Effect of polymerization catalysts on the microstructure of P(LLA-co-εCL)

The copolymerization of L ,L -lactide and ε-caprolactone was carried out using antimony trioxide and stannous octoate as catalysts. The effect of polymerization catalysts on the physical and the chemical microstructures of this copolymer was investigated by ¹³C NMR and DSC analysis. Antimony trioxide causes more random sequence distribution within the copolymer chain due to its higher transesterification characteristic than stannous octoate. The copolymer samples made with the antimony trioxide catalyst seem to have more amorphous phase structure, than those prepared using stannous octoate which are semicrystalline for the entire compositional range due to blocky copolymer sequences. © 1994 John Wiley & Sons, Inc.     

Study of dielectric and ac impedance properties of citrate-gel synthesized Li0.35Zn0.3Fe2.35O4 ferrite

Nano-crystals of Li_0.35Zn_0.3Fe_2.35O₄ ferrite have been synthesized using citrate precursor method. The sample synthesized was sintered at different temperatures in order to vary their crystallite size. The average crystallite size was found in the range 24?C57?nm by varying the temperature from 300 to 1,100?°C. X-ray diffraction measurements confirmed the formation of cubic spinel structure at all the sintering temperatures in this work. The high frequency performance of the ferrite samples were estimated by measuring the frequency dispersion of the dielectric constant, dielectric loss and ac electrical conductivity. The dielectric constant has been observed to show normal behavior with frequency and decreases with the decrease in crystallite size. It is also observed that decrease in dielectric constant depends on sintering temperature because of lithium evaporation at higher temperature. A low value of dielectric constant and dielectric loss has been found, which makes them applicable for high frequency applications by decreasing the skin effect. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of Li_0.35Zn_0.3Fe_2.35O₄ ferrite. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the prepared samples.     

Highly stable dielectric frequency response of chemically synthesized Mn-substituted ZnFe2O4

The tenacity of the present study was to develop a material using an economical chemical route, having balance between magnetic and dielectric order parameters for maximum transmittance of electromagnetic waves in order to use them in shielding materials. In this context, Mn-Zn ferrites were prepared using a wet chemical based sol-gel auto-combustion technique. X-ray diffraction confirmed the pure phase formation of samples, while some impurity peaks were also present for the higher value of Mn substitution. Field emission scanning electron microscopy revealed a decrease in grain size with increasing Mn substitution. While energy dispersive X-ray spectroscopy confirmed the elemental composition of pure and Mn substituted samples, the dielectric constant, dielectric loss and tangent loss were decreased with increasing frequency and increasing Mn substitution. The complex electric modulus was found to be a function of frequency and values of complex electric modulus were increased with increasing frequency and Mn substitution. The complex impedance of RC series circuit and RC parallel circuit was also decreased with increasing both the parameters while AC conductivity was increased in the series. Dielectric frequency response was also studied for the prepared samples and the best match was found with expected results. The Nyquist and Cole-Cole plots revealed the semi-conductive behavior at higher frequency and Mn substitution also yielded the same results. The relative stability of the samples to be used as dielectric materials was also studied using Bode and Nichols plots, and a comparatively high gain margin was observed, well suitable for potential applications in electromagnetic shielding.     

Evaluation of the (Allyl alcohol 1,2‐butoxylate‐block‐etoxylate)‐b‐PMMA Copolymers Composition by the Dielectric Measurements

Redox initiated free‐radical polymerization of methyl methacrylate (MMA) with allyl alcohol 1,2‐butoxylate‐block‐etoxylate (AABE) was carried out to yield AABE‐b‐PMMA copolymers at elevated temperatures. The composition of the copolymers depending on the polymerization temperature was qualitatively estimated by the dielectric measurements. It has been seen that AABE segment quantity decreased and PMMA segment quantity increased with increasing the polymerization temperature. The dielectric constant and the dissipation factor of the copolymers were investigated as a function of frequency and temperature. The dielectric constant and the dissipation factor were found to be strongly affected by the polymerization temperature. The highest dielectric constant in all studied temperatures and frequencies was obtained in the case of the copolymer which was prepared at 313 K. The dipolar C‐O and OH groups of the AABE segment have the primary effect on the dielectric constant. The copolymer which was prepared at 323 K, showed the highest dissipation factor near the relaxation temperature of PMMA.     

A dielectric study of the amorphous fraction in oriented nylon 610

Since some dielectric loss peaks can be assigned to amorphous phase motions, it would appear that dielectric measurements should be a good probe to study the effects of macroscopic plastic deformation on the structure of the amorphous phase. We report here results of measurements of dielectric constant and loss in nylon 610 measured both parallel (∥) to and perpendicular (⊥) to the extrusion direction in specimens prepared by extrusion through a die at 160°. It is found that the intensity of the (∥) loss was nearly zero and the (⊥) loss intensity was greatly reduced compared to unoriented material. The other relaxation parameters (loss width and location) were largely unaffected. Since the amide dipole is nearly perpendicular to the chain axis and alternates in direction in the extended conformation, a considerable degree of chain alignment parallel to the draw direction is indicated. These results therefore show considerable permanent amorphous phase orientation even though the deformation was carried out 100° above its glass temperature.     

锶、铈和镧共掺杂铈钛酸锶钡陶瓷的固溶性和介电性能的研究

采用冷压陶瓷技术,制备了(Ba1-x-0.02SrxLa0.02)(Ti1-yCey)O3(x=0.05,0.10,0.15;y=0.03,0.04,0.05)(BSLTC)陶瓷,通过X射线衍射(XRD)和介电温谱测试对BSLTC陶瓷的固溶性和介电性能进行调查。XRD结果表明,所有样品均为立方钙钛矿结构,La掺杂量为2%时,能够将(Sr,Ce)组合的固溶提高到(x=0.10,y=0.04)或(x=0.15,y=0.03)。介电结果表明,各样品的居里温度都在室温附近,介电峰明显宽化。其中,(x=0.05,y=0.03)组合的样品显示出最高的室温介电常数(ε’RT≈6000)和低介电损耗(tanδ<0.02),满足美国EIA"Y5V"高介电陶瓷材料指标。     

Time evolution of dielectric parameters during the crosslink of epoxy resins

The d.c. conductivity and the dielectric constant of an epoxy resin cured with a diamine were examined in a frequency interval extended up to 10¹⁰ Hz. The analysis of the dielectric behavior has permitted gaining a better knowledge of the relationships between dielectric parameters and the physical and chemical modifications of the systems. The results indicate that the chemical kinetics of the crosslink process is closely paralleled by the change in time of the dielectric parameters so that dielectrometry provides valuable information on conversion, relaxation times, glass transition temperature and gelation.     

新铌酸盐Sr5NdTi3Nb7O30的合成与介电特性

一些铁电铌酸盐具有优良的电光性能和非线性光学性能,因此该类化合物的人工合成、结构与性能的研究受到了重视,其中钨青铜结构的系列晶体(例如SBN、KNSBN、SCNN)在材料的制备和器件的设计方面都取得了很大进展,它们在实时全息存储、集成     

Thermal and electrical behaviors of acid functionalized MWCNT/ABC-type triblock copolymer grafted-MWCNT nanocomposites

In this work, ABC-type triblock copolymer grafted onto the surface of the MWCNT/acid functionalized MWCNT (MWCNT-COOH) composites were prepared and the properties of nanocomposites were characterized extensively using differential scanning calorimetry (DSC), scanning electronic microscopy (SEM), thermogravimetric analysis (TGA), ac electrical conductivity and dielectrical measurements.

DSC study showed that the glass transition temperatures of the nanocomposites are a some higher than that of the matrix polymer. The increase in oxidized MWCNT in the nanocomposite improved the thermal stability of the composite, according to initial decomposition temperatures. The ac electrical conductivity has increased moderately with increasing frequency, but has increased slowly with increase in the oxidized MWCNT content in the nanocomposites. The electrical conductivity increases slowly with increasing temperature to about the glass transition temperature, then it increases faster. The dielectric constants for the matrix polymer and all the composites decreases slightly with increasing frequency from 0.1 kHz to 2.0 kHz. The dielectric constant increases slightly with increasing temperature up to about the glass transition temperature region and then the increase in temperature is accelerated the increase in the dielectric constant.     

Polycondensation rate of poly(ethylene terephthalate). I. Polycondensation catalyzed by antimony trioxide in presence of reserve reaction

The technique of G. Challa was employed for following the reaction of oligomeric poly(ethylene terephthalate) in the presence of antimony trioxide. This technique is based on determining the changes in glycol and bishydroxyethyl terephthalate concentrations with time at a given temperature. The linear trimer of ethylene terephthalate, prepared from terephthaloyl chloride and bishydroxyethyl terephthalate, was employed as starting material. Rate constants were not corrected for catalyst concentrations. The reaction was studied over the temperature range 221–251°C. The equilibrium constant k_p/k′ for the polycondensation reaction was found to be 0.36. Observed reaction rates were low but faster than in the uncatalyzed reaction studied by Challa. At 231°C. the values of the rate constants found in the presence of 0.025% by weight antimony trioxide were k_p = 0.151.mole^?1 hr.^?1 and k′ = 0.33 l./mole-hr., and the redistribution rate constant was k_R = 0.11 l./mole-hr. From data at four temperatures the estimated activation energies were E_p = 29 kcal. and E_R = 24 kcal. The polycondensation rates were low compared with rates calculated from literature data. A mechanism to explain the difference requires that bishydroxyethyl terephthalate endgroups compete successfully with oligomer endgroups for antimony trioxide catalyst and that the bishydroxyethyl terephthalate catalyst product is unreactive in polymerization.     

Effect of ZnNiCrFe<Subscript>2</Subscript>O<Subscript>4</Subscript> content on structural and dielectric properties of polyaniline based nanocomposites,synthesized via in-situ chemical polymerization

Zn_0.5Ni_0.4Cr_0.1Fe₂O₄ nanopowder and its composite with polyaniline were successfully prepared by using wow sol-gel and in situ chemical polymerization respectively. The samples were characterized by X-ray diffraction and field emission scanning electron microscopy. Dielectric properties were investigated as function of frequency by using impedance analyzer. The results showed the presence of the two intended phases. The ac conductivity was found to obey Jonscher’s universal power law. The dielectric constant and loss showed dispersion in low frequency region. Impedance analysis revealed the semiconducting behavior of the investigated samples.     

Study of structural,electrical and dielectric behavior of cadmium selenide quantum dots/polyaniline nanocomposites

In this study, the cadmium selenide quantum dots/polyaniline nanocomposites were prepared by in-situ polymerization technique and they were characterized using X-ray diffraction and Fourier transform infrared spectroscopy. The surface morphology was studied by scanning electron microscopy. The electrical and dielectric properties were investigated by using 4-probe mechanism and LCR meter respectively. The increase in dielectric constant was observed with the increase of quantum dots content in the nanocomposites. The frequency dependent AC conductivity followed the universal power law. The temperature dependent DC conductivity was found to be a typical for a semiconducting behavior following Mott’s 1D variable range hoping model.     

新铌酸盐Ba5LnZnNb9O30(Ln=Y,La)的合成、结构与介电性能

在BaO-Ln2O3-ZnO-Nb2O5(Ln=Y,La)体系中通过固相反应法合成了填满型钨青铜结构的新铌酸盐Ba5YZnNb9O30与Ba5LaZnNb9O30.采用X射线衍射分析和扫描电镜进行了结构分析,并进行了介电性能测试.结果表明,Ba5YZnNb9O30为弛豫性铁电体,10kHz时的居里温度为25℃;室温时为四方钨青铜结构铁电相,晶胞参数a=1.25255(4)nm,c=0.39530(2)nm;1MHz时陶瓷体的室温相对介电常数为456.Ba5YZnNb9O30在室温下为四方钨青铜结构顺电相,晶胞参数a=1.25731(3)nm,c=0.39812(2)nm;频率为1MHz时,其陶瓷的室温相对介电常数为316.     

A Synthetic Strategy of Ultrathin High-κ Antimony Oxide Single Crystals

Ultrathin antimony oxide single crystals with high dielectric constant and large breakdown electric field were synthesized via an expressly substrate-buffer-controlled chemical vapor deposition strategy. This strategy is also versatile for the synthesis of other ultrathin oxides.     

新铌酸盐Sr5LnTi3Nb7O30的结构与介电性能

在SrO-Ln2O3-TiO2-Nb2O5(Ln=La, Y)体系中,通过固相反应法，合成了填满型钨青铜结构新铌酸盐Sr5LaTi3Nb7O30与Sr5YTi3Nb7O30.分别采用X射线衍射分析、扫描电镜进行了结构分析,并进行了介电性能测试.结果表明, Sr5LaTi3Nb7O30室温时为四方钨青铜结构顺电相,晶胞参数a=1.233 60(4) nm, c=0.388 01(2) nm;频率为1 MHz时,其陶瓷的室温相对介电常数为466,介电损耗约为5×10－3.Sr5YTi3Nb7O30为弛豫性铁电体, 10 kHz时居里温度为260 ℃;室温时为四方钨青铜结构铁电相,晶胞参数a=1.228 80(4) nm, c=0.387 05(2) nm; 1 MHz时,陶瓷体的室温相对介电常数为290.     

巯基乙酸锑(Ⅲ)配合物合成与晶体结构

以三氧化二锑和巯基乙酸在水溶液中反应合成了配合物HSb(SCH2COO)2,并通过元素分析、红外光谱、X射线粉末衍射进行了表征,利用单晶X射线四圆衍射法测定了晶体结构,结果表明该配合物晶体属于单斜晶系,C2/c空间群,晶胞参数为:a=1.40005(8)nm,b=1.19121(8)nm,c=1.23588(8)nm,β=126.822(1)°,V=1.6499(2)nm^3,Dc=2.439g·cm^-3,Z=8,R1=0.0250。并对X射线粉末衍射数据进行了指标化,其结果与单晶数据吻合。     

Barium strontium titanate-based perovskite materials for microwave applications

Ba_1−xSr_xTiO₃ solid solutions were prepared by solid-state reaction from raw materials. Four compositions with x = 0.25, 0.50, 0.75 and 0.90 have been investigated. The perovskite type and polycrystalline structure of the BST samples were revealed by X-ray diffraction data. The morphology, grain size distribution, porous structure and elemental composition of the sintered ceramics were analyzed by using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) microanalysis. The temperature dependence of permittivity and of dielectric loss tangent at low frequency (1 kHz) showed decrease of Curie temperature with increase of Sr content. Microwave measurements (1 GHz) showed substantial decrease of the dielectric constant from about 1600 to 200 and also of the losses from 12% to less than 2% with the Sr concentration increase from x = 0.25 to x = 0.90. Moreover, the addition of MgO and MnO₂ 1 wt.% each, improved sintering process and lowered the microwave losses up to 0.2%.     

弛豫基铁电单晶0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3的制备与 性能研究

用改进的Bridgman法,在加入助熔剂的条件下,生长出最大尺寸为φ30×25mm^3的铌锌酸铅-钛酸铅固溶体单晶[0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3]。无宏观缺陷的晶片的典型尺寸为20×15mm^2。晶体的单晶性及其结构用X射线衍射法加以研究。所得晶体用Laue衍射法定向,取(001)晶片进行性能表征。研究了材料的介电性能,并用偏光显微镜观察了(001)晶片的电畴结构。结果表明,室温下材料的介电常数较大,为2500～5000。随着温度的升高,材料发生四方铁电相—立方顺电相的相变,相变温度为190℃左右。单晶的介电温谱呈现明显的频率色钐现象,同时,介电常数最大值的温度tm随着频率的升高而降低,观察到了电畴结构的不均匀与孪生现象。     

不同制备条件对锑钨改性钒基中低温SCR催化剂性能的影响

结合锑较优的抗水抗硫能力与钨提高钒基催化剂活性的能力,采用浸渍法制备了以锑-钨为双助剂的V-W-Sb/Ti催化剂,并探究了不同制备条件对改性催化剂脱硝性能的影响。催化剂脱硝活性测试及抗水抗硫实验在固定床反应器中进行,并利用氮气物理吸附-脱附测试、X射线衍射、NH_3-TPD测试和H_2-TPR测试对催化剂进行表征。结果表明,在洁净气氛下,以3V_2O_5-5WO_3-2Sb_2O_3/90TiO_2为例,采用醋酸锑为前驱物制备的催化剂脱硝效率高于以氯化锑为前驱物制备的催化剂; 400℃下焙烧制备的催化剂较500℃焙烧制备的催化剂有更优的脱硝活性;而浸渍步数的差异对催化剂的脱硝活性影响有限。在180℃的测试条件下,通入10%H_2O与0.01%SO_2后,以醋酸锑为前驱物、采用两步浸渍并在400℃下焙烧制备的催化剂的脱硝活性仅比以氯化锑为前驱物、采用一步浸渍并在400℃下焙烧制备的催化剂脱硝活性高2%,而后者制备过程相对简单方便,因此,其更具有工业应用价值。     

Synthesis and dielectric properties of the Aurivillius oxide BaBi4Ti4O15 by the Pechini method

Barium bismuth titanate (BaBi₄Ti₄O₁₅, BBT) powders were synthesized using the Pechini method, and related ceramics were prepared using a solid-state reaction method. The structure and morphology of the BBT specimens were examined by the X-ray diffraction and scanning electron microscopy. Complexation of citric acid with metallic cations was detected by Fourier-transform infrared spectroscopy. Thermal analyses of the obtained gels were performed using differential thermal gravimetry. Pure BBT powders could be obtained at a relatively low temperature of 500 °C. A broad dielectric peak around Curie temperature and significant frequency diffusion of the maximum dielectric permittivity were observed. The diffuseness parameter was 1.82, and relaxation was attributed to cationic disordering at the A-site of BBT.