Thermal Properties of Polycrystalline ZnIn2Se4

A pulse method was used to measure the thermal conductivity, specific heat capacity C _p and thermal diffusivityξ of polycrystalline ZnIn₂Se₄ in the temperature range 300–600 K. The temperature dependence of λ, C _p and ξ demonstrated a light decrease for this material in the temperature range 300–600 K, indicating that there is not a significant change in the structure in this temperature range; this was confirmed by DTA measurements. The results showed that the mechanism of heat transfer is due mainly to phonons; the contributions of electrons and dipoles are very small. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal expansion of MNbO<Subscript>4</Subscript> phases where <Emphasis Type="Italic">M</Emphasis>=Al,Cr, Fe,Ga

Studies on thermal expansion of the MNbO₄ type phases where M=Al, Cr, Fe, Ga have been carried out in the high-temperature X-ray diffraction attachment. In the case of isotypic AlNbO₄ and GaNbO4 compounds the structure of which consists of the ReO₃ type blocks, the direction of minimal thermal expansion is consistent with the direction in which these blocks spread to infinity. In the case of CrNbO₄, the maximal thermal expansion direction is consistent with the [001] direction parallel to which the edge shared octahedra building its structure form infinite chains. FeNbO₄ has the highest coefficients of thermal expansion in this group of compounds.     

第一性原理计算钛酸铅A位掺杂对其负热膨胀性的影响

近年来,实验发现钛酸铅基材料具有负热膨胀性,且其热膨胀程度会受到掺杂元素的影响. 目前所研究的A位掺杂体系中,仅Cd原子掺杂能使钛酸铅负热膨胀性增强. 所以研究A位掺杂钛酸铅,比较Cd原子与其他原子在掺杂钛酸铅时化学键的异同,有助于深刻理解钛酸铅负热膨胀的本质. 本文利用第一性原理,分别优化了Sr、Ba、Cd掺杂钛酸铅的晶格常数,计算了它们的态密度和电荷密度. 结果表明Cd―O键的共价性强于Pb―O键,而Ba―O键和Sr―O键几乎呈离子性,Ba/Sr对Pb的替代削弱了化合物的共价性,降低了自发极化强度. 与实验测量的热膨胀系数对比可以发现,A位原子与氧原子之间的共价性增强,化合物负热膨胀程度升高;若A位原子与氧原子之间的共价性削弱,负热膨胀程度降低. 可见A位原子与氧原子之间的共价性影响了钛酸铅基化合物负热膨胀性.     

C80 Calorimeter Studies of the Thermal Behavior of LiPF 6 Solutions

The thermal behavior of several LiPF₆ solutions was studied using a C80 calorimeter. It was found that oxygen might react with the solvents and decrease their thermal stability. The dissolution of LiPF₆ influences the thermal behavior remarkably with more heat generation and a lower onset temperature. Furthermore, the exothermic peak of LiPF₆ based on an electrolyte containing diethyl carbonate (DEC) was found around 185 ^∘C, which is 9.5–13.6 ^∘C lower than that containing dimethyl carbonate (DMC), which may be due to the relative activity of C₂H₅— and CH₃— in DEC and DMC, respectively.     

纳米氧化铪的制备与热膨胀性

制备了尺寸为4 nm的HfO2纳米颗粒,并借助X射线原子对分布函数方法,研究了尺寸约4 nm和体相HfO2颗粒的晶格热膨胀。结果表明,在纳米尺度的HfO2中,晶格沿a,c轴的热膨胀性增大,b轴热膨胀性稍微减小,体积热膨胀性增大。同时纳米HfO2晶格热膨胀的各向异性比体相大。该现象是由于尺寸效应导致结构畸变变大,尤其是次近邻Hf-O-Hf键角减小,随后升温过程中该畸变发生热弛豫趋向恢复至平衡位置导致的。     

Thermal Property and Phase Transition of the Synthesized New Cubic Leucite-type Compounds

New cubic leucite-type compounds, CsMSi₂O₆ (M=B_0.2Al_0.8,Al_0.2Fe_0.8), Cs₂MSi₅O₁₂ (M=Cd, Mg, Ni, Zn) have been synthesized by the two-stage heat treatment of the solid-state reaction. The thermal expansion properties of the synthesized leucite-type compounds have been studied with HTXRD and LTXRD in the temperature range of 123 to 1273 K. The thermal expansion rate of CsB_0.2Al_0.8Si₂O₆ was found to be considerably smaller than that of CsAlSi₂O₆, while the thermal expansion property of Cs₂MSi₅O₁₂ (M=Mg, Zn, Cd) was found to have a linear relationship in the temperature range of 298 to 1273 K. By using Rietveld analysis it was found that the thermal expansion rate decreased with increasing the Si—O—M(Si) angle for cubic leucite-type compounds at 298 K, and that the phase transitions of CsAlSi₂O₆ and Cs_0.9Al_0.9Si_2.1O₆ were due to the relationship between the bond angle of Si—O—M(Si) of the three-dimensional framework structure and the space ratio in the unit cell at 298 K.This revised version was published online in November 2005 with corrections to the Cover Date.     

EDTA assisted sol–gel synthesis of ZrW2O8

A novel sol–gel synthetic route using water-soluble precursor salts is presented as a synthetic path for a high-purity negative thermal expansion material, ZrW₂O₈. This synthetic route involves a sol–gel method with the use of EDTA as complexing agent. The aqueous solution is transformed into a ceramic material after a two-step heat treatment: gelation at 60 °C and reactive sintering at 1,180 °C. The decomposition of the gel is monitored with infrared spectroscopy and TGA. The high-temperature heat treatment results in ZrW₂O₈ with its characteristic negative thermal expansion behaviour α_{[75–130 °C]}: −9.8 ± 1.6 μm/m °C and α_{[175–300 °C]}: −1.2 ± 0.2 μm/m °C.     

Cr2(WO4)3和Cr2(MoO4)3的负热膨胀特性研究

用液相反应-前驱物烧结法制备了Cr₂(WO₄)₃和Cr₂(MoO₄)₃粉体。298～1 073 K的原位粉末X射线衍射数据表明Cr₂(WO₄)₃和Cr₂(MoO₄)₃的晶胞体积随温度的升高而增大, 本征线热膨胀系数分别为(1.274±0.003)×10^-6 K^-1和(1.612±0.003)×10^-6 K^-1。用热膨胀仪研究了Cr₂(WO₄)₃和Cr₂(MoO₄)₃在静态空气中298～1 073 K范围内热膨胀行为，即开始表现为正热膨胀，随后在相转变点达到最大值，最后表现为负热膨胀，其负热膨胀系数分别为(-7.033±0.014)×10^-6 K^-1和(-9.282±0.019)×10^-6 K^-1。     

Thermal expansion of phases formed in the system Nb2O5-MoO3

Studies on thermal expansion of phases formed in the system Nb₂O₅-MoO₃ (WO₃) have been carried out in the high-temperature X-ray diffraction attachment. In the case of Nb₁₄Mo₃O₄₄, Nb₁₂MoO₃₃ and Nb₁₂WO₃₃ the structure that consists of ReO₃ type blocks, the direction of minimal thermal expansion is consistent with direction in which the chains of corner-sharing polyhedra spread to infinity. On the contrary, for Nb₂Mo₃O₁₄, the structure of which resembles the structure of tetragonal tungsten bronzes, the maximal thermal expansion direction is consistent with above mentioned direction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal Stability of C60-Methylnaphthalenes Solvates

The crystalline solvates containing fullerenes and (di)methylnaphthalenes were investigated by thermal analyses and X-ray diffraction methods. It was found that C₆₀ with (di)methylnaphthalenes forms two types of stable solvates: either at the molar ratio 1:2 decomposing at temperatures close to 100°C or at 1:1 molar ratio decomposing in the temperature range 120–214°C. Crystalline lattice and thermal stability of the solvates depends on the structure of the solvent molecules. The strong solute-solvent interaction is also manifested by the modification of the C₆₀ absorption spectra in solution. The results are discussed using semiempirical quantum chemistry methods. This revised version was published online in August 2006 with corrections to the Cover Date.     

Prototype of Low Thermal Expansion Materials: Fabrication of Mesoporous Silica/Polymer Composites with Densely Filled Polymer inside Mesopore Space

A prototype of novel low thermal expansion materials using mesoporous silica particles is demonstrated. Mesoporous silica/polymer composites with densely filled polymer inside the mesopore space are fabricated by mechanically mixing both organically modified mesoporous silica and epoxy polymer. The mesopores are easily penetrated by polymers as a result of the capillary force during the mechanical composite processing. Furthermore, we propose a new model of polymer mobility restriction using mesoporous silica with a large pore space. The robust inorganic frameworks covering the polymer effectively restrict the polymer mobility against thermal energy. As a result, the degree of total thermal expansion of the composites is drastically decreased. From the mass‐normalized thermal mechanical analysis (TMA) charts of various composites with different amounts of mesoporous silica particles, it is observed that the coefficient of thermal expansion (CTE) values gradually increase with an increase of the polymer amount outside the mesopores. It is proven that the CTE values in the range over the glass‐transition temperatures (T_g) are perfectly proportional to the outside polymer amounts. Importantly, the Y‐intercept of the relation equation obtained by a least‐square method is the CTE value and is almost zero. This means that thermal expansion does not occur if no polymers are outside the mesopores. Through such a quantative discussion, we clarify that only the outside polymer affects the thermal expansion of the composites, that is, the embedded polymers inside the mesopores do not expand at all during the thermal treatment.     

Thermal properties of some precursor oxides of YBCO superconductor

The lattice parameters of BaCuO₂ and CuO were measured as a function of temperature from 10 to 300 K. The fitting of the parameters led to the thermal expansion coefficients and the anisotropy characteristics of the polycrystalline materials. The accuracy of these measurements is discussed, as is the use of thermal expansion measurements, even if not particularly accurate, with a view to calculation ofC _v fromC _p.

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Zusammenfassung Die Gitterparameter von BaCuO₂ and CuO wurden als eine Funktion der Temperatur zwischen 10 und 300 K bestimmt. Das Fitting der Parameter führte zu den thermischen Ausdehnungskoeffizienten und der Anisotropieeigenschaften dieser polykristallinen Substanzen. Die Genauigkeit der Messungen und die Verwendung von—wenn auch nicht genauen—thermischen experimentellen Messungen zur Berechnung vonC _v ausC _p wird diskutiert.

     

Zero Thermal Expansion in K0.46Co1.27[Fe(CN)6]·5.5H2O

Thermal expansion of materials is a comparatively easy‐understood physical property. Prussian blue analogues are of particular interest in engineering as new zero thermal expansion materials. We investigated the thermal expansion in K_0.46Co_1.27[Fe(CN)₆] · 5.5H₂O by x‐ray powder diffraction. This compound is a good example of a zero thermal expansion material. The origin of zero thermal expansion is considered to be the low frequency transverse vibrational motion of the cyano bridges.     

Phase Transitions,Prominent Dielectric Anomalies,and Negative Thermal Expansion in Three High Thermally Stable Ammonium Magnesium–Formate Frameworks

We present three Mg–formate frameworks, incorporating three different ammoniums: [NH₄][Mg(HCOO)₃] ( 1 ), [CH₃CH₂NH₃][Mg(HCOO)₃] ( 2 ) and [NH₃(CH₂)₄NH₃][Mg₂(HCOO)₆] ( 3 ). They display structural phase transitions accompanied by prominent dielectric anomalies and anisotropic and negative thermal expansion. The temperature‐dependent structures, covering the whole temperature region in which the phase transitions occur, reveal detailed structural changes, and structure–property relationships are established. Compound 1 is a chiral Mg–formate framework with the NH₄⁺ cations located in the channels. Above 255 K, the NH₄⁺ cation vibrates quickly between two positions of shallow energy minima. Below 255 K, the cations undergo two steps of freezing of their vibrations, caused by the different inner profiles of the channels, producing non‐compensated antipolarization. These lead to significant negative thermal expansion and a relaxor‐like dielectric response. In perovskite 2 , the orthorhombic phase below 374 K possesses ordered CH₃CH₂NH₃⁺ cations in the cubic cavities of the Mg–formate framework. Above 374 K, the structure becomes trigonal, with trigonally disordered cations, and above 426 K, another phase transition occurs and the cation changes to a two‐fold disordered state. The two transitions are accompanied by prominent dielectric anomalies and negative and positive thermal expansion, contributing to the large regulation of the framework coupled the order–disorder transition of CH₃CH₂NH₃⁺. For niccolite 3 , the gradually enhanced flipping movement of the middle ethylene of [NH₃(CH₂)₄NH₃]²⁺ in the elongated framework cavity finally leads to the phase transition with a critical temperature of 412 K, and the trigonally disordered cations and relevant framework change, providing the basis for the very strong dielectric dispersion, high dielectric constant (comparable to inorganic oxides), and large negative thermal expansion. The spontaneous polarizations for the low‐temperature polar phases are 1.15, 3.43 and 1.51 μC cm^?2 for 1 , 2 and 3 , respectively, as estimated by the shifts of the cations related to the anionic frameworks. Thermal and variable‐temperature powder X‐ray diffraction studies confirm the phase transitions, and the materials are all found to be thermally stable up to 470 K.     

Anomalous changes in the specific heat and thermal conductivity of LiKSO4 crystals around the high-temperature phase

The thermophysical properties of LiKSO₄ crystals were studied around the high-temperature phase, atT _{c 2}=943 K. A Heraeus (DSC) technique was used to measure the specific heat,c _p, while the thermal conductivity,K, was measured by the linear heat flow steady-state method. The measured parameters showed an anomaly in the temperature dependence of bothc _p andK. Anisotropy in the thermal conductivity coefficient was also observed in the different crystallographic axes.     

Thermal decomposition analysis of organic peroxides using model-free simulation

To understand better the thermal decomposition characteristics of organic peroxides, a C80 heat flux calorimeter was used and the decomposition pattern of cumene hydroperoxide and di-tert-butylperoxide were classified as auto-catalytic and n ^th order reaction, respectively. Based on the scanning results with the C80 at several differing rates of heating, the thermal decomposition behavior of organic peroxides under isothermal storage at lower temperature was simulated with a model-free simulation. Simulated results showed that the calculated conversion of cumene hydroperoxide as a function of time was in good agreement with experimental data obtained with the TAM-III high sensitivity thermal activity monitor.     

Thermal expansion of crosslinked closed-cell polyethylene foams

An experimental study on the thermal expansion of a collection of crosslinked low-density polyethylene (LDPE) foams with closed-cell structure is presented. The thermal characterization of these materials, the relationships between the linear thermal expansion coefficient and the structure of the foams, and the determination of the variables that can modify the thermal properties of these products are the goals of this work. The experimental results show that the linear thermal expansion coefficient decreases when the density of the foamed material increases. The gas expansion inside the cells is a mechanism that should be taken into account. Moreover, the thermal expansion also depends on the cellular structure. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2587–2596, 1998     

Switching Between Giant Positive and Negative Thermal Expansions of a YFe(CN)6-based Prussian Blue Analogue Induced by Guest Species

The control of thermal expansion of solid compounds is intriguing but remains challenging. The effect of guests on the thermal expansion of open-framework structures was investigated. Notably, the presence of guest ions (K⁺) and molecules (H₂O) can substantially switch thermal expansion of YFe(CN)₆ from negative (α_v=−33.67×10⁻⁶ K⁻¹) to positive (α_v=+42.72×10⁻⁶ K⁻¹)—a range that covers the thermal expansion of most inorganic compounds. The mechanism of such substantial thermal expansion switching is revealed by joint studies with synchrotron X-ray diffraction, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations. The presence of guest ions or molecules plays a critical damping effect on transverse vibrations, thus inhibiting negative thermal expansion. An effective method is demonstrated to control the thermal expansion in open-framework materials by adjusting the presence of guests.     

Comparative Study of the Decomposition of CCl4 in Cold and Thermal Plasma

Decomposition of carbon tetrachloride was studied in an inductively coupled thermal plasma reactor and in a low temperature, non-equilibrium plasma reactor, in neutral and oxidative conditions, respectively. In neutral conditions formation of solid soot, aliphatic- and cyclodienes was observed in equilibrium, and products, such as Cl₂ and C₂Cl₆ were detected in non-equilibrium plasma. Feeding of oxygen into the thermal plasma reactor depressed both soot and dienes formation and induced the formation of oxygen containing intermediates and products. GC-MS analyses of the gaseous products and the extract of the soot referred to as complex decomposition and recombination mechanism at given conditions. Presence of oxygen in the low temperature plasma reactor results in the formation of carbonyl compounds as intermediers. CO₂ and Cl₂ revealed as final products of CCl₄ decomposition in cold plasma.     

Thermal explosion and runaway reaction simulation of lauroyl peroxide by DSC tests

Lauroyl peroxide (LPO) is a typical organic peroxide that has caused many thermal runaway reactions and explosions. Differential scanning calorimetry (DSC) was employed to determine the fundamental thermokinetic parameters that involved exothermic onset temperature (T₀), heat of decomposition (ΔH_d), and other safety parameters for loss prevention of runaway reactions and thermal explosions. Frequency factor (A) and activation energy (E_a) were calculated by Kissinger model, Ozawa equation, and thermal safety software (TSS) series via DSC experimental data. Liquid thermal explosion (LTE) by TSS was employed to simulate the thermal explosion development for various types of storage tank. In view of loss prevention, calorimetric application and model analysis to integrate thermal hazard development were necessary and useful for inherently safer design.