Constant Rate Thermal Analysis (CRTA) as a Tool for the Synthesis of Materials with Controlled Texture and Structure

Some examples of the use of the CRTA method for the synthesis of materials with controlled texture and structure are given. BaTiO₃ has been obtained from the thermal decomposition of Barium Titanyl Oxalate (BTO) and Barium Titanyl Citrate (BTC) by controlling the reaction temperature in such a way that the partial pressure of the gases generated in the reaction was maintained constant at a value close to 10^-2 mbar. It has been shown that this method allows getting BaTiO₃ with crystal sizes considerably lower than those obtained by decomposing the same precursors by conventional methods. This small crystal sizes lead to the stabilisation of the metastable cubic phase with regards to the tetragonal phase. It has been also shown that the control of the CO generated in the carbothermal reduction of silica allows tailoring the phase composition of the silicon nitride obtained as final product. This revised version was published online in August 2006 with corrections to the Cover Date.     

Controlled Rate Thermal Analysis of kaolinite dehydroxylation: effect of water vapour pressure on the mechanism

The present kinetic study is focused on one aspect of kaolinite dehydroxylation, namely the influence of water vapour pressure in the 10⁻³ to 5 hPa range and in the presence of crystalline defects. The experimental problem of keeping, throughout the dehydroxylation, the pressure gradients negligible around and within the sample is solved by means of Controlled Rate Evolved Gas Detection (CR-EGD). The dehydroxylation rate selected is as low as 0.014 h⁻¹ (which corresponds to a duration of 70 h for the whole experiment). Moreover, more than 20 independent measurements of the apparent Arrhenius energy of activation are carried out all along the dehydroxylation, with help of the rate–jump method, and therefore, without any assumption about the rate law of the determining step. In these conditions, the apparent Arrhenius energy of activation measured during the dehydroxylation of a poorly crystallised kaolinite is shown to be constant in the range 0.02<<0.84 (under 10⁻³ hPa) and in the range 0.18<<0.80 (under 5 hPa), indicating that the rate law obeys the Arrhenius law in this range of extent of reaction. The corresponding activation energies obtained are (233±15) kJ/mol under 10⁻³ hPa and only (188±10) kJ/mol under 5 hPa. Although this decrease is in contradiction with previously published results, it can be interpreted by considering that, under 10⁻³ hPa, diffusion is the limiting step whereas, under 5 hPa, the part of water desorption probably becomes predominant.     

Kinetic Analysis of the Thermal Decomposition of Synthetic Malachite by CRTA

The kinetic behavior of the thermal decomposition of synthetic malachite was investigated by means of CRTA under different conditions of reduced pressure, flowing gases and quasi-isobaric atmospheres. The thermal decomposition was found to proceed at lower temperatures under the influence of the self-generated gases, CO₂ and H₂O. From a viewpoint of chemical equilibrium, the normal and opposite effects on the overall kinetics were observed for the self-generated CO₂ and H₂O, respectively. The complexity of the present reaction is also reflected by the variations of the apparent kinetic parameters which depend on the applied and self-generated atmospheric conditions. The practical usefulness of CRTA when applied to a complicated thermal decomposition is discussed as exemplified by the kinetic approaches to the present reaction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Sample Controlled Thermal Analysis Temperature Programmed Reduction of Bulk and Supported Copper Oxide

The reduction of bulk and supported copper oxide was investigated using Constant Rate-Temperature Programmed Reduction (CR-TPR) and conventional linear heating rate TPR. Linear heating profiles indicated that the reduction of supported samples was more facile than that of the bulk oxide. CRTA results revealed that both supported and bulk oxide samples were reduced via a mechanism involving a nucleation step and/or auto-catalysis. The increased reducibility of the supported samples is attributed to a higher dispersion which provides a larger reactive surface area and a high concentration of defects at which reduction is initiated. This revised version was published online in August 2006 with corrections to the Cover Date.     

含氯磷硼酸酯聚合物的摩擦性能

合成了标题聚合物,对其进行了IR表征。利用四球试验机测定其作为润滑剂的摩擦学性能。结果表明,含磷硼酸酯的最大无卡咬负荷值大于含氯硼酸酯,而且减摩抗磨效果好;含长链疏水基团的硼酸酯的最大无卡咬负荷比短链疏水基团的高,承载能力高。EDAX能谱分析表明,其抗磨性能与摩擦表面中存在硼、氯、磷活性元素有关。     

水合硼酸锶的制备及脱水热分解动力学

以氯化锶和硼酸氢铵为原料,采用液相沉淀法制备了片状水合硼酸锶（SrB6O10•5H2O）粉体,并用XRD、FT-IR及SEM进行了表征。利用热重分析法对片状纳米硼酸锶粉体的脱水热分解动力学进行了研究,分别采用 Coats- Redfern 方程和 Flynn-Wall-Ozawa（FWO）法对热重分析数据进行了处理和拟合,初步确定了水合硼酸锶的四步脱水过程及相应的热分解反应机理,得到各步反应的表观活化能和指前因子。     

Green Colorants Based on Energetic Azole Borates

The investigation of green‐burning boron‐based compounds as colorants in pyrotechnic formulations as alternative for barium nitrate, which is a hazard to health and to the environment, is reported. Metal‐free and nitrogen‐rich dihydrobis(5‐aminotetrazolyl)borate salts and dihydrobis(1,3,4‐triazolyl)borate salts have been synthesized and characterized by NMR spectroscopy, elemental analysis, mass spectrometry, and vibrational spectroscopy. Their thermal and energetic properties have been determined as well. Several pyrotechnic compositions using selected azolyl borate salts as green colorants were investigated. Formulations with ammonium dinitramide and ammonium nitrate as oxidizers and boron and magnesium as fuels were tested. The burn time, dominant wavelength, spectral purity, luminous intensity, and luminous efficiency as well as the thermal and energetic properties of these compositions were measured.     

硼-锗骨架无机微孔材料研究进展

作为潜在的新型功能材料，硼-锗骨架微孔化合物近年来得到科学家的普遍关注。本文从结构化学的角度，分别讨论了硼酸盐、锗酸盐及硼锗酸盐研究体系，并对其中的典型结构进行了描述、归类与总结；针对硼、锗易成簇聚集的特点，探讨了硼-锗骨架无机微孔化合物研究的发展趋势。     

稀土硼酸盐的结构及其真空紫外(VUV)荧光性质

稀土硼酸盐在真空紫外(VUV)波段的吸收与化合物中硼酸根离子的结构有关,扩展Hückel计算表明,硼酸根离子中的B-O成键轨道与B-O反键轨道能量差按BO₄^5->B₂O₅^4->BO₃^3->B₃O₉^9-顺序减小,这与VUV激发光谱的实验结果一致,表明VUV激发过程对应于电子从B-O成键轨道向反键轨道的跃迁.     

Functionalization of closo‐Borates via Iodonium Zwitterions

A simple method for the functionalization of closo‐borates [closo‐B₁₀H₁₀]^2? ( 1 ), [closo‐1‐CB₉H₁₀]^? ( 2 ), [closo‐B₁₂H₁₂]^2? ( 3 ), [closo‐1‐CB₁₁H₁₂]^? ( 4 ), and [3,3′‐Co(1,2‐C₂B₉H₁₁)₂]^? ( 5 ) is described. Treatment of the anions and their derivatives with ArI(OAc)₂ gave aryliodonium zwitterions, which were sufficiently stable for chromatographic purification. The reactions of these zwitterions with nucleophiles provided facile access to pyridinium, sulfonium, thiol, carbonitrile, acetoxy, and amino derivatives. The synthetic results are augmented by mechanistic considerations.     

Kinetic Analysis of Solid State Reactions by Means of Stepwise Isothermal Analysis (SIA) and Constant Rate Thermal Analysis (CRTA) A comparative study

The results obtained in this work point out that both SIA and CRTA traces, simulated assuming the same values of the reaction rate, are identical provided that either 'n order’ or diffusion controlled kinetics are concerned. The α-T CRTA plots of solid state reactions fitting an Avrami-Erofeev kinetic model show that the temperature decreases with increasing α until reaching a minimum at a value of the reacted fraction α = αmin characteristic of the Avrami-Erofeev exponent. The shapes of the corresponding SIA diagrams are quite different and a very long isothermal step is obtained. Moreover, the reaction rate is not maintained constant during the isothermal period but it shows a maximum at a value of α depending on the Avrami-Erofeev coefficient. It is important to point out that this αmax value agrees with the corresponding αmin calculated from a CRTA curve by assuming the same Avrami-Erofeev exponent. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal Analysis of Hydrated Fly Ash-Lime Pastes

Fly ash is the ash precipitated from the exhaust fumes of coal-fired power stations. It consists mainly of active silica and alumina, with large amounts of glass. The aim of this work was to study the pozzolanic activity of fly ash with lime by means of DTA as a function of the lime content and the curing time. The curves revealed that the rate of hydration, as indicated by the residual lime and hydrated compounds, increases with increasing lime content and curing time. It is concluded that fly ash can be used as a blending material in pozzolanic cement. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrocatalysts Based on Transition Metal Borides and Borates for the Oxygen Evolution Reaction

Electrochemical water splitting is a clean and sustainable process for hydrogen production on a large scale as the electrical power required can be obtained from various renewable energy resources. The key challenge in electrochemical water splitting process is to develop low-cost electrocatalysts with high catalytic activity for the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode. OER is the most important half-reaction involved in water splitting, which has been extensively studied since the last century and a large amount of electrocatalysts including noble and non-noble metal-based materials have been developed. Among them, transition metal borides and borates (TMBs)-based compounds with various structures have attracted increasing attention owing to their excellent OER performance. In recent years, many efforts have been devoted to exploring the OER mechanism of TMBs and to improving the OER activity and stability of TMBs. In this review, recent research progress made in TMBs as efficient electrocatalysts for OER is summarized. The chemical properties, synthetic methodologies, catalytic performance evaluation, and improvement strategy of TMBs as OER electrocatalysts are discussed. The electrochemistry fundamentals of OER are first introduced in brief, followed by a summary of the preparation and performance of TMBs-based OER electrocatalysts. Finally, current challenges and future directions for TMBs-based OER electrocatalysts are discussed.     

Controlled rate thermal analysis of hydromagnesite

The reaction of magnesium minerals such as brucite with CO₂ is important in the sequestration of CO₂. The study of the thermal stability of hydromagnesite and diagenetically related compounds is of fundamental importance to this sequestration. The understanding of the thermal stability of magnesium carbonates and the relative metastability of hydrous carbonates including hydromagnesite, artinite, nesquehonite, barringtonite and lansfordite is extremely important to the sequestration process for the removal of atmospheric CO₂. This work makes a comparison of the dynamic and controlled rate thermal analysis of hydromagnesite and nesquehonite. The dynamic thermal analysis of synthetic hydromagnesite proves that dehydration takes place in two steps at 135 and 184°C, dehydroxylation at 412°C and decarbonation at 474°C. Controlled rate thermal analysis shows the first dehydration step is isothermal and the second quasi-isothermal at 108 and 145°C, respectively. In the CRTA experiment both water and carbon dioxide are evolved in an isothermal decomposition at 376°C. CRTA technology offers better resolution and a more detailed interpretation of the decomposition processes of magnesium carbonates such as nesquehonite via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of non-isothermal nature reveal partial nesquehonite structure.     

水合铵硼氧酸盐及其饱和溶液的FTIR和Raman光谱研究

研究了NH₄B₅O₈·4H₂O和(NH₄)₂B₈O₁₃·6H₂O及其饱和溶液于20℃的FTIR和Raman光谱,对振动频率进行了归属.根据振动光谱特征,预测(NH₄)₂B₈O₁₃·6H₂O中所含基本结构单元为[B₇O ₁₁(OH)·B(OH)₃]^2-.首次将Raman光谱中516cm^-1处的强散射峰归属为这一多聚硼氧配阴离子的对称脉冲振动峰,并对以上2种铵硼氧酸盐饱和溶液中硼氧配阴离子的存在形式{B(OH)₃,[B₃O₃(OH)₄]^-和[B₅O₆(OH)₄]^-}和相互作用机理进行了探讨.     

Analysis of the Thermokinetics Under Dynamic Conditions by Relative Rate of Thermal Decomposition

A novel parameter of the relative rate of thermal decomposition has been defined on the basis of an analysis of equation relating the logarithm of the conversion degree on the temperature. The dependence of this parameter on temperature in the dynamic conditions has been analyzed and discussed. The dependence of the relative rate of thermal decomposition is a linear relationship involving two coefficients. These coefficients can be related to the enthalpy and activation energy. The parameter developed has been used for the analysis of a series of consecutive reactions of thermal decomposition of calcium oxalate monohydrate. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Modification of Hydroxyl Surfaces of Formamide-Intercalated Kaolinites Synthesized by Controlled Rate Thermal Analysis

Controlled rate thermal analysis (CRTA) technology made possible the separation of adsorbed formamide from intercalated formamide in formamide-intercalated kaolinites. X-ray diffraction shows that the CRTA-treated formamide-intercalated kaolinites remain expanded after CRTA treatment. The Raman spectra of the CRTA-treated formamide-intercalated kaolinites are significantly different from those of the intercalated kaolinites with both intercalated and adsorbed formamide. An intense band is observed at 3629 cm(-1), attributed to the inner surface hydroxyls hydrogen bonded to the formamide. Broad bands are observed at 3600 and 3639 cm(-1) and are attributed to the inner surface hydroxyls, which are hydrogen bonded to the adsorbed water molecules. The hydroxyl stretching band of the inner hydroxyl is readily observed at 3621 cm(-1) in the Raman spectra of the CRTA-treated formamide-intercalated kaolinites. The results of thermal analysis show that the amount of intercalated formamide between the kaolinite layers is independent of the presence of water. The Raman bands of the formamide in the CRTA-treated intercalated kaolinites are readily observed. Copyright 2001 Academic Press.     

Lanthanide Nitrido Borates with Six-Membered B3N6 Rings: Ln3B3N6

Metal compounds with heteroatomic ring systems of main group elements are a domain of coordination chemistry. However, lanthanide nitrido borates Ln₃B₃N₆ (Ln=La or Ce; see structure) are synthesized by the reaction of hexagonal boron nitride with LnN. The compounds contain the six-membered B₃N₆ ring, which can be seen as a fragment from one layer of the hexagonal BN structure.     

Polymorphs of the Gadolinite-Type Borates ZrB2O5 and HfB2O5 Under Extreme Pressure

Based on the results from previous high-pressure experiments on the gadolinite-type mineral datolite, CaBSiO₄(OH), the behavior of the isostructural borates β-HfB₂O₅ and β-ZrB₂O₅ have been studied by synchrotron-based in situ high-pressure single-crystal X-ray diffraction experiments. On compression to 120 GPa, both borate layer-structures are preserved. Additionally, at ≈114 GPa, the formation of a second phase can be observed in both compounds. The new high-pressure modification γ-ZrB₂O₅ features a rearrangement of the corner-sharing BO₄ tetrahedra, while still maintaining the four- and eight-membered rings. The new phase γ-HfB₂O₅ contains ten-membered rings including the rare structural motif of edge-sharing BO₄ tetrahedra with exceptionally short B−O and B⋅⋅⋅B distances. For both structures, unusually high coordination numbers are found for the transition metal cations, with ninefold coordinated Hf⁴⁺, and tenfold coordinated Zr⁴⁺, respectively. These findings remarkably show the potential of cold compression as a low-energy pathway to discover metastable structures that exhibit new coordinations and structural motifs.     

Influence of the Heating Rate in the Thermal Decomposition of HMX

This work refers to a study of the thermal decomposition of octahydro-1,3,5,7-tetranitro-1,3,5,7 tetrazocine (HMX) by differential scanning calorimetry (DSC) in non-isothermal conditions, with heating rates from1 to 25°C min⁻¹. The influence of the heating rate, the particle and the sample size were verified. The activation energy was calculated using the peak temperature shift method, proposed by Ozawa and a significant variation in the results was observed according to the range of the used heating rates. As the heating rate was increased, different conversions and self-heating effects were observed at the respective DSC peaks, indicating that the use of this method was inadequate and it may lead to incorrect results, which, in turn, could explain the wide range of activation energy values published in literature. At lower heating rates HMX decomposition occurs on the solid state and at higher ones decomposition occurs after melting practically at the same temperature, which does not depend on the heating rate. This revised version was published online in July 2006 with corrections to the Cover Date.