Preparation of coated sand for selective laser sintering and optimization of baking process of sand moulds

A cold method was used to prepare coated sand for application in the selective laser sintering(SLS)process.Tensile strength,loss on ignition,gas evolution,and accuracy of the SLS samples were tested and analyzed,and the baking process was thoroughly investigated.Compared with coated sand prepared by the hot method,the cold method yields a more uniform and complete resin film on the sand's surface,resulting in enhanced tensile strength and accuracy.Additionally,the cold method requires a lower binder content to meet the same strength requirements,thereby minimizing gas evolution,reducing porosity defects,and ultimately improving casting quality.The coated sand samples prepared through the cold method exhibit superior accuracy,with a size error of within±0.4 mm.In contrast,the coated sand samples prepared by the hot method display a lower accuracy,with an average negative error of 2.1993 mm.The highest tensile strength could be attained by controlling the baking temperature within a suitable range(180-190°C),which can effectively reduce the generation of gas,thus contributing to improved overall performance.

Sintering Densification of Boron Carbide Materials and Their Research Progress

Boron carbide(B_(4)C)has excellent high-temperature oxidation resistance,high hardness,low relative density,high melting point and excellent abrasive resistance,which is widely used in fields such as refractories,wear-resistant materials and lightweight protective materials.The research progress and application of B_(4)C materials in China and overseas in recent years were summarized.The influences of sintering processes(pressureless sintering,hot-pressing sintering,hot isostatic pressing sintering,spark plasma sintering and microwave sintering)and sintering additives(simple substances,oxides and carbides)on the B_(4)C densification were analyzed.The development of B_(4)C materials was prospected.

Sintering zone prediction in direct metal laser sintering by finite element method

A three-dimensional finite element thermal model in direct metal laser sintering(DMLS) including the effect of powder-to-solid transition were established to predict sintering zone, which benefited the determination of suitable process parameters in DMLS. The nonlinear transient model of the metals thermal conductivity for powder-to-solid transition was developed. The model uses solid thermal properties of material in liquid-phase zone, transitional ones in sintering or sintered zone and powder ones in unsintered zones of powder bed to predict, respectively. Sintering zone boundary was estimated by maximum temperature history profile. Experiments were carried out using multi-component Cu-based metal powder. Compared experimental and predicted results, the mean error of sintering depth and width are 7.8% and 14.4%, respectively, which confirms the accuracy of the FEM prediction.

Effects of various sintering methods on microstructure and mechanical properties of CP-Ti powder consolidations

Effects of various sintering methods such as spark plasma sintering(SPS), hot pressing(HP) and electric resistance sintering(ERS) on the microstructure and mechanical properties of commercial pure titanium(CP-Ti) powder consolidations with particle size of <147 μm, <74 μm and <43 μm were studied. The smaller particle powders are densified to proceed at a higher rate. Dense titanium with relative density up to 99% is found to take place at 850 °C under 30 MPa of SPS and HP condition. However, in case of ERS, CP-Ti powders were densified almost at 950 °C under 30 MPa. The microstructure of sintered titanium is composed of equiaxed grains at 850-950 °C. The yield strength of sintered body composed of <43 μm powder is 858 MPa by using SPS at 850 °C under 30 MPa. When there is a higher content of small particle, the higher yield strength value is obtained both by using SPS and HP. However, when ERS is introduced, the highest yield strength is 441 MPa at 950 °C under 30 MPa, which shows much lower values than those by SPS and HP methods. ERS method takes much less sintering time compared with SPS and HP. Nevertheless, higher sintering temperature results in lower strength and elongation because of brittle fracture.

Fabrication of Au/SiO_(2) Nanocomposite Films by Self-Assembly Multilayer Method

Gold colloid was prepared by chemical reduction of hydrogen tetrachloroaurate, polyelectrolyte/gold nanoparticle/silica nanoparticie composite films were fabricated via an electrostatic self-assembly multilayer method, and composite films of gold nanoparticle dispersed in silica matrix were formed by heat-treating the polyelectrolyte/gold nanoparticle/silica nanoparticle composite films to eliminate the polyelectrolyte. The obtained composite films were investigated with UV-vis, TEM, AFM and XRD. The results show that the self-assembly multilayer method is a promising process to produce composite films of gold nanoparticle-dispersed in organic and/or inorganic matrixes.

Preparation of PVA/GO/h-BN Janus Film with High Thermal Conductivity and Excellent Flexibility via a Density Deposition Self-assembly Method

Janus films with asymmetric physical/chemical properties have attracted con siderable attention due to their promising applications in personal thermal management,electronic skin s,sensors,actuators,etc.However,traditional methods for fabricating Janus films conventionally need the assistance of an interface or auxiliary equipment,which are usually complex and time-consuming.Herein,flexible poly(vinyl alcohol)(PVA)/graphene oxide(GO)/h-BN(recorded as PVA/GO/h-BN)Janus films with thermally,optically,and electrically anisotropic properties are fabricated by a simple density deposition self-assem bly method,which just utilizes the density difference between GO and h-BN during water evaporation.Experimental results show that the two sides of the acquired Janus films have obvious asymmetric characteristics.In the original state of the PVA/GO/h-BN Janus films,the thermal conductivity of the GO side(10.06 W·m^(-1)·K^(-1))is generally lower than that of the h-BN side(10.48W·m^(-1)·K^(-1)).But after GO is reduced,the thermal conductivity of the rGO side reaches 12.17 W·m^(-1)·K^(-1),surpassing that of the h-BN side.In addition,the relative reflectance of the h-BN side of Janus film is also significantly higher than that of the rGO side,and the su rface resistance difference between the two sides is about 4 orders of magnitude.The prepared PVA/GO/h-BN Janus films show great application potential in human thermal management,light conversion switches,and electronic skins.This study provides a simple and versatile strategy for fabricating Janus films with multifunctional(such as thermal,optical,and electrical)anisotropies.

UPPER BOUND METHOD FOR SINTERED POWDER MATERIALS IN PLANE STRAIN

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Dielectric properties of spark plasma sintering AlN-W composite ceramics

In order to develop AlN composites suitable for high average power electronic tube, AlN-W materials were prepared by spark plasma sintering. The effects of manufacture parameters on dielectric loss tangent and permittivity constant were investigated, which include powder-mixed method, milling time of high-energy ball milling, starting powder particle size, sintering temperature and holding time and adding amount of the conductive second phase. The results showed that A1N-W materials sintered at the temperature of 1700℃ holding for 5 min with 10 vol.% W showed the best dielectric loss tangent larger than 0.81 at the frequency 1 kHz-1 MHz. In addition, magnetic stirring mixed powder and lower sintering temperature led to the better propelties because of the higher porosity. The samples sintered from the starting AlN powder with smaller particle size also had the better properties.

A Facile Self-assembly Synthesis of Hexagonal ZnO Nanosheet Films and Their Photoelectrochemical Properties

Here, large-scale and uniform hexagonal zinc oxide(ZnO) nanosheet films were deposited onto indium tin oxide(ITO)-coated transparent conducting glass substrates via a facile galvanic displacement deposition process. Compared with other commonly used solution methods, this process avoids high temperature and electric power as well as supporting agents to make it simple and cost-effective. The as-fabricated ZnO nanosheet films have uniform hexagonal wurtzite structure. The photoelectrochemical(PEC) cell based on ZnO nanosheet film/ITO photoelectrode was also fabricated and its performance was improved by optimizing the solution concentration. A higher photocurrent density of*500 l A cm^(-2)under AM 1.5 G simulated illumination of 100 m W cm^(-2)with zero bias potential(vs. Ag/AgCl electrode) was obtained, which may ascribe to the increased surface-to-volume ratio of disordered Zn O nanosheet arrays. Our developed method may be used to deposit other oxide semiconductors, and the Zn O nanosheet film/ITO PEC cell can be used to design low-cost optoelectronic and photoelectrochemical devices.

The effect of agglomerate on micro-structural evolution in solid-state sintering

Discrete element method （DEM） is used in the present paper to simulate the microstructural evolution of a planar layer of copper particles during sintering. Formation of agglomerates and the effect of their rearrangement on densification are mainly focused on. Comparing to the existing experimental observations, we find that agglomerate can form spontaneously in sintering and its rearrangement could accelerate the densification of compacts. Snapshots of numerical simulations agree qualitatively well with experimental observations. The method could be readily extended to investigate the effect of agglomerate on sintering in a three- dimensional model, which should be very useful for understanding the evolution of microstructure of sintering systems.

SLIP LINE METHOD FOR SINTERED POWDER MATERIALS UNDER CONDITION OF AXIAL SYMMETRY DEFORMATION

Slip line method for sintered powder materials under condition of axial symmetry is proposed based on the simplified yield condition of sintered powder materials and Haar-von Karman perfect plastic criterion. The equations of slip line and stress along slip line are derived, and numerical solutions are given. Deformation load in closed die upsetting of sintered copper cylinder is calculated by slip line method, and theoretical solutions are compared with experimental results.

Influences of Sintering Process of Sprayed Precursors on the Struc-ture and Electrochemical Properties of LiMn_2O_4 Cathode Material

Well-defined spinel LiMn2O4 powders were synthesized via sintering a precursor, which was prepared by spraydrying method. The effects of sintering process on the structure and electrochemical properties of LiMn2O4 were discussed. It was found that a single sintering could not synthesize a pure LiMn2O4 compound, while two-step sintering procedure consisting of decomposing sprayed precursors at 350℃ and further sintering at an elevated temperature leads to the formation of a single-phased LiMn2O4 with homogeneous particle size distribution. As compared to that sintered in air, the two-step sintered LiMn2O4 in oxygen shows tighter structure and more uniform particle size, as well as better electrochemical properties. It delivers an initial discharge capacity of 131 mAh·g^-1 （1/10C）, and still has excellent cycling stability at higher rate （1/5C）.

Finite-Element Simulation and Experimental Research of the Hot-Press Sintering of Nano-Sized Ceramic Powders

Numerical simulation of hot-press sintering of nano-sized ceramic powders was introduced by the commercial finite element code MSC.MARC. The powder plastic model and the thermo-mechanical coupled quadrilateral element were developed and adopted in the simulation. The mechanical and thermal properties of the nano-sized alumina based powders were determined.In addition, the experimental research and numerical simulation of the sintering process of different initial densities were carried out. The stress state in sintering of green compacts with different initial densities was analyzed by the densification theory.The reason for the density fluctuations of as-sintered ceramic bulks was found out.

A Simple Deposition Method for Self-Assembling Single Crystalline Hybrid Perovskite Nanostructures

A sequential deposition method is developed, where the hybrid organic-inorganic halide perovskite （CH3NH3Pb （I1-xBrx）3 ） is synthesized using precursor solutions containing CH3NH3I and PbBr2 with different mole ratios and reaction times. The perovskite achieved here is quite stable in the atmosphere for a relatively long time without noticeable degradation, and the perovskite nanowires are proved to be single crystalline structure, based on transmission electron microscopy.Furthermore, strong red photoluminescence from perovskite is observed in the wavelength range from 746nm to 770nm with the increase of the reaction time, on account of the exchanges between I- ions and Br- ions in the perovskite crystal. Lastly, the influences of concentration and reaction time of the precursor solutions are discussed, which are important for evolution of hybrid perovskite from nanocuboid to nanowire and nanosheet.

Finite element method simulation for tensile process of sintered iron-base material

Different material properties leads to different metal fracture behaviors. Even if the powder material is composed of plastic metal, the fracture still does not show macroscopic plastic deformation characteristics if the material contains a large number of voids. Eight node isoparametric elastic plastic finite element method was used to simulate the tensile process of sintered powder material. By setting a number of voids in the analyzed metal cuboid, the initial density was taken into consideration. The material properties of the three dimensional solid for the tensile simulation were defined with reference to the known pure iron material parameters. The load displacement curves during elongation were obtained with a universal testing machine, and then the simulated curves were compared with the experimental results. The factors that cause the stress concentration and strength decrease were analyzed according to the simulated equivalent von Mises stress distribution.

碱烧结法回收二次铝灰中氧化铝

二次铝灰中蕴含大量的氧化铝、铝和氮化铝等,还有部分氟化物和氯化物盐类,属于危险废弃物。目前常用的铝回收技术有酸浸法和碱浸法,均存在工艺条件苛刻、回收率低、能耗高、物耗大和产品附加值较低等问题。本研究采用碱烧结法从二次铝灰中回收氧化铝,考察烧结温度对氮化铝转化率的影响,碱铝比、烧结时间和成型方式对氧化铝回收率的影响,得到以下主要结论:在烧结温度1423 K、烧结时间40 min、碱铝比1.1、干压成型的试验条件下,氧化铝溶出率最大为94.23%;通过对二次铝灰铝酸钠熟料浸出渣成分以及物相进行分析,得到二次铝灰经过碱烧结法焙烧后熟料主要成分为Al_(2)O_(3)和Na_(2)O,分别占到49.70%和28.06%,熟料标准浸出后残渣主要成分为CaO、Al_(2)O_(3)和MgO,含量分别为25.46%、20.27%和15.80%;二次铝灰中AlN分解率较高,可达到95%以上,最终浸出渣中氟离子浸出毒性在30 mg/L以下。综上所述,本文采用碱烧结法资源化利用二次铝灰,实现了二次铝灰脱氮固氟的无害化处置,具有环境友好、资源利用率高的优势,为后续大规模工业化推广奠定了理论基础。

放电等离子烧结细晶0.8PMN-0.2PT陶瓷的介电性能

铌镁酸铅-钛酸铅[Pb(Mg_(1/3)Nb_(2/3))O_(3)-PbTiO_(3),PMN-PT]陶瓷是一种典型的驰豫铁电体。为了研究晶粒尺寸对PMN-PT陶瓷介电性能的影响,采用两步固相反应法合成了0.8PMN-0.2PT粉体,使用放电等离子烧结技术在不同温度下制备0.8PMN-0.2PT陶瓷,并分析了陶瓷的晶体结构、微观形貌和介电性能。结果表明:采用两步法能得到纯0.8PMN-0.2PT粉体,其晶粒尺寸约为0.36μm;当烧结温度为1100℃时,采用放电等离子烧结技术可以得到晶粒细小的纯0.8PMN-0.2PT陶瓷,晶粒尺寸约为0.69μm,密度为8.02 g/cm^(3),但是其介电常数较低,最大值为12708,介电-顺电转变峰为一个较宽的峰,且随频率增加,介电-顺电转变温度(T_(c))向高温方向移动,呈现典型的弛豫特性。此外,在频率为1×10^(2)~2×10^(6)Hz时,随温度升高,介电常数和介电损耗都升高。因此,采用放电等离子烧结法可以得到细晶致密且介电常数较高的0.8PMN-0.2PT陶瓷。

极地高密度烧结雪层无侧限压缩离散元分析

从新雪变成老雪的过程叫作“烧结”,对雪层进行人工压实和烧结是建设极地机场跑道的主要方法,雪层烧结使雪颗粒间胶结尺寸增加,雪层强度增强。为探究人工压实烧结雪层在无侧限压缩荷载下的胶结接触破坏规律和承力机理,本研究采用三维离散元方法模拟了密实烧结雪层无侧限压缩试验,在室内试验结果基础上构建雪的胶结接触模型并标定接触参数,通过建立压实烧结雪离散元模型,进行了无侧限压缩加载模拟。模拟的结果验证了离散元模拟的有效性并表明:(1)在无侧限压缩下,密实烧结雪试样在剪切带区域发生集中胶结破坏,胶结断裂主要归因于接触在受拉状态下受剪;(2)冰雪颗粒的破碎会影响冰雪道面的抗剪强度,因此需要慎重考虑高应力对内摩擦角的影响。本研究将为极地地区通过压实烧结雪层简单现场试验预测其复杂应力路径下的力学特征提供科学支持。

国内高铝粉煤灰提取氧化铝主要技术进展及发展建议

粉煤灰已成为我国排放量最大的工业固废,目前堆存量估计在 30 亿吨以上,如果不及时处理或处理不当,将会对环境造成严重危害。由于其具有高含量的氧化铝,特别是高铝粉煤灰,含量超过38%,被认为是取代铝土矿,生产氧化铝的理想原料之一。近几十年来,高铝粉煤生产氧化铝是一项热门的研究课题,各企业及科研院校均投入了大量的人力和物力,并且取得了一定的成就,提出了不同的工艺路线,部分工艺已经进行了中试试验或者工业化生产。目前,初步形成了三种主要的工艺路线:预脱硅-碱石灰石烧结法、石灰石烧结法、一步酸溶法。文章主要综述了以上三种技术的原理、工艺流程及技术进展,分析其发展难点,同时提出评价和建议。

两种方法制备微晶玻璃的介电与储能特性的影响

作为一种有潜质的电介质材料,微晶玻璃材料得以广泛研究。目前开展的相关研究都聚焦于材料体系与结构的优化,而有关制备工艺对性能的影响鲜有报导。本研究在之前工作基础上,系统性的研究了熔融法与烧结法两种不同的制备工艺对磷酸盐体系微晶玻璃的结构、介电与储能特性的影响。结果表明,采用熔融法所制备的微晶玻璃具有更有序的显微结构,同时获得了更高的介电常数和更低的介电损耗,兼具最佳的能量释放特性与能量效率的特性。