陶瓷干燥、烧成收缩与总收缩的关系

本根据陶瓷制造各阶段的收缩率计算公式,推导了它们之间的关系,提出了总收缩等干干燥收缩与烧成收缩之和与两之积的差异。‘     

Densification and Shrinkage During Liquid-Phase Sintering

The process of densification and shrinkage during the final stage of liquid-phase sintering is described. The densification occurs by the liquid filling of pores during grain growth. The pore filling results in an instantaneous drop of liquid pressure in the compact and causes gradual accommodation of grain shape. The grain shape accommodation by the growth causes the specimen shrinkage. At the same time, the grains tend to restore their spherical shape, resulting in microstructure homogenization around filled pores. The process of densification and shrinkage appears to be determined by the growth of grains during sintering.     

压实密度与烧成收缩率的探讨

探讨了压制成型后产品压实密度与烧成收缩率的关系，提出了在各种原材料烧失量稳定的情况下，用控制压实密度代替用控制压力来保证坯件收缩的新理念。     

Pore Growth during Initial-Stage Sintering

A quantitative model for pore growth during initial-stage sintering is proposed. During initial-stage sintering, neck formation leads to surface rounding of the pores, thereby causing a decrease in the surface area of the system. The decrease in surface area, without a concomitant decrease in pore volume, leads to a spurious increase in pore size, as calculated by the gas adsorption technique. Geometrical calculations predict a final pore size that is a function of the initial (green) density of the compact. For the 62% dense compacts of the present study, the model predicts a factor of 1.28 increase in pore size, compared to the factor of 1.27, which is experimentally observed. Interestingly, a common, constant factor of 1.27 can also be observed in pore growth data reported by a number of other researchers.     

Thermodynamics of Densification: I, Sintering of Simple Particle Arrays, Equilibrium Configurations, Pore Stability, and Shrinkage

Equilibrium configurations for linear and closed arrays (rings and regular polyhedra containing a single pore) of identical particles (cylinders or spheres) were determined by minimizing the array's surface and grain-boundary energies with the assumption that each particle conserves its mass. The change in free energy between the initial and equilibrium configuration increases with dihedral angle (i.e., the equilibrium angle). More significantly, it is shown that pores will shrink to an equilibrium size if the number ( n ) of coordinating particles is greater than a critical value. The critical pore coordination number ( n _c) increases with the dihedral angle. Only pores with n n _c are thermodynamically unstable during sintering. It is also shown that any mass-transport mechanism can lead to pore shrinkage while a connecting path to the pore surface remains open. The effective sintering "stress" (i.e., driving force) increases with the dihedral angle and decreases to zero as the equilibrium configuration is reached. Sintering stresses increase with decreasing coordination number. It is also shown that the shrinkage strain for closed arrays increases with the pore coordination number. Rearrangement phenomena within a powder compact are discussed with regard to resultant sintering forces on nonsymmetrically coordinated particles.     

Lamination and Sintering Shrinkage Behavior in Multilayered Ceramics

Variations in lamination conditions, such as pressure, temperature, and time, changed the laminated density of multilayered alumina but had no effect on the sintered density. The present results showed that sintering shrinkage values differ with lamination conditions and vary inversely with laminated density. When lamination was accomplished using a press die, the difference in shrinkage between the X-Y and the Z directions was <1%. The effect of the press die could be explained by introducing a new factor, the SDF (shear deformation factor), which represents the ratio of area change in the X-Y direction before and after lamination. The lamination of green sheets exhibited almost the same behavior as did the compression of granules. A linear relationship also was found between laminated density and the logarithm of lamination pressure. Results for sintering shrinkage in the overall range of measured laminated densities showed that sintering shrinkage behavior could be divided into three regions; that is, the laminates had three packing structures with different laminated densities. A new factor ( k ), related to packing structure values before and after sintering, was introduced to explain the sintering shrinkage behavior. Each k value was obtained from the relationship between laminated density and sintering shrinkage. Comparing k factors for the laminated densities ( X-Y and Z ) under various lamination conditions made it possible to systematically analyze variations in the sintering shrinkage behavior of laminates with processing conditions. An estimation of sintering shrinkage was possible from that analysis.     

碎粒烧结法玻璃陶瓷的气孔问题探讨

认为碎粒烧结法玻璃陶瓷的气孔率主要由玻璃液的澄清、粒度级配、杂质、烧成制度和化学组成决定。采用严格合理的澄清制度,以酸洗等方式剔除杂质或予以掺杂,建立适宜的烧结模型和碎粒紧密堆积所要求的粒度级配,优化配方,加人一定的Na2O等助熔剂,可显著减轻玻璃陶瓷的气孔率。     

Zero Shrinkage of LTCC by Self-Constrained Sintering

Low shrinkage in x and y direction and low tolerances of shrinkage are an indispensable precondition for high-density component configuration. Therefore, zero shrinkage sintering technologies as pressure-assisted sintering and sacrificial tapes have been introduced in the low-temperature co-fired ceramics (LTCC) production by different manufacturers. Disadvantages of these methods are high costs of sintering equipment and an additional process step to remove the sacrificial tapes. In this article, newly developed self-constrained sintering methods are presented. The new technology, HeraLock^®, delivers LTCC modules with a sintering shrinkage in x and y direction of less than 0.2% and with a shrinkage tolerance of ±0.02% without sacrificial layers and external pressure. Each tape is self-constrained by integration of a layer showing no shrinkage in the sintering temperature range of the LTCC. Large area metallization, integration of channels, cavities and passive electronic components are possible without waviness and camber. Self-constrained laminates are an alternative way to produce zero shrinkage LTCC. They consist of tapes sintering at different temperature intervals. Precondition for a successful production of a self-constrained LTCC laminate is the development of well-adapted material and tapes, respectively. This task is very challenging, because sintering range, high-temperature reactivity and thermal expansion coefficient have to be matched and each tape has to fulfill specific functions in the final component, which requires the tailoring of many properties as permittivity, dielectric loss, mechanical strength, and roughness. A self-constrained laminate is introduced in this article. It consists of inner tapes sintering at especially low-temperature range between 650°C and 720°C and outer tapes with an as-fired surface suitable for thin-film processes.     

成型压力与坯体烧成线收缩的关系

采用８种不同的等静压力成型压力对３种喷雾干燥法制备的氧化铝陶瓷粉料进行烧结试验,获成型压力与产品烧成线收缩的对应关系。结果表明,不同粉料、不同成型压力与烧成线收缩的对应关系是不同的,通过改变成型压力可在一定范围内调整坯体的烧成线收缩。为控制等静压成型陶瓷制品的外形尺寸找到一种有效的途径。     

Experimental Assessment of Pore Breakaway During Sintering

Experimental measurements of intragranular pore-size distributions and of the shapes of distorted pores attached to grain boundaries were used to examine the podgrain-boundary separation problem. In situ evaluation of the ratio of the boundary mobility-to-surface diffusivity from pore distortion measurements, coupled with prior analytic expressions for the critical pore breakaway condition, permitted specific intragranular pore sizes generated by separation to be predicted. The predictions lie within the distribution of measured intragranular pore sizes.     

Particle Rearrangement and Pore Space Coarsening During Solid-State Sintering

Coarsening of porosity during sintering has been observed in powder compacts of metallic, ceramic, and amorphous materials. Monitoring and modelling of the growth of individual (closed) pores in the late sintering stages are well established. Porosity is interconnected up to very high densities. Coarsening of the continuous pore space takes place during the initial and intermediate sintering stages. This coarsening is caused by localized transport of atoms or molecules (diffusion or viscous flow) as well as by bulk particle movement (rearrangement). Its quantitative exploration poses problems both experimentally and theoretically. Ways to characterize the geometry of the interconnected pore space and of closed pores are discussed with emphasis on stereological parameters. Recent and classical approaches, experimental findings with 2D model arrangements (as the formation and opening up of particle contacts, pore coarsening, and particle rearrangement) and some advances of computer simulations are discussed together with open questions.     

Pore Evolution During Glow Discharge Sintering of Alumina

A series of experiments have been performed to demonstrate the applicability of small-angle neutron scattering to the study of pore evolution during sintering. Samples of α-Al₂O₃ which had been zone sintered in a hydrogen hollow cathode glow discharge to densities exceeding 94% of theoretical were employed in these preliminary measurements. The neutron scattering results indicate that densification during the final stages of glow discharge sintering occurs primarily through a reduction in the number of pores present with only a small change in the average pore size.     

Anisotropic Sintering Shrinkage in Alumina Ceramics Containing Oriented Platelets

The effect of oriented α-Al₂O₃ platelets on densification and shrinkage behavior of tape-cast α-Al₂O₃ during solid-state sintering was studied using dilatometric sintering shrinkage experiments and microstructural characterization. Platelet-free alumina samples exhibit isotropic shrinkage with an ∼17% linear final shrinkage. The addition of platelets retards densification, and results in anisotropic shrinkage. Initially during sintering, shrinkage is isotropic for platelet-containing systems; however, it becomes highly anisotropic by inhibition of shrinkage in the platelet orientation direction. In the 10 vol% platelet-containing system, shrinkage was ∼12% in the casting and the transverse directions and ∼20% in the direction perpendicular to the casting plane.     

Effect of Heating Rate on Pore Shrinkage in Yttria-doped Zirconia

The advantage of fast firing depends not only on the finer grain-size effect at the sintering temperature, but also on the smaller pore-size effect which results from the restricted pore growth. Pore growth due to the differential sintering during heating was observed to occur at relatively low temperature regions, and rapid passing through the region restricted the pore growth and enhanced the densification.     

Diffusion Sintering: I, Initial Stage Sintering Models and Their Application to Shrinkage of Powder Compacts

Consideration is given to several geometrical models that contribute to shrinkage. Various shapes of particles, vacancy sinks, and diffusion paths are described as they affect sintering shrinkage. These simplified models are extended to compacts of nonuniform particles so that much of the kinetics of sintering of a substance can be determined by measuring shrinkage rates of powder compacts. A nonideal compact may sinter as though it had once been an ideal compact after a specific amount of shrinkage has occurred. This shrinkage is characteristic of the particular compact and its origin and is independent of temperature.     

近球状氧化铝微粉压制品中各向同性烧结收缩

粉末的晶粒形状在收缩性能中是重要的，而且各向同性的收缩率可以通过二次CIP处理来达到。     

Sintering Stress of Nonlinear Viscous Materials

An analytical model for the sintering stress of materials characterized by a nonlinear viscous behavior during densification is proposed. The growing applications in the field of nanosized powders processing (in particular, consolidation of high surface area components used in supercapacitors, rechargeable batteries, gas absorbers) have renewed the interest in this fundamental parameter of sintering science, because of the sintering stress’ characteristic inverse proportionality with respect to the powder particles radius. This increase in the magnitude of the sintering stress is also responsible for power‐law creep being the mechanism that underlies densification even without the application of any additional external load, and therefore for a nonlinear viscous behavior of the solid material. The analytical treatment of problems involving nonlinear viscous materials has traditionally involved complex self‐consistent methods and approximations, unless the local case of an isolated pore embedded in a fully dense skeleton was considered. The paper proposes a simple first‐order iterative method that allows the derivation of both bulk modulus and sintering stress of a material containing an arbitrary amount of pores, as functions of porosity and of the material's nonlinearity parameter, namely strain rate sensitivity. An expression for densification kinetics is also obtained and compared with experimental data.     

Effect of Shear Stress on Sintering

The effect of small uniaxial stresses on the sintering of CdO powder compacts was studied using a loading dilatometer. Compacts of two different green densities were sintered at 1123 K and subjected to stresses between 0 and 0.25 MPa. Densification and creep occur simultaneously, and the effects of these two processes can be separated. Between relative densities of 0.5 and 0.9, the dependence of the uniaxial creep rate on density can be described in terms of a stress intensification factor which depends exponentially on the porosity but is independent of the grain size. Comparison of the densification and creep rates permits definition of the sintering stress, which is found to decrease with increasing density, and verification of the Zener relation. The stress and grain size dependence of the creep rate, and the grain size dependence of the densification rate, support grain-boundary diffusion as the rate-controlling step in both processes.     

镍铁渣砂对泡沫混凝土孔结构及收缩开裂性能的影响

本文采用镍铁渣机制砂(简称镍铁渣砂)制备泡沫混凝土,研究镍铁渣砂对泡沫混凝土抗压强度、变形及收缩开裂的影响,并采用SEM、X-CT研究泡沫混凝土微观结构。结果表明,镍铁渣砂掺量为5%(质量分数,下同)时泡沫混凝土的抗压强度最高,而镍铁渣砂掺量超过10%时会引入界面缺陷,降低混凝土的强度。镍铁渣砂能够约束基体的变形,减少水泥用量,降低泡沫混凝土的收缩,提高其抗裂性能。当镍铁渣砂掺量由0%增加到20%时,泡沫混凝土的抗裂等级由V级提高到II级。镍铁渣砂具有作为泡沫混凝土生产原料的潜力。