Synthesis of Silicon Nitride by a Combustion Reaction under High Nitrogen Pressure

Fine Si₃N₄ powders were prepared by the combustion reaction of an Si powder compact undez 10 MPa nitrogen pressure. Addition of Si₃N₄ powder to the starting Si promoted conversion of the reactants to homogeneous Si₃N₄ particles. Submicrometer SisN₄powders with a uniform size distribution around 0.5 μm were obtained from a 1.8Si-0.4Si₃N₄ mixture (molar ratio); they were free of residual Si.     

Growth Mechanisms for SiC–AlN Solid Solution Crystals Prepared by Combustion Synthesis

AlN–SiC solid solution particles with a variety of morphologies including faceted polyhedrons with or without ledges; hexagonal platelets; hexagonal columns with a hexagonal plate or a pyramidal cap; and interpenetrating cones, have been found in the combustion products of a mixture of Al, Si, and carbon black under a nitrogen pressure of 10 MPa. Combustion temperature (the growth temperature of crystals) is the most important factor controlling the morphology of crystals formed in the combustion product. When temperatures are close to the melting point of the solid solutions, a small driving force for nucleation and long distances of surface migration make nucleation on the basal plane difficult, and thus the solid solution particles tend to grow as platelets. Supersaturation is the second key factor influencing crystal growth. At relatively low temperatures, a low supersaturation at the large pores renders nucleation difficult and the solid solution particles tend to grow as platelets. At relatively low temperatures and high supersaturation, a relatively high driving force for nucleation and short mean distances of surface migration promote the growth of AlN–SiC solid solutions as polyhedrons. The formation of the ledges on the polyhedral particles is attributed to the differences in the evaporation rates and the deposition rates between Al and Si. At relatively low temperatures and an intermediate supersaturation, the solid solution particles grow as prismatic columns. The formation of the prismatic columns with a hexagonal plate, or a pyramidal cap, is attributed to a sudden change of temperature during combustion. A possible growth mechanism for the AlN–SiC solid solution cones is proposed.     

Growth of Quasi-Aligned AlN Nanofibers by Nitriding Combustion Synthesis

Quasi-aligned AlN nanofibers were formed by the nitriding combustion synthesis according to a unique micro-reactor model. A charge composed of aluminum and aluminum nitride diluent powders (40/60 mol%) with a mixture of yttria and ammonium chloride as additives (5 wt% each) was combusted at low nitrogen gas pressures of 0.25 MPa. The FE-SEM images of as-synthesized AlN product showed the formation of ball-like grains (same shape and size as the original Al reactant) that consisted of a thin surface nitride layer or crust cover quasi-aligned AlN nanofibers grown in the interior. The cross-sectional view is sea anemone like. Formation of this novel morphology is believed to occur through a two-stage process. The first one occurs at the preliminary stage of the combustion outside Al particles. After the ignition, the heat generated causes the sublimation and dissociation of ammonium chloride into various gaseous species. This effectively interrupts the combustion and slows down the increase of reaction temperature. In addition, yttria interacts with the native oxide layer present on the surface of Al particles and forms a stable Al–N–Y–O crust. The second stage begins by the infiltration of various gaseous species such as HCl_(g), NH_3(g), and N_2(g) through the crust into the molten Al cores. The "crust–core" systems function as "micro-reactors" where both the nitridation and growth processes occur inside. The molten Al cores are spontaneously halogenated to AlCl₃ vapors and the nitridation proceeds by the gas–gas reaction of AlCl₃ and NH₃/N₂ vapors. The AlN nanofibers are then grown from the vapor phase quasi-aligned inside the micro-reactors by VLS and VS mechanisms.     

Preparation of Nanoparticles via Cellulose-Assisted Combustion Synthesis

Cellulose-assisted combustion synthesis is a simple and effective continuous synthesis method that emerged almost a decade ago and now witnessing exponential growth in research interest and application. The simplicity of the process and easily scalable nature makes it appropriate for industrial scale synthesis of nanomaterials with high surface area and large porosity. Over the past few years, catalysis community has devoted considerable efforts in exploiting these properties borne naturally in combustion-synthesized materials such as complex surface structure and large porous volume providing suitable surface sites for catalytic reactions. In this review article, we plan to gather this development in the cellulose-assisted combustion synthesis and gain some insights related to possible future outlook and research needs.     

自蔓延燃烧法制备AlN陶瓷粉体的研究进展

概述了自蔓延燃烧法(Self-propagating high-temperature synthesis,简称SHS)制备氮化铝(A1N)粉体的原理及方法.重点综述了纳米A1N陶瓷粉体的最新制备工艺研究进展,并对SHS法制备A1N粉体的发展进行了展望.     

碳基固体酸催化Micheal加成反应的研究

碳基固体酸催化剂以其优良的催化活性和较强的选择性等优点,被广泛地应用到催化化学的各个领域,成为当前热点催化剂之一。通过水热碳化呋喃甲醛与羟乙基磺酸,合成了一类新型碳基固体酸材料,并以其为催化剂,进行Micheal加成反应。     

Humic Acids as a Complexible Fuel for Combustion Synthesis of Ceramic Nanoparticles

A novel combustion synthesis of ceramic powder has been developed using humic acids (HA) as the complexible fuel. Synthesis of mixed oxide (Sm_0.5Sr_0.5CoO₃ (SSC)) ceramic nanoparticles was chosen to illustrate synthetic procedures. SSC nanoparticles obtained by combustion of HA–metal complexes exhibited particle sizes of 50–100 nm, and good electrochemical performance as the cathode material for solid oxide fuel cells. The process of using renewable HA as the complexible fuel is of low cost, and thus suitable for sustainable production of various oxide ceramic materials.     

F掺杂SnO2纳米粉体的燃烧合成

以金属锡和HF为原料采用燃烧合成法合成得到了F掺杂SnO2纳米颗粒。燃烧反应中甘氨酸为燃料,硝酸为氧化剂。采用XRD,BET,TEM和EDS等分析手段对合成得到的FTO进行了表征。结果显示,F的掺杂促进了SnO2基体晶粒的长大,所得到的粉体团聚较少,呈单分散状态。     

Morphological Control of Zirconia Nanoparticles through Combustion Aerosol Synthesis

Ceramic oxide nanoparticles produced by flame-based processes are typically agglomerated, which can limit their use in some applications. In this paper, a novel combustion synthesis method that utilizes the spraying of combustible droplets into a premixed flame to produce nanoscale crystalline particles of agglomerated and unagglomerated morphologies is described. Although the same flame-based experimental setup is used in both cases, variation in peak flame temperatures results in a corresponding variation between fractallike agglomerates and single isolated spherical particles. TEM/ED analysis shows that both classes of particles are the tetragonal crystal phase of zirconia. In the case of the unagglomerated spherical particles, results indicate that each precursor solution droplet, which acts as the feed, produces multiple spherical ceramic nanoparticles with a number mean diameter of 90 nm. The use of an inertial impaction stage in the precursor feed line to eliminate large feed droplets leads to a decrease in the number mean diameter to 60 nm, suggesting that crystalline spherical nanoparticles can be produced in a continuous flame-based process through control of the feed droplet size.     

Combustion Synthesis of Silicon Carbide in Nitrogen Atmosphere

Fine SiC powders were synthesized by burning the mixed reactionts Si and C in a nitrogen atmosphere of 3 to 10 MPa. The exothermic synthesis reaction propagated spontaneously after igniting the reactants at room temperature. The SiC powders obtained had a uniform size distribution of about 0.2 μm. The combustion velocity was 0.8 to 1.5 mm / s. The maximum temperature measured at the reaction was 2500 K, which was higher than the adiabatic combustion temperature of SiC, but slightly lower than the decomposition temperature of Si₃N₄ under nitrogen pressure.     

Synthesis of Sinterable β-SiC Powders by a Solid Combustion Method

A solid combustion method for obtaining sinterable β-SiC powders from Si and C mixtures is described. The method is similar to the thermal explosion method in which a proper preheating of the reactive mixture enables a substantial increase in the temperature of the mixture, due to the heat of reaction, in a short time. The characteristics of the powders obtained by the method are given. Both a high chemical purity of the powders, which is due to the high temperature of synthesis, and an adequate BET surface area of the powders, obtained after vibratory milling, enable these powders to be used for producing sintered SiC materials.     

一种新型稀土纳米粒子的可控合成

纳米粒子的粒径与其应用联系紧密,研究发现六方相稀土纳米晶的上转换效率比立方相高,为可控合成一定粒径、六方相的稀土纳米粒子,文章采用溶剂热法,以Na YF4为基质,掺入一定比例在稀土元素Yb及Er,并改变反应温度、稀土掺杂量等影响因素,得出在300℃条件下,合成分散性好的六方相稀土纳米粒子。为稀土纳米粒子的可控合成提供依据,同时对研究稀土纳米材料的应用提供参考。     

Effect of Glucose on the Synthesis of Iron Carbide Nanoparticles from Combustion Synthesis Precursors

Iron carbide (Fe₃C) nanoparticles have been successfully synthesized by combining low‐temperature combustion synthesis method with carbothermal reduction. A homogeneous precursor powder (Fe₂O₃ + C) derived from iron nitrate, glycine, and glucose mixed solution was subsequently calcined under nitrogen at 450°C–700°C for 2 h. Effects of glucose on the size and morphology of the precursors as well as the synthesized Fe₃C powders were studied in details. The results showed a regular variation in the particle size and morphology of the precursors and Fe₃C powders with the increasing molar ratio of glucose to iron nitrate (G/Fe). XRD analysis indicated that the initial transformation of the precursor for (G/Fe = 1) to Fe₃C occurred at 500°C. Meanwhile, magnetic properties of the Fe₃C have been tested by vibrating sample magnetometer (VSM). The saturation magnetization (Ms) of Fe₃C powders synthesized using different G/Fe ratios (G/Fe = 1, 2, 3) was 51.2, 37.0, and 27.1 emu/g, respectively. This made the Fe₃C a promising candidate for magnetic materials.     

Two‐Step Carbothermal Synthesis of AlN–SiC Solid Solution Powder Using Combustion Synthesized Precursor

In the present work, a two‐step carbothermal reduction method is employed to prepare the AlN–SiC solid solution (AlN–SiCss) powders by using a combustion synthesized precursor. The precursor is prepared by low‐temperature combustion synthesis (LCS) method using a mixed solution of aluminum nitrate, silicic acid, polyacrylamide, glucose, and urea. The synthesized LCS precursor exhibits a porous and foamy uniform mixture of Al₂O₃ + SiO₂ + C consisting of flaky particles. The carbothermal reduction in the LCS precursor is carried out in two steps. First, the precursors are calcined at 1600°C in argon for 3 h. Subsequently, the precursors are further calcined at 1600°C–1900°C in nitrogen for 3 h. The results indicate that the precursor calcined at and above 1850°C in nitrogen for 3 h yields the single‐phase AlN–SiCss powders. The synthesized AlN–SiCss powder exhibits near‐spherical particles with diameter of 200–500 nm. The experimental and thermodynamical results reveal that the formation of AlN–SiCss occurs via the diffusion of AlN into SiC by virtue of formation of a highly defective β′ intermediate during the second step reaction.     

Nucleation with Simultaneous Chemical Reaction in the Vapor-Phase Synthesis of AlN Ultrafine Powders

Using the classical (one-component) homogeneous nucleation theory approach of a constrained equilibrium distribution of sub- critical particles, a nucleation equation that considers the effect of heterogeneous reaction is derived for the AlN system. In the absence of more detailed information about the aluminum nitride (AlN) reaction kinetics, a simplified surface reaction model is formulated. Clusters, which can be formed by AlN and pure Al, are assumed to grow from the condensation of Al vapor followed by surface reaction between condensed Al and absorbed NH₃. The composition of any subcritical particle is assumed to be determined by the rates at which Al and AlN species are added to their surface and thus made entirely dependent upon local gas concentrations. This assumption resulted in important simplifications that allowed this development to be cast in a form resembling the one used by the classical theory for one-species nucleation. An analysis intended to evaluate the dependence of this nucleation equation on temperature, concentration of reacting species, and degree of aluminum supersaturation is presented. Considerable changes of the nucleation rate were observed between low and high concentration ratio of reacting species. These changes are directly related to the changes of the surface energy and bulk free energy terms determined as a function of the free-Al content in a nucleus.     

反应萃取法生产氯化胆碱

利用萃取、反萃取相结合的技术,以乙烯次氯酸化的主产物氯乙醇为主要原料来生产氯化胆碱,并通过正交实验确定出最佳反应条件。结果表明,当反应温度为50℃,催化剂浓度为1％,三甲胺与氯乙醇的摩尔比为1．1：1,反应时间为60min时,氯化胆碱的收率可达到90％,并且可以解决乙烯次氯酸化反应中氯醇类物质分离困难、能耗高以及用石灰乳中和产物中的氯化氢时会产生大量废水、废渣等缺陷。     

固相法合成甘氨酸钙络合物

以甘氨酸和氢氧化钙为原料、用固相化学法合成了甘氨酸钙络合物,用正交实验选择了最佳合成条件。当投料量n(甘氨酸)∶n(氢氧化钙)=1 00∶0 75,反应温度90℃,反应时间3h时,产率为93 4%。用元素分析、红外光谱、热重-差热分析、X射线粉末衍射对络合物进行了表征,其组成为[Ca(NH2CH2COO)2]·H2O。     

萃取反应法合成二甲基乙醇胺

采用萃取反应法合成二甲基乙醇胺.实验过程中,首先通过测定分配系数和反应速率,验证了萃取技术的可行性;又利用正交实验探索出了反应的最佳工艺条件为:温度为60 ℃,水与萃取剂的质量比为1:1,反应时间为120 min,二甲胺与氯乙醇的物质的量的比为3.5:1,在此条件下产物收率可达95.71%.     

Synthesis of Titanate Derivatives Using Ion-Exchange Reaction

Two types of titanate derivatives, layered hydrous titanium dioxide (H₂Ti₄O₉· n H₂O) and potassium octatitanate (K₂Ti₈O₁₇) with a tunnellike structure, were synthesized using an ion-exchange reaction. Fibrous potassium tetratitanate (K₂Ti₄O₉· n H₂O) was prepared by calcination of a mixture of K₂CO₃ and TiO₂ with a molar ratio of 2.8 at 1050°C for 3 h, followed by boiling-water treatment of the calcined products for 10 h. The material then was transformed to layered H₂Ti₄O₉· n H₂O through an exchange of K⁺ ions with H⁺ ions using HCl. K₂Ti₈O₁₇ was formed by a thermal treatment of KHTi₄O₉· n H₂O. Pure KHTi₄O₉· n H₂O phase was effectively produced by a treatment of K₂Ti₄O₉ with 0.005 M HCl solution for 30 min. Thermal treatment at 250°–500°C for 3 h resulted in formation of only K₂Ti₈O₁₇.     

Combustion Synthesis of AlON–BN Composites under Low Nitrogen Pressure

Hexagonal boron nitride (hBN) and aluminum oxinitride (AlON) composites were synthesized by combustion reaction of powder mixtures of Al–B₂O₃–AlN systems under a low pressure of nitrogen gas (0.5 MPa). Explosive combustion reaction of Al–B₂O₃ systems under the same nitrogen pressure produced alumina, aluminum borate, AlN, and AlON depending on the binary mixing ratio, but no trace of BN phases could be identified. Most of the elemental boron product remained unreacted and amorphous. On the other hand, AlN addition as a diluent in the range of 15–30 wt% was effective in producing hBN phase and forming AlON–BN composites. In the composition range of the ternary mixture of Al, B₂O₃, and AlN, where significant BN formation was identified, the primary role of AlN was to react with B₂O₃ to produce BN and α-Al₂O₃. The temperature profile obtained during the combustion reaction by a thermocouple imbedded in the middle of the powder bed revealed that the initial nitridation reaction of aluminum metal provides the heat required for the combustion reaction, creating a state of a "chemical oven." The reaction product, α-Al₂O₃, reacted subsequently with AlN to produce AlON phases to give final AlON–BN composites. The combustion reaction was highly unstable and followed a mixed mode with a regularly reversing spinning mode for aluminum nitridation reaction in the surface region and an oscillatory mode for the BN formation reaction in the subsurface region.