New model for shrinkage compensation in selective laser sintering

This paper presents a new model for shrinkage and a new approach for shrinkage compensation to enhance the accuracy of parts produced by selective laser sintering (SLS)–a solid freeform fabrication process. The present prevailing approach as proposed by machine manufacturers is simple but not accurate. A new shrinkage model which accounts for part geometry as well as beam offset is proposed in this work. A new compensation scheme which accounts for nonlinear shrinkage is proposed, implemented and validated. The proposed compensation scheme compensates for shrinkage at every layer and at every hatch length, unlike a uniform compensation scheme applied to entire part. A new algorithm which accounts for this is developed and implemented. Experiments carried out with the new shrinkage model as well as with the new compensation scheme have shown significant improvement in the accuracy of the parts produced which establishes the effectiveness of the proposed methodology.     

Hybrid processing and anisotropic sintering shrinkage in textured ZnO ceramics

Abstract

We have studied the combined effects of the templated grain growth and magnetic alignment processes on sintering, anisotropic sintering shrinkage, microstructure development and texture in ZnO ceramics. Suspensions of 0–10 vol % ZnO template particles were slip cast in a 12 T rotating magnetic field. Sintering and texture characteristics were investigated via thermomechanical analysis and electron backscatter diffraction, respectively. Sintering as well as texture characteristics depend on template concentration. For the studied ZnO system, there is a critical template concentration (2 vol % in this study) above which densification is limited by the templates owing to constrained sintering. Below this limit, the densification is enhanced and the anisotropic shrinkage is reduced, which is attributed to densifying characteristics of the templates.     

钛酸钡陶瓷烧结动力学曲线的研究

以超重力反应沉淀法(HGRP)制备的纳米钛酸钡粉体为原料,以热膨胀仪为测试手段,对钛酸钡陶瓷的烧结动力学曲线特征进行了研究.结果表明,烧结温度和成型坯片密度都会对钛酸钡陶瓷的烧结过程产生影响,所表现出的烧结动力学曲线特征也不相同,而升温速率几乎不对坯片的烧结动力学曲线特征产生影响.随着烧结温度的升高,坯片收缩率增大,当烧结温度由1150℃升到1300℃时,收缩率由6.7%增大到23.2%;坯片的密度增大,收缩率减小,当成型坯片相对密度由61.08%增大到64.42%时,收缩率由17.5%减少到14.4%;升温速率增大,坯片开始收缩的温度及收缩率几乎不变.     

Kinetics and sintering mechanisms of hydro-thermally obtained hydroxyapatite

In this paper, the kinetics and sintering mechanism of hydro-thermally synthesized hydroxyapatite in the temperature range 900–1100 °C are presented. It was found that at 900 °C, the sintering process is very slow, whereas at 1000, 1050, and 1100 °C is much faster, caused by activated sintering with liquid phase/phase of solution on spots with concentrated material impurity and the formation of reaction/polyeutectic mixtures dominates. The activation energies of sintering were also determined. At all the investigated sintering temperatures, an approximate value of 90 kJ mol⁻¹ was found.     

Rapid pattern based powder sintering technique and related shrinkage control

Rapid Pattern Based Powder Sintering (RPBPS) is a new rapid tooling technique proposed by the authors. It is an integration of three techniques: rapid prototyping; lost wax casting; and powder sintering. The main advantages of the RPBPS technique compared with other tooling techniques are low investment and production cost, short production cycle and a variety of materials of products. One of the key technical problems is how to calculate and control the shrinkage during three processes in the RPBPS to improve the accuracy of products. The total linear shrinkage of the products made with the RPBPS can be controlled within 3.5% by using the selected binder type and binder ratio, suitable compacting pressure and specially designed ramp temperature program in the sintering and infiltration processes.     

Densification of pure magnesium by spark plasma sintering-discussion of sintering mechanism

In this study, high-quantity pure magnesium was prepared by SPS, and the sintering densification mechanism was discussed by using numerical simulation and simultaneous experiment. Results show that a layer of dense magnesium oxide was formed at the surface of Mg particles, and the oxide reduction can be observed owing to the effect of oxide film removal in SPS. The high energy pulsed current flows preferentially through particle contact surfaces, which provides the conditions for the generation of micro-arc between particles and the temperature at the particles contact point can be up to 1979 °C. With the low pressure at the initial sintering stage, local high temperature induced by micro-arc makes the melting (even evaporation) appearance. At the same sintering temperature of 570 °C, the obvious difference in bending strength also demonstrates the significance of spark discharge in SPS. The formation process of sintering neck demonstrates that sintering process is the reflection of melting (even evaporation), diffusion and plastic deformation. In order to realize tiny area and high-quality connection between powder particles, an innovative powder sintering technology using high-frequency pulse electric current (High frequency pulse current assisted sintering) is proposed based on the enlightenment of the skin, proximity and arc discharge effect of high-frequency pulse current. This technology is also instructive for other alloys and engineering materials.     

污泥陶粒制备工艺烧结温度影响研究

为明确陶粒制备工艺的烧成温度范围,考察焙烧机制,进行了不同温度污泥烧制页岩陶粒的实验研究。结果表明,工艺控制参数耐火度:Al2O3/+SiO2Fe2O3+CaO+MgO+Na2O+K2O可以表征混合料烧成开始温度;1000℃以下烧制陶粒的孔径分布为单峰曲线,1050～1100℃烧制陶粒的孔径呈现双峰结构,焙烧机制对陶粒孔隙分布有如下影响:提高焙烧温度,峰值孔径增大,延长焙烧时间,峰值孔径相对位置增加;XRD分析表明,陶粒主要晶相为石英和蓝晶石,在1050℃以上烧结,孔分布出现双峰曲线,可能与氧化铁分解有关。     

Investigations of failure mechanisms at Ta and TaO diffusion barriers by secondary neutral mass spectrometry

One of the most important processes in Cu metallization for highly integrated circuits is to fabricate reliable diffusion barriers. Recently, thin films made of refractory metals and their compounds have been widely used in solid-state electronics as barriers because of their good electric properties, favourable thermal properties and chemical stability. Thermal stability of Tantalum (Ta) and Tantalum-oxide (TaO_x) layers as a diffusion barrier in Si/Ta/Cu, Si/TaO_x/Cu and Si/Ta-TaO_x/Cu systems have been investigated. Si/Ta (10 nm)/Cu (25 nm)/W (10 nm), Si/TaO_x (10 nm)/Cu (25 nm)/W (10 nm) and Si/Ta (5 nm)TaO_x (5 nm)/Cu (25 nm)/W (10 nm) thin layers were prepared by DC magnetron sputtering. A tungsten cap layer was applied to prevent the oxidation of the samples during the annealing process. The samples were annealed at various temperatures (473 K-973 K) in vacuum. Transmission Electron Microscopy, X-ray diffraction, X-Ray Photoelectron Spectroscopy and Secondary Neutral Mass Spectrometry were used to characterize the microstructure and diffusion properties of the thin films. Our results show that at the beginning phase of the degradation of the Si/Ta/Cu system Ta atoms migrate through the copper film to the W/Cu interface. In the Si/TaO_x/Cu system the crystallization of TaO and the diffusion of Si through the barrier determine the thermal stability. The Ta-TaO bilayer proved to be an excellent barrier layer between the Si and Cu films up to 1023 K. The observed outstanding performance of the combined film is explained by the continuous oxidation of Ta film in the TaO_x-Ta bilayer.     

复相铁氧体材料的的放电等离子体烧结合成

采用化学共沉淀法制备了名义组成为xFe+BaFe12O19(0≤x≤1,△x=0.2)的共沉淀前驱体,研究了该前驱体在放电等离子体烧结条件下形成复相铁氧体材料的结晶行为和烧结体的磁性能.结果表明,所有烧结体中均没有Fe2O3中间相形成,x=0时烧结体为单相M型钡铁氧体(BaM),0相似文献   

A research on sintering characteristics and mechanisms of dried sewage sludge

This study investigates the sintering behavior of dried sewage sludge and the related sintering mechanisms, considering sintering temperature and sintering time. Experimental results indicate that the characteristics are primarily influenced by sintering temperature. When the sintering temperature is increased from 1020 to 1050 °C, the specimens’ compressive strength and bulk density increase significantly, while water absorption decreases obviously, indicating an improvement of densification due to sintering. However, the compressive strength cannot meet the requirement for traditional ceramic products due to the release of organic matters and the formation of big pores in the products. Phosphorus in sewage sludge initially takes reactions with the formation of calcium magnesium phosphate and aluminum phosphate during sintering, which are helpful for enhancing the compressive strength. So, some materials with high contents of Al could be used to enhance the compressive strength of products. Heavy metals are fixed primarily inside the sintered specimens, with the As, Pb, Cd, Cr, Ni, Cu, Zn concentrations in the leachate found to be in the range of China regulatory requirements. These results reveal the feasibility of recycling dried sewage sludge by sintering as a construction material.     

A comparison study of Portland cement hydration kinetics as measured by chemical shrinkage and isothermal calorimetry

Two different methods of evaluating cement hydration kinetics, namely chemical shrinkage and isothermal calorimetry tests, are used to investigate the early stage hydration of different classes of oilwell cement at various temperatures. For a given cement paste, the hydration kinetics curves measured by the two methods are proportional to each other at the same curing temperature. The ratio of heat of hydration to chemical shrinkage for different cements used in this study ranges from 7500 J/mL to 8000 J/mL at 25 °C and increases almost linearly with increasing curing temperature at a rate that varies only slightly with cement composition (approximately 58 J/mL per °C). A previously proposed scale factor model for simulating the effect of curing temperature and pressure on cement hydration kinetics is further validated in this study for its temperature aspect. The model is shown to be particularly helpful in correcting for slight temperature errors in the experiments.     

Image deblurring based on ForIcM: Fourier shrinkage and incomplete measurements

Abstract

In this paper, we propose a new deblurring algorithm, which is based on image reconstruction from incomplete measurements in Fourier domain. Our algorithm has two steps. Firstly, an initial estimator is obtained using Fourier regularised inverse operator. Secondly, parts of the estimator’s Fourier coefficients are saved, and the others are removed to suppress noise energy, and then the remaining coefficients are used to recover image based on the sparse constraints. This image reconstruction problem is an optimisation problem which is solved by a fast algorithm named split Bregman iteration. Our algorithm combines two different regularisation strategies efficiently by applying a selection matrix. The tests using images with different blurs and noise produce good results. The experiment shows that our method gives better performance than many other competitive deblurring methods.     

A comparative study of nickel and alumina sintering using spark plasma sintering (SPS)

Nickel and alumina powders with particle sizes of 4.3 μm and 108 nm, respectively, have been sintered in moulds of different sizes. Only the diameter (inner and/or outer) of moulds has been varied. The dimension modification is responsible for sample microstructure variation because set temperature, heating rate, dwell time and pressure were identical in all experiments. The influence of the die (or sample) dimension on the sample microstructure appears to depend strongly on the electric characteristics of the powder. The present paper is an attempt to correlate the bulk microstructure evolution with the die and sample size and the electrical current distribution within the system.     

粉末材料的爆炸烧结

论述了爆炸烧结的方法及装置，讨论了烧结过程的机理及影响其过程的主要参数。     

真空下低价化合物法提取铝的新工艺--原料烧结实验研究

概述了我国炼铝工艺的现状以及铝冶炼方法的研究状况，并初步讨论与研究了煤烧结的一些基本规律。基于常规冶金存在的局限性以及真空冶金的优势，我们探索一种新炼铝的工艺，对低价化合物直接提取法作了一定的研究，低价化合物提取法主要包括了原料烧结实验、提取实验两大部分。本文着重研究了原料烧结实验部分中不同煤的配比，焦结温度对烧结团块性能的影响，得到最佳煤配比，焦结温度等重要的工艺条件和参数。它们是实验成功的关键。实验得到物料的最佳配比为10^#煤：5^#煤：16^#煤：工业Al2O3=3：7：10：5，最佳烧结温度为520℃。     

Evaluation of diffusion barrier and electrical properties of tantalum oxynitride thin films for silver metallization

The thermal stability and the diffusion barrier properties of DC reactively sputtered tantalum oxynitride (Ta-O-N) thin films, between silver (Ag) and silicon (Si) p⁺n diodes were investigated. Both materials characterization (X-ray diffraction analysis, Rutherford backscattering spectrometry (RBS), Auger depth profiling) and electrical measurements (reverse-biased junction leakage current-density) were used to evaluate diffusion barrier properties of the thin films. The leakage current density of p⁺n diodes with the barrier (Ta-O-N) was approximately four orders of magnitude lower than those without barriers after a 30 min, 400 °C back contact anneal. The Ta-O-N barriers were stable up to 500 °C, 30 min anneals. However, this was not the case for the 600 °C anneal. RBS spectra and cross-sectional transmission electron microscopy of as-deposited and vacuum annealed samples of Ag/barrier (Ta-O-N)/Si indicate the absence of any interfacial interaction between the barrier and substrate (silicon). The failure of the Ta-O-N barriers has been attributed to thermally induced stresses, which cause the thin film to crack at elevated temperatures.     

体积收缩率对光固化树脂基复合材料收缩应力的数值模拟

依据SiC晶须增强树脂基复合材料的光固化实验结果，运用材料的温度应力原理，利用降温等效体积应变与光固化体积收缩率关系的假设，建立了有限元模型。研究并分析了树脂基体中组分配比、光固化层厚和晶须增强体对成型件收缩应力的影响。结果表明：树脂基体中组分配比和晶须增强体对收缩应力的影响很大，而光固化层厚对收缩应力的影响较小。     

硬质覆层材料的液相烧结工艺研究

采用液相烧结法成功地制备了钢结硬质合金-钢硬质耐磨覆层材料FC-Ⅰ和FC-Ⅱ，对其液相烧结工艺进行了研究，并利用扫描电镜和X射线衍射分析了两种覆层材料的微观结构。结果表明：热处理后FC-Ⅰ和FC-Ⅱ的表面维氏硬度分别为6.418MPa和5.580MPa;FC-I的烧结温度为1300℃，FC-Ⅱ的烧结温度为1280℃，升温速度在固相烧结阶段是10-15℃/min，液相阶段是5℃/min，保温时间为20-60min,FC-I的组织为体心立方相的α-Fe和面心立方相的Fe3W3C;FC-Ⅱ的组织为Ni基固液体，面心立方相的Fe3W3C和体心立方相的αF-Fe。     

Understanding the drying shrinkage performance of alkali-activated slag mortars

In this work, drying shrinkage of four alkali-activated slag (AAS) mortars, prepared using various types/dosages of activator, was characterized at four different levels of relative humidity (RH) and two drying regimes (i.e. direct and step-wise drying). The results show that drying shrinkage values of AAS are significantly dependent on the drying rate, as AAS shrinks more when the RH is decreased gradually, instead of directly. At high RH, the drying shrinkage of AAS exhibits a considerable visco-elastic/visco-plastic behavior, in comparison to ordinary portland cement (OPC). It is concluded that the cause of high-magnitude shrinkage in AAS mortar is due to the high visco-elastic/visco-plastic compliance (low creep modulus) of its solid skeleton. Furthermore, the activator affects the shrinkage behaviors of AAS by influencing the pore structure and mechanical properties.     

Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering

The retention of nanocrystallinity in dense ceramic materials is still a challenge, even with the application of external pressure during sintering. The compaction behavior of high purity and acetate enriched zinc oxide (ZnO) nano-powders was investigated. It was found that acetate in combination with water plays a key role during the compaction into green bodies at moderate temperatures. Application of constant pressure resulted in a homogeneous green body with superior packing density (86% of theoretical value) at moderate temperature (85 °C) in the presence of water. In contrast, no improvement in density could be achieved if pure ZnO powder was used. This compaction behavior offers superior packing of the particles, resulting in a high relative density of the consolidated compact with negligible coarsening. Dissolution accompanying creep diffusion based matter transport is suggested to strongly support reorientation of ZnO particles towards densities beyond the theoretical limit for packing of ideal monosized spheres. Finally, the sintering trajectory reveals that grain growth is retarded compared to conventional processing up to 90% of theoretical density. Moreover, nearly no radial shrinkage was observed after sinter-forging for bodies performed with this advanced processing method.