Tailored graded pore structure in zirconia toughened alumina ceramics using double-side cooling freeze casting

Ceramics with graded and continuously aligned open pores were investigated using a double-side cooling freeze casting setup. The ceramic preforms with tailored lamellae spacing (wavelength), wall thickness and graded pore structure were used to infiltrate with a second phase for the fabrication of graded interpenetrating phase composites. The effects of solid content, temperature setting and gradient, cooling rate and the introduction of electrophoretic deposition (EPD) on the freezing velocity of the ceramic suspension were analysed. On the bottom of the ceramic specimen, a dense layer was formed and tailored with the use of EPD. The ceramic was characterised by a graded open pore structure with wavelengths up to 115 μm and interconnected microstructure. The effect of solid content on the degree of supercooling and the effect of temperature gradient on the average freezing velocity were investigated. The addition of EPD before freeze casting affected significantly the microstructure, the wavelength decreased and the wavelength gradient became smaller compared to simple freeze casting.     

Gradient-controlled freeze casting of preceramic polymers

Solidification is the foundation upon which freeze casting is built, and this work seeks to understand the solidification process in freeze casting, especially with respect to the growth of dendrites. To this end, two solidification parameters, freezing front velocity and temperature gradient, were independently controlled using a gradient-controlled freeze casting setup to investigate the effects of each parameter. Changes in dendritic pore size and morphology with solidification parameters were investigated by scanning electron microscopy and mercury intrusion porosimetry. The results agree with dendrite growth theory. The theory of constitutional supercooling is used to describe the morphological changes of dendritic pores to cellular pores by the control of freezing front velocity, temperature gradient, and preceramic polymer concentrations.     

挤出吹塑制品冷却和固化阶段的模拟及优化方法

在挤出吹塑过程中,冷却和固化是一个非常重要的阶段,它对挤出吹塑制品的性能及生产效率均有很大影响。本文从挤出吹塑制品冷却和固化阶段聚合物的相变、吹塑制品的微观结构与宏观性能之间的关系、模具与制品间传热系数对吹塑制品冷却和固化的影响以及吹塑制品冷却和固化阶段的温度场及结晶度的模拟四方面综述了这一复杂过程的研究进展。     

Solidification structure in pure zirconia liquid molten phase

Up to now few studies concerning the crystallographic features of zirconia materials (HZ) used as refractory material have been performed. The elaboration process through raw material fusion followed by a controlled cooling step lead to a material containing 94 wt.% of dendritic monoclinic zirconia. During cooling, ZrO₂ passes through three different structural phase transitions (SPTs), inducing formation of a complex microstructure made of different crystallographic domains called variants. This study focus on formation of these different monoclinic variants in a non-doped zirconia based material and the possibility to reconstruct initial cubic parent crystals formed at high temperature using optical microscopy and Electron Back Scattering Diffraction (EBSD). EBSD experiments allow to identify 24 monoclinic variants issued from tetragonal parent crystal and to highlight the contours of the parent cubic crystal. Then, the microstructure appearing during the first steps of the solidification of the material can be evidenced.     

Schmelzkristallisation – theoretische Voraussetzungen und technische Grenzen

Melt crystallization – Theoretical presumptions and technical limitations. The fundamentals of the design of melt crystallization processes on cooled surfaces are presented. Determination of the energy requirement is the essential point. During each freezing step a multiple of the heat of solidification must be removed and must be re-introduced during the subsequent melting step. Consequently, discontinuous processes of this nature can only compete with other thermal separation techniques – in which a continuous countercurrent operation can be realised – if the plant is well designed and the process is precisely controlled.     

喷雾冷冻法单个液滴冻结过程模拟

喷雾冷冻液滴的冻结过程决定着干燥产品的微结构。本文以单个雾化液滴为研究对象,利用数值模拟的方法研究了液滴大小、气体流速和环境温度3个参数对其冻结过程的影响。结果发现,液滴越大冻结时所需的形核时间和完全固化时间越长,而且冻结过程随着气体流速的增大和环境温度的降低而缩短。通过方差分析发现,液滴大小较气体流速和环境温度对液滴完全固化时间的影响有较显著差异。液滴冷冻过程中,其质量损失率随着液滴大小的增大而略有减小,随着气体流速的增加及环境温度的降低而减小,其中环境温度对液滴质量损失率的影响最大。     

Cooling and Solidification of Melted Drops in an Immiscible Cooling Medium

The cooling and solidification of melted drops during their movement in an immiscible cooling medium is widely employed for granulation in the chemical industry, and a study of these processes to provides a basis for the design of the granulation tower height and the temperature of the cooling medium is reported. A physical model of the cooling and solidification of the drop is established and the numerical calculation is performed. The influences of the key factors in the solidification, i.e., Bi number, drop diameter, temperature of the cooling medium, etc. are presented. The cooling and solidification during wax granulation in a water‐cooling tower and during urea granulation in an air‐cooling tower (spraying tower) are described in detail. Characteristics of the solidification and temperature distribution within the particle at different times are shown. The model and calculations can be used for structure design of the granulation tower and optimization of the operation parameters.     

基于低温制冷机的流体液化与凝固过程可视化装置

液化和凝固是流体系统工程中常见的两种基本现象。由于低温流体液化点和凝固点温度都非常低，给受控条件下观测流体的液化和凝固过程带来了困难。设计并搭建了基于G-M低温制冷机的流体受控液化和凝固过程可视化实验装置，并对氮气（44~80 K）、氩气（50~90 K）、氧气（50~90 K）3种低温流体的液化和凝固过程进行控制观测，获得了3种流体在一定温度下的相变过程的视频图像。实验结果表明，3种低温流体的凝固过程表现出较明显的固化行为特性差异。     

Preparation of a Dense/Porous BiLayered Ceramic by Applying an Electric Field During Freeze Casting

Dense/porous bilayered ceramics were fabricated by introducing an electric field into the freeze-casting process. A unidirectional solidification was performed by placing an aqueous ceramic on refrigerating equipment, at a constant cooling rate of 6°C/min. When the bottom temperature of the slurry was reduced to 0°C, an electric field along the vertical direction was applied until the freezing process was completed. After drying and sintering of the green part, an as-prepared ceramic with dense/porous bilayered architectures was obtained. It was shown that the thickness of the dense layer in the sample could be controlled by tuning the electric field intensity from 51 to 155 μm as the electric field increased from 15 to 90 V. On the other hand, all samples exhibited almost an identical lamellae thickness and interlayer distance in the porous region, as large as ∼14 and ∼24 μm, respectively.     

Influence of Freezing Temperature on Product Parameters of Solid Dosage Forms Prepared via the Freeze‐Casting Technique

The freeze‐casting technique was used to formulate porous solid bodies containing theophylline as an active pharmaceutical ingredient, potato starch as a filler and citric acid or saccharose as binding agents. Aqueous suspensions of the ingredients were frozen at three different temperatures. The aim of this work was to investigate the influence of the freezing temperature on the ice crystal growth rate of the suspensions and the resulting porosity of the freeze‐casted samples. The impact of the freezing temperature on the solidification of the suspensions was analyzed via contact angle measurements. The rate of ice crystal growth was expressed as an overall linear growth rate. The porosity of the freeze‐casted bodies was determined by high‐pressure mercury porosimetry. A close correlation was found between the freezing temperature and the investigated product parameters. Lower cooling temperatures resulted in higher final porosities. The lowest temperature, at which final product properties could be regulated by varying the cooling regimes, was –30 °C. The influence of the freezing temperature and the impact of the additives were not remarkable at a freezing temperature lower than –30 °C. Therefore, it can be concluded that the optimum freezing temperature of the suspensions investigated in this study is in the temperature range between –20 °C and –30 °C.     

Reducing residual stresses in molded parts

Means of reducing the flow-induced residual stresses in injection molded parts through optimization of the thermal history of the process are presented. An approach through the use of a passive insulation layer with low thermal inertia on the cavity surface was investigated. The passive insulation layer prevents the polymer melt from freezing during mold filling and allows the flow-induced stresses to relax after the filling. The criteria for the optimal thermal properties and the required thickness of the layer are presented. A numerical simulation model of non-isothermal filling and cooling of viscoelastic materials was also used to understand the molding process and to evaluate this approach. This model predicts the stress development and relaxation in the molding cycle. Both simulation and experimental results show that the final stresses in the molded parts can be reduced significantly with the use of an insulation layer. This technique can also be applied to other molding or forming processes in order to decouple the material flow and cooling process for minimum residual stresses in the molded parts.     

Microstructure of rapidly-quenched ZrO2-SiO2 glass-ceramics fabricated by container-less aerodynamic levitation technology

In this work, an aerodynamic levitation technology (ALT) was utilized to prepare ZrO₂-SiO₂ glass-ceramics with two different ZrO₂ contents, that is, 35 mol% and 50 mol%. The glass-ceramics were partially melted at ∼2000°C or fully melted at ∼3000°C by ALT, followed by rapid quenching to obtain spherical glass-ceramic beads. The phase compositions and microstructures of the glass-ceramics were characterized. Crystallization of ZrO₂ occurred during the solidification process and ZrO₂ content, processing temperature, and the addition of yttrium (3 mol%) affected the crystalline phase of ZrO₂. No ZrSiO₄ or crystalline SiO₂ were formed during the solidification process and the glass-ceramics were away from thermodynamic equilibrium due to rapid quenching. The glass-ceramics showed a microstructure of irregular-shaped ZrO₂ micro-aggregates embedded in an amorphous SiO₂ matrix, with lamellar twins and lattice defects formed within ZrO₂ crystals. For samples prepared at ∼3000°C, a liquid-liquid phase separation occurred in the melt, which eventually resulted in the formation of large and irregular-shaped ZrO₂ aggregates. In comparison, for samples prepared at ∼2000°C, pre-existed ZrO₂ crystals formed during heating acted as nucleation sites during the cooling process, followed by grain growth to form large ZrO₂ aggregates. Solidification and microstructure formation mechanisms were proposed to elucidate the solidification process during rapid cooling and the microstructure of the glass-ceramics obtained.     

EXERGY ANALYSIS FOR THE FREEZING STAGE OF THE FREEZE DRYING PROCESS

The mathematical expressions for exergy and the exergy analysis of the freezing stage of the freeze drying process are presented. The exergy analysis indicates that very substantial reductions in the magnitudes of the total exergy loss and of the exergy input due to the heat that must be removed during the freezing stage, can be obtained when the freezing stage is operated through the use of a rational distribution in the magnitude of the temperature of the cooling source. The rational distribution in the magnitude of the temperature of the cooling source should provide significant savings in the utilization of energy during the freezing stage of the freeze drying process as well as satisfactory freezing rates that form ice crystals that are continuous and highly connected and their shape and size are such that the pores of the porous matrix of the dried layer generated by sublimation during the primary drying stage, have a pore size distribution, pore shape, and pore connectivity that are appropriate to allow high rates for mass and heat transfer during the primary and secondary drying stages of the lyophilization process.     

Swelling mechanism of porous P(VP‐co‐MAA)/PNIPAM semi‐IPN hydrogels with various pore sizes prepared by a freeze treatment

BACKGROUND: The objective of the work reported was to investigate the effect of gel microstructure on the swelling mechanism. A series of porous gels with various pore sizes were prepared by freeze‐treating a conventional hydrogel that contained various amounts of water at ? 20 °C. Scanning electron microscopy and differential scanning calorimetry were used to characterize the microstructure of the porous gels. RESULTS: The experimental results showed that the water content during freezing was the key factor controlling the microstructure. Measurement of swelling kinetics showed that a greater amount of water during the freezing process would lead to a rapid swelling rate. The apparent diffusion coefficients (D_p) at all times during the swelling process were obtained by fitting the experimental data to the diffusion equation. The values of D_p suggested that the swelling mechanism of the gels depends on the pore size. The diffusion exponent (n) obtained by fitting the fractional mass change (M_t/M_∞) to equation of Fick's law further confirmed that the swelling mechanism of the gels is determined by the pore size. CONCLUSION: The swelling mechanism of the gels is determined by the microstructure related to the pore size and the thickness of struts. The microstructure can be controlled by adjusting the water content of the hydrogels by changing the pH of the swelling medium prior to freezing. Copyright © 2008 Society of Chemical Industry     

Heat transfer in blow molding operations

A one-dimensional model of unsteady state heat conduction has been applied to the cooling and solidification of a polymer in a blow molding process. The approach includes a temperature-dependent specific heat term to account for latent heat effects during the phase change. The model is used to predict temperature profiles in a thick-walled component of high-density polyethylene. These profiles lead to a clearer understanding of the heat transfer process. It is further shown that these temperature distributions can be used to study the influence of the major process variables upon the cooling of the molded component.     

Cracking mechanism in laser directed energy deposition of melt growth alumina/aluminum titanate ceramics

Thanks to its advantages of high efficiency and near-net shaping, laser directed energy deposition (LDED) is rapidly becoming a remarkable preparation technology for high-purity ceramics. However, the cracking problem in shaping process is always a great challenge for LDED to achieve industrial application. For this purpose, alumina/aluminum titanate melt-growth ceramics (A/AT MGCs) were prepared using LDED system, and the corresponding finite element thermal analysis model was developed. The solidification behavior and cracking mechanism of A/AT MGCs were investigated based on the thermal analysis model, and the influence of process parameters on the cracking characteristics was revealed with experiments. Results show that the crack morphology and distribution are controlled by microstructure and temperature gradient together. The scanning speed of 100–150 mm/min, with better microstructure and lower temperature gradient, is a preferred process window. This study provides theoretical guidance and technical support for the cracking suppression during LDED shaping of ceramics.     

连铸凝固过程热模拟实验方法及应用案例

凝固是制约冶金产品质量的重要环节，但因高温、不透明、大规模和连续化的生产特点，连铸生产条件下的凝固问题研究极为困难。目前的研究方法主要包括数值模拟、物理模拟和热模拟，其中热模拟方法因可以直接获取接近生产条件的实验数据而备受关注。本工作系统介绍了连铸凝固热模拟研究方法，简述了热模拟技术原理，并对结晶器热模拟方法及特征单元热模拟方法的原理和应用进行总结。其中，基于特征单元热相似性提出的连铸坯枝晶生长热模拟及凝固裂纹热模拟等方法成功地将十几吨铸坯的凝固过程“浓缩”到实验室用百克钢研究，不仅可以揭示钢液成分、浇注和冷却条件等因素对凝固过程、组织和元素分布的影响规律，而且还可以获得铸坯固液界面形貌、界面前沿溶质扩散和夹杂物演变、凝固裂纹形成的可能性及条件等其他手段无法得到而冶金界非常关注的问题。     

Prediction of residual stress distribution in plastic pipe extrusion

In thermoplastic pipe extrusion, the extrudate emerging from the die is typically sized and cooled from outside in a quench tank. This process causes quick solidification of the external layer, while the inner mass of molten material cools only gradually. The slow cooling and crystallization, and the associated shrinkage of this material, can lead to build-up of severe stresses in the final part that can affect the long term service performance. In this paper, a simple theoretical analysis of this process of residual stress build-up is presented. The pipe undergoing quenching is modeled as an annular cylinder of molten polymer being cooled at a controlled rate from outside. The overall stresses are derived numerically by adding up the stress contributions due to incremental advance of the solidification boundary. The results of the analysis are found to be in qualitative accord with the experimentally measured stress profiles in thermoplastic pipe.     

UHMWPE/矿物油体系热致相分离过程的研究

采用POM观察超高摩尔质量聚乙烯(UHMWPE)/矿物油体系的热致相分离过程,结合SEM观察流延铸片断面的形貌结构,构建了UHMWPE/矿物油体系的蜂窝状结构模型.研究了不同冷却速率影响相分离程度而得到的流延铸片的结构形态,对比了不同冷却条件下流延铸片经过双向拉伸之后的薄膜的微观结构.     

Eutectic solidification of the quasi binary system of isotactic polypropylene and 1, 2,4, 5-tetrachlorobenzene

This paper describes a study on eutectic solidification of the quasi binary system of un-fractionated isotactic polypropylene and the faceted growing diluent 1,2,4,5-tetrachlorobenzene. According to the Flory-Huggins theory of melting point depression the phase diagram of the present system does not exhibit a eutectic point for equilibrium conditions. However, under experimental circumstances a simple “eutectic-type phase diagram” was found, in which the location of the eutectic point was determined by kinetics to a large extent. The very nature of the eutectic solidification process was found to depend critically on the rate of cooling. At cooling rates lower than 0.5°C/ min the simultaneous crystallisation of the polypropylene and the tetrachlorobenzene proceeded in an uncoupled manner, producing a microstructure consisting of randomly mixed polypropylene spherulites and diluent needles with a characteristic dimension of about 0.1 mm. At speeds of cooling exceeding 0.5°C/min coupled growth of the polymer and the solvent occurred, forming a unique eutectic microstructure of curved diluent crystals homogeneously dispersed in a polymer matrix. The transverse dimension of these particular tetrachlorobenzene crystals was found to be in the micron range.