New data on the thermal analysis of diphasic mullite gel

A diphasic mullite gel has been synthesized by using Ludox and Al(NO₃)₃-9H₂O in ammoniacal solution. Both DTA and DDTA studies have been performed with a sensitive differential thermal analyzer system. Besides usual exotherm due to mullite formation, a pronounced exotherm between 800–1300°C has been noted. Considering previous XRD data, it is explained as due to formation of Si-Al spinel phase, which is subsequently transformed exothermally at 1320°C to mullite.     

固相萃取-超高效液相色谱-串联质谱法同时测定大气降水中9种全氟化合物前体物质

采用固相萃取-超高效液相色谱-电喷雾串联三重四极杆质谱联用技术,建立了大气降水中9种全氟化合物前体物质的高通量检测方法。使用HLB固相萃取柱富集和净化降水样品中的目标化合物,以HSS T_3色谱柱(100mm×2.1 mm,1.7μm)为分析柱,甲醇和水作为流动相进行梯度洗脱。质谱以电喷雾负离子电离,采用多反应监测模式检测。9种目标化合物在0.05~5.00μg/L、0.5~50.0μg/L或5.00~500μg/L浓度范围内线性良好,相关系数为0.992 1~0.999 5,方法的检出限为0.05~7.9 ng/L;高、中、低3个添加水平的回收率为76.0%~106%,相对标准偏差为0.72%~13.7%。实验结果表明,该方法灵敏、准确,且具有检测范围广、分析速度快等特点,是一种适用于大气降水样品中全氟化合物前体物质检测分析的方法。     

Thermal evolution of TEOS-Al(secBuO)3-etac mullite gels

Mullite gels have been prepared using tetraethyl orthosilicate and aluminium tri-sec-butoxide chemically modified by ethylacetoacetate as precursors. The hydrolysis conditions are shown to have a very noticeable influence on the evolution of both phase and microstructure of the resulting xerogels. The optimization of thermal treatment allows well crystallized mullite samples with a density more than 90% of the theoretical value after sintering at 1200°C for 4 hours to be obtained.     

Synthetic approach to potential precursors of sclerophytin A

A direct approach to simple bicyclic analogues of the antitumor natural diterpene, sclerophytin A, is described. The readily available bicyclic ketone 8 (prepared from furan and trichloroacetone) was enantioselectively methylated to give the optically active ketone 11. Regioselective allylation using Negishi's method afforded the α,α-dialkyl ketone 12, which was converted to the chloroacetate 15 by hydroboration-oxidation and protection. Regioselective Baeyer-Villiger oxidation afforded the lactone 7a which could be transformed into the silyl ether 7b. Tebbe olefination furnished a mixture of two enol ethers in which the desired product 17 was the minor isomer. Several attempts to use the major endocyclic enol ether 18 to give the tricyclic analogues of sclerophytin proved unsuccessful. Opening of the lactone of 18 and selective protection of the diol afforded the primary alcohol 24 which was oxidized to the keto aldehyde 25. Unfortunately pinacol coupling of 25 did not give any cyclic product. The diene 27 was also prepared from 25 but all attempts at ring-closing metathesis of 27 met with the same fate. The failure of these various cyclization methods underscores the difficulty in forming medium-sized ring systems, especially those cis-fused at the 2- and 5-positions of a tetrahydrofuran ring.     

Thermal behaviour of sericite clays as precursors of mullite materials

Thermal analysis of some sericite clays, from several deposits in Spain, which are not exploited at this time, has been studied. The samples have been previously characterized by mineralogical and chemical analysis. Sericite clays have interesting properties, with implications in ceramics and advanced materials, in particular concerning the formation of mullite by heating. According to this investigation by differential thermal and thermogravimetric analysis (DTA-TG), the sericite clay samples can be classified as: Group (I), sericite–kaolinite clays, with high or medium sericite content, characterized by an endothermic DTA peak of dehydroxylation of kaolinite with mass loss, which overlapped with dehydroxylation of sericite, and Group (II), sericite–kaolinite–pyrophyllite clays, with broader endothermic DTA peaks, in which kaolinite is dehydroxylated first and later sericite and pyrophyllite with the main mass loss, appearing the peaks overlapped. X-ray diffraction analysis of the heated sericite clay samples evidenced the decomposition of dehydroxylated sericite and its disappearance at 1050 °C, with formation of mullite, the progressive disappearance of quartz and the formation of amorphous glassy phase. The vitrification temperature is ~ 1250 °C in all these samples, with slight variations in the temperatures of maximum apparent density (2.41–2.52 g mL^?1) in the range 1200–1300 °C. The fine-grained sericite content and the presence of some mineralogical components contribute to the formation of mullite and the increase in the glassy phase by heating. Mullite is the only crystalline phase detected at 1400 °C with good crystallinity. SEM revealed the dense network of rod-shaped and elongated needle-like mullite crystals in the thermally treated samples. These characteristics are advantageous when sericite clays are applied as ceramic raw materials.     

Formation of sol-gel nanostructured mullite by additions of fluoride ion

Summary The subject of this study was to investigate the effect of fluoride ions addition on the temperature of sol gel mullite formation based on the hypotheses that the presence of fluoride ions can decrease the temperature of mullite formation (in respect to common 980°C, in sol-gel processing). Polymeric sols were prepared by mixing TEOS and aluminum nitrate nanohydrate and by adding fluoride ions (from 2 to 5 mass%). DTA, TG, XRD and SEM were used for characterisation of mullite gel and crystalline mullite. The experimental results confirmed that the addition of fluoride ions decrease the temperature of mullite formation up to 890°C for the fluorine concentration of 3.5 mass%. Experimental results showed that the temperature of mullite formation is not a simple function of the fluoride ion content. The mechanism of fluorine effect was discussed in terms of the gelling process, gel structure and the phase separation before the mullite formation.     

Thermal properties of fluoride glasses and their gel precursors

Xerogels were prepared from zirconium, barium, aluminum, lanthanum and lithium acetates, corresponding to a Li containing ZBLA composition. The study of their thermal properties (DSC, TG/DTG, FT-IR) showed that they might be used as chemically stable precursors in the preparation of fluoride glasses. Hydrofluoric acid in solution was chosen as a mild fluorinating agent. This newly proposed technique of fluorinating allowed to obtain high quality ZBLALi glass which presents the advantage of higher thermal stability and homogeneity in comparison with the glass obtained using individual commercial fluorides.This revised version was published online in November 2005 with corrections to the Cover Date.     

Synthesis of modified vitamin D precursors

Modified vitamin D precursors with oxygen-containing substituents at various positions in the side-chain were obtained using previously synthesized 20-isoxazolin-3-yl-and 20-isoxazolin-5-ylsteroids.Institute of Bioorganic Chemistry, National Academy of Sciences of Belarus, 220141 Minsk, Belarus;Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 230–235, February, 2000.     

Development of mullite and mullite/Al2O3 precursor sols for electrophoretic deposition of oxidation protection coatings

Mullite and mullite/Al₂O₃ precursor sols have been developed for the deposition of oxidation barrier coatings on carbon fibre reinforced composites using a combination of sol–gel synthesis and electrophoresis.The sols were synthesised by controlled hydrolysis and condensation of TEOS (tetraethoxysilane) and Al(OBu^s)₃ (aluminium tri-sec-butylate). The main objective was the definition of synthesis conditions which yield sols suitable for the electrophoretic deposition (EPD). Measurements of the Electrokinetic Sonic Amplitude were used to investigate the electrokinetic properties of the sols in the as-prepared state and depending on the later addition of H₂O. ²⁹Si CP/MAS NMR spectra of dried precursor samples were recorded to study the homogeneity of Al/Si mixture. The progress of crystallisation with increasing temperature of heat treatment was examined by XRD. Oxidation protection coatings on C/C–SiC composites were prepared by EPD.Whereas a low H₂O to TEOS ratio during the sol synthesis was advantageous for a low mullite formation temperature, a high H₂O to TEOS ratio was necessary to enable the EPD. The synthesis of a sol with a low H₂O to TEOS ratio in the first step and the later modification of this sol by the addition of water was a successful method to combine the required electrokinetic properties and mullitisation temperatures below 1200 °C.     

Effects of metal salts on the thermal decomposition of edta-gel precursors for ferroelectric ceramic powders

Raw chemicals such as metal nitrates and chlorides were found to affect the thermal decomposition behaviour of EDTA-gel precursors used for the production of ceramic powders. Fine, homogeneous ceramic powders were produced from nitrate solutions while chlorides gave segregated phases. In studies on the production of lead zirconate titanate (PZT) using chlorides, the segregation and loss of lead was observed and shown to be caused by the formation and evaporation of PbCl₂. Thermal analysis (DTA/TG) quantitatively proved the suggested reaction mechanism for this phase segregation. Crystallization of the desired perovskite phase of lead zirconate titanate (PZT) and barium titanate (BT) initiated at temperatures as low as 250°C in the nitrate-EDTA precursors. Water of crystallization and formation of BaCO₃ in the barium titanate precursor were suggested to account for differences in the observed decompositional behaviours of the BT and PZT precursors.     

Short and efficient preparations of isoxazole-3-carboxylic acid and imino-oxopentanoic acid potent precursors of 4-hydroxyisoleucine

Herein, we describe short and efficient syntheses of isoxazole-3-carboxylic acid 3a and imino-oxopentanoic acid 8 achiral precursors of 4-hydroxyisoleucine. The developed procedures involve readily available and cheap starting materials and can easily be transposed to the large scale.     

Synthesis and characterization of CuInS2 single source precursors for chemical vapor deposition

A family of single source precursors, for the spray chemical vapor deposition (CVD) of chalcopyrite thin films (CuInS₂), has been synthesized in good yields (ca. 65%). Newly synthesized compounds include [{L}₂Cu(SR)₂In(SR)₂], (R=alkyl, aryl; L=neutral donor ligand). The use of the single source precursors provides an attractive alternative over conventionally used multi-source precursors, which are often toxic, air sensitive and pyrophoric. However, it is desirable that these thin films be processed on flexible polymer substrates such as Kapton^TM. Therefore, milder deposition temperatures are needed to maintain the structural integrity of the underlying polymer substrates. By selective manipulation of the steric and electronic properties of the precursor, milder processing temperatures may be employed, while mainting the desired stoichiometry of the deposited films. Elucidation of the structures have been confirmed by the use of NMR. Thermal analytical techniques, differential scanning calorimetry (DSC) and thermogravimetric analysis (TG), have been employed to determine thermal profiles of each candidate compound. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cyclodextrin inclusion complexes as novel MOCVD precursors for potential cobalt oxide deposition

The potential use of the inclusion complexes of β‐cyclodextrins with metal halides as novel precursors in MOCVD applications was examined in terms of microstructure, thermal stability and chemical modifications during heating. The investigation was especially focused on the inclusion complex of β‐cyclodextrin with cobalt iodide for cobalt oxide thin film deposition. The general composition assigned to the dextrin's inclusion complex was: (β‐CD)₂^?CoI₇^?11H₂O. It was found that the inclusion complex of β‐cyclodextrin with CoI₂ may prove a promising alternative to traditional metalorganic or organometallic Co‐precursors for precise CVD applications. The sublimation temperature must be preferably in the range 70–125 °C, and the decomposition temperature (substrate temperature) in the range of 350–400 °C. Three distinct regions can be recognized by heating: transformation of tightly bound water molecules into easily movable ones, sublimation of iodine ions and Co atoms oscillation and thermal decomposition of the glycositic ring into volatile by‐products. Copyright © 2009 John Wiley & Sons, Ltd.     

某些分析化学实验教材中均相沉淀概念的误区

目前,均相沉淀法在大学分析化学实验中已很少采用。高锰酸钾间接滴定法测定钙含量是大学分析化学实验中的经典实验之一,历史上存在均相沉淀法和传统沉淀法2种不同的沉淀操作。虽然我国现有大学分析化学实验中已不采用均相沉淀法进行该实验,但在某些分析化学实验教材中,却存在概念混乱的现象,即把传统沉淀法称作均相沉淀法。对这种现象进行了探讨和总结。     

Practical synthesis of bis-substituted tetrazines with two pendant 2-pyrrolyl or 2-thienyl groups, precursors of new conjugated polymers

Novel linear oligoheterocycles based on substituted tetrazines are described. The desired compounds have been accomplished by a variation of the original Pinner [Ann. Chem., 297 (1897) 221] synthesis in which the aromatic nitrile reacted with hydrazine in an aqueous solution to give bis(pyrrolyl)tetrazines or bis(phenyl)tetrazines. The bis(phenyl)tetrazines reacted with 3,4-ethylenedioxy-2-(trimethyltin)thiophene or 2-(trimethyltin)thiophene in the presence of Pd(PPh₃)₂Cl₂ or Pd(PPh₃)₄ as catalyst to give the desired compounds. Quantum-chemical calculations were performed to assess the usefulness of the synthesized compounds for electropolymerization. Studies have indicated qualitative difference between bis-pyrrole tetrazine and bis-phenyl tetrazines regarding the electronic density rearrangement due to the loss of an electron.     

均匀沉淀法制备氧化锌纳米棒

采用均匀沉淀法制备了氧化锌纳米棒,用XRD,TEM,PL等检测手段对样品进行了表征.结果表明:所得样品为长约100 nm,宽约30 nm的纤锌矿结构氧化锌纳米棒,颗粒分布均匀.其在可见光区比紫外区的荧光发射显著增强.     

热解单源前驱体方法可控制备纳米金属硫化物

本文简要综述了本研究组近年来在利用热解单源前驱体合成方法制备纳米金属硫化物方面的相关工作．通过采用不同的反应前驱体,改变反应时间、反应温度,选择不同的表面配体分子及反应溶剂等手段实现了对纳米金属硫化物形貌、尺寸、组成和晶相的精确调控．除了对合成方法和过程进行介绍外,本文还简要讨论了具有特定形貌的金属硫化物的形成机理,并对几类典型硫化物的应用研究进行了总结．     

Homogeneous precipitation of basic aluminium benzoate with a buffer solution

Addition of an ammonium benzoate-benzoic acid buffer solution to a solution of aluminium ions gives a homogeneous precipitation of basic aluminium benzoate. This has advantages over the urea method for the gravimetric determination of aluminium in the presence of large quantities of manganese, calcium, cadmium, nickel, cobalt, copper, and zinc ions.     

均匀沉淀法制备一维结构的氧化锌

以硝酸锌和尿素为反应原料,利用均匀沉淀法在水溶液中直接制备了多种形貌的氧化锌．考察了反应物浓度,加热方式等对产物形貌和结构的影响,利用透射电子显微镜,X射线衍射仪等分析手段对所得产物的形貌和结构进行了表征．     

Precipitation Under Homogeneous or Heterogeneous Conditions by Photochemical Action

Abstract

Precipitation reagents may be formed in situ by photochemistry but the formation cannot be completely homogeneous due to the exponential nature of light absorption (Beer's Law). By proper adjustment of concentrations, one may vary the conditions from near homogeneous to heterogeneous formation of photoprecipitate.