thermal expansion behavior. The thermal expansion behaviors of all Bi-doped SmCrO₃ are similar to that of SmCrO₃, but unexpectedly, negative thermal expansion behaviors were observed in the b direction above 200 K in the highly Bi doped samples. The dielectric measurement shows that there are two thermal activation processes in all Sm_1-xBi_xCrO₃ (x = 0, 0.1, 0.2, 0.3) respectively, with the low-temperature activation corresponding to the grain itself, while the high-temperature activation comes from the grain boundary. After low-temperature thermal activation, with increasing temperature, the enhanced vibration of the corner oxygen atoms of the CrO₆ octahedron is mainly perpendicular to the b axis, which leads to the near zero thermal expansion behavior in the b direction. Moreover, after high-temperature thermal activation, as the temperature increases, the enhanced interaction between Bi³⁺ and the free oxygen atom (space oxygen atom) at the grain boundary will further lead to the negative thermal expansion behavior in the b-direction of the high concentration doped samples.     

磷灰石晶格热膨胀性质的实验研究

本文采用高温粉晶X射线衍射技术 ,对溶胶—凝胶法合成羟基磷灰石 (Hap) ,提拉法合成氟磷灰石 (Fap)和天然沉积型碳氟磷灰石 (CFap)从室温到 95 0℃之间的晶格热膨胀系数进行了测定。实验结果表明 :在测试温度范围内 ,各样品的晶格膨胀系数与温度呈线性关系。Hap的△a/a与△c/c分别为 13 .8× 10 - 6 C- 1 和 15 .3× 10 - 6 C- 1 ,Fap的△a/a与△c/c分别为 15 .8× 10 - 6 C- 1 和 18.2× 10 - 6 C- 1 ,CFap的△a/a与△c/c分别为 15 .3× 10 - 6 C- 1 和 18.5× 10 - 6 C- 1 。在实验温度区间内未发现样品有明显的相变或结构转变现象 ,高温实验后样品的傅立叶变换红外光谱测定证实了这一推断。     

The thermal expansion behaviour of highly oriented polyethylene

The thermal expansion behaviour of oriented polyethylene has been studied over the temperature range ?50°C to +20°C. In general the values for the thermal expansion coefficient in the orientation direction are negative and of a greater magnitude than the c-axis expansion for the crystal, which is approached at ?50°C. This remarkable effect has been quantitatively related to the influence of an internal shrinkage force, and the changes in expansivity with temperature shown to relate primarily to the change in modulus with temperature.     

Mechanisms of ultralow and anisotropic thermal expansion in cordierite Mg2Al4Si5O18: Insight from phonon behaviors

Materials with negative or ultralow thermal expansion are of crucial importance for technological applications since they make it possible to tailor the coefficient of thermal expansion (CTE) of composite to a specific positive, negative or even zero value. In this work, first‐principle calculations were performed to investigate the thermal expansion behavior in cordierite Mg₂Al₄Si₅O₁₈, which is a representative silicate widely used in the ceramic industry and of promising application due to its ultralow CTE and good thermal shock resistance. According to the quasi‐harmonic approximation and the Grüneisen theory, temperature dependences of linear CTEs along a, b, and c directions were predicted. The transverse acoustic modes and low‐energy optic modes are identified to take the most of the responsibility for the negative CTE, especially at low temperatures while the high‐energy optic modes contribute positively to the thermal expansion, leading to increasing CTE at higher temperatures. The ultralow linear CTEs result from the weighted average of all the modal contributions with negative or positive Grüneisen parameters. In addition, the anisotropy of thermal expansion originates from its layered crystal structure containing rigid tetrahedron rings in a‐b plane staking along c direction. This work provides an insight into the mechanism of ultralow and anisotropic thermal expansion in Mg₂Al₄Si₅O₁₈ and further enriches the scope of material design for use in applications needing to control thermal expansion.     

Tuning stoichiometry of high-entropy oxides for tailorable thermal expansion coefficients and low thermal conductivity

Thermal barrier coating materials with proper thermal expansion coefficient (TEC), low thermal conductivity, and good high-temperature stability are of great significance for their applications in next-generation turbine engines. Herein, we report a new class of high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x with different Ce⁴⁺ contents synthesized by a solid-state reaction method. They exhibit different crystal structures at different Ce⁴⁺ content, including a bixbyite single phase without Ce⁴⁺ doping (x = 0), bixbyite-fluorite dual-phase in the RE₂O₃-rich region (0 < x < 2), and fluorite single phase in the stoichiometric (x = 2) and CeO₂-rich region (x > 2). The high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x exhibit tailorable TECs at a large range of 9.04 × 10^–6–13.12 × 10^–6 °C^–1 and engineered low thermal conductivity of 1.79–2.63 W·m^–1·K^–1. They also possess good sintering resistance and high-temperature phase stability. These results reveal that the high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x are promising candidates for thermal barrier coating materials as well as thermally insulating materials and refractories.     

Negative thermal expansion properties of Cu1.5Mg0.5V2O7

Cu_1.5Mg_0.5V₂O₇ was prepared by a solid state method. Its phase, microstructure, thermal expansion property, and Raman spectra were analyzed in detail. Results show that Cu_1.5Mg_0.5V₂O₇ maintains a monoclinic crystal structure and exhibits an excellent linear negative thermal-expansion property with coefficient of thermal expansion of ?8.72?×?10^?6?K^?1 over a wide temperature range of 153–673?K. The mechanism underlying the negative thermal expansion of Cu_1.5Mg_0.5V₂O₇ involves the coupling effect of the tetrahedron caused by the lateral vibration of the bridge oxygen atom and the tensile effect of the tetrahedron, The partial collapse caused by the loss of the oxygen atoms also plays an important role in the mechanism.     

The Distortion‐Adjusted Change of Thermal Expansion Behavior of Cubic Magnetic Semiconductor (Sc1−xMx)F3 (M = Al,Fe)

For the study of negative thermal expansion (NTE) compounds, it is critical to effectively control the thermal expansion. In this letter, a chemical approach has been taken to control the thermal expansion behavior in ScF₃ which has a strong NTE. Owing to the difference of radius of substituting ions, local distortion inevitably emerges in the lattice matrix, which is verified by pair distribution function analysis of high‐resolution synchrotron X‐ray scattering. It is a valuable clue that the thermal expansion behaviors in the ScF₃ based systems and other trifluorides are correlated closely to structural distortion of metal‐F‐metal linkages. In addition, the introduction of 3d transition‐metal enables its semiconductor and ferromagnetic characteristics. This study provides important reference opinion for the control of thermal expansion and introduction of multifunctionalization for those NTE compounds with open framework structure.     

Anodic lithium ion battery material with negative thermal expansion

Negative thermal expansion materials will effectively counteract possible severe expansion and contraction due to the insertion and extraction of Li ions in lithium ion batteries. Herein, negative thermal expansion ZrScMo₂VO₁₂ and its carbon-coating composites are prepared as electrode material in lithium ion batteries by a heating treatment route. The galvanostatic charge/discharge process, cyclic voltammetry measurement and electrochemical impedance spectroscopy are tested to relate their thermal expansion and electrochemical properties. The initial specific capacity reaching 1062 mA h g^-1 at the current density of 0.2 A g^-1 is obtained with ideal negative thermal expansion properties. The reversible specific capacity still remains stable at 310 mA h g^-1 for that material coated with carbon after 100 cycles. The corresponding theoretical simulations and in situ XRD patterns propose a Li ion storage mechanism based on Li ion insertion process in open framework structure. As a proof-of-concept research, this work paves a way to the promising application of negative thermal expansion materials in lithium ion batteries and other energy storage systems.     

Re-examination of the relationship between packing coefficient and thermal expansion coefficient for aromatic polyimides

The existence of a possible relationship between molecular packing coefficient and thermal expansion coefficient for various aromatic polyimides was investigated. Rod-like low-thermal-expansion polyimides without side groups were seen to have very high packing coefficients, pointing to free volume as a factor in lowering their thermal expansion coefficients. But the small packing coefficients for low-thermal-expansion polyimides with side groups indicated that this was not so. Also, even if the large packing coefficients tended to increase the Young's moduli for these polyimides without side groups, the rod-like polyimides with side groups have small packing coefficients and large Young's moduli. The polyimides with low packing coefficients were found to have very small diffusion coefficients for water vapour.     

Correlation between crystallinity and thermal expansion in LiAlSiO4 ceramics prepared by spark plasma sintering

LiAlSiO₄ (LAS) ceramics are prepared by using the sol-gel method followed by spark plasma sintering. XRD patterns and SEM images verify that the ceramics contain amorphous and LAS phases and that microcracks appear in the sample prepared at 900 °C due to its larger grain size. Compared with applied pressure and soaking time, sintering temperature has a greater impact on the crystallinity and density of the ceramics during sintering. High-temperature XRD results reveal that the LAS phase exhibits its intrinsic negative thermal expansion independently in all samples regardless of crystallinity. The coefficients of thermal expansion (CTE) measured by the dilatometric method change from positive values in samples prepared at 600 and 650 °C to near zero in samples prepared at 700 and 800 °C and then to a negative value in the sample prepared at 900 °C. The combined effects of an amorphous phase with a positive CTE and the LAS phase with a negative CTE are responsible for the observed transformation of thermal expansion in the samples. The calculated total CTEs of the glass-ceramic bulks are in agreement with the results measured through the dilatometric method in samples prepared at 650–800 °C. Microcracks in the sample prepared at 900 °C cause a more negative bulk CTE than the calculated CTE.     

Evolution of the coefficient of thermal expansion of a thermosetting polymer during cure reaction

The evolution of the coefficient of thermal expansion (CTE) of a thermosetting polymer during cure reaction is an important parameter for industrial applications such as composite processing since it influences the development of internal stresses in the material. The CTE being almost impossible to measure on a reacting thermoset, we propose to use an indirect method based on the modelling of ionic conductivity by a modified WLF equation, allowing to calculate the evolution of CTE from dielectric spectroscopy measurements. This method is applied to a dicyanate ester thermosetting polymer, leading to encouraging results both qualitatively and quantitatively.