Phenomenological study and modelling of wick debinding

Wick debinding employs capillary suction (via a surrounding wicking powder) to remove the liquid binder phase from powder injection moulded parts (known as a compact). Experimental measurements of binder distribution within the compact during debinding highlight flaws in previous wick debinding models. The spatially uniform distribution of binder observed consistently during debinding indicates that it is removed in order of pore size regardless of location in the compact. A model is proposed which gives good agreement with 1-D experimental data of binder distribution. Key parameters of the model are the permeability of the wicking powder and the relationship between the capillary pressure, saturation and relative permeability of the compact.     

Micro Structural Investigations and Mechanical Properties of Macro Porous Ceramic Materials from Capillary Suspensions

Recently, we have introduced a novel, material‐independent processing method for producing macro porous ceramics with capillary suspensions as a stable precursor. A capillary suspension is a three‐phase system where a small amount of an immiscible secondary liquid is added to a suspension resulting in the formation of a sample spanning particle network. This technology provides open porosities well above 50% and pore sizes ranging from 0.5–100 μm. Here we focus on microstructure formation in the capillary suspensions and its impact on mechanical strength of the corresponding sintered parts. Based on the rheological data and SEM‐images, three regimes (I, II, III) are identified with distinctly different flow properties of the wet suspension and characteristic structural features of the sintered ceramic parts depending on the amount of added secondary liquid phase. The average pore size increases and the pore size distribution changes from monomodal (I) to bimodal (II) and broad multimodal (III) with increasing amount of secondary liquid phase. A clear correlation between the yield stress of the wet suspension and the porosity and pore size is observed for regime (I) and (II). Compressive and flexural strength as well as the Young's modulus monotonically decrease with increasing amount of the secondary liquid phase. Absolute values are mainly determined by the porosity and are well predicted by the Gibson & Ashby model for samples corresponding to regime (I) and (II). The broad pore size distribution in regime (III) results in a significantly lower mechanical strength.     

Modeling of mass transfers in a porous green compact with two-component binder during thermal debinding

Mass transfers and phase changes of two-component binder in a porous green compact during thermal debinding process are modeled. The evaporation of low molecular weight (LMW) component and volatile fragments, the thermal degradation of high molecular weight (HMW) component, the capillary driven and pressure driven liquid phase transports, the binary diffusion in solutions, the convection and diffusion of gas phases, and the heat transfer in a porous medium are captured in the model. The model is validated with experimental data. The simulated results show that mass transfers during the early stage of thermal debinding are mainly due to capillary driven and pressure driven liquid transports. During the final stage of thermal debinding, both convective liquid and gas transports are important in binder removal. The developed model provides physical understanding of binder removal mechanisms that are essential for process optimization.     

Numerical and experimental investigations on thermal debinding of polymeric binder of powder injection molding compact

A mathematical model is established to describe the thermal debinding process of polymeric binder from a powder injection molding compact. The model takes into account of the thermal degradation of liquid polymer into liquid volatile fragment, the evaporation of liquid volatile fragment, the capillary driven liquid phase transport, the binary diffusion in solution, the convection and diffusion of gas phases, and the heat transfer in a porous medium. The proposed model is solved numerically based on a finite volume method and validated with experimental data. Based on the numerical results, the binder removal, the pressure buildup, the binder distribution, the mass transfers, and the removal mechanisms during thermal debinding are studied.     

Kinetics and equilibrium of Eco-debinding of PZT ceramics shaped by thermoplastic extrusion

Thermoplastic extrusion can present an alternative shaping process to a well-established powder pressing or tape casting method for lead zirconate titanate (PZT) ceramic sensor structures, if a complex debinding step can be achieved in an economic way.In this study PZT ceramic parts were made by thermoplastic extrusion using ethylene vinyl acetate and paraffin as a binder. Samples were thermally debinded according to different programs with and without a powder bed. A modified second order decomposition kinetics model is proposed which is especially suitable for describing partial debinding. Also a novel simplified model for wick-debinding is proposed to evaluate the contribution of a capillary extraction mechanism.Results show that isothermal debinding leads to partial debinded parts with a well-defined equilibrium residual binder, which is a function of a time-temperature program. We demonstrate that 90% of the binder can be removed at only 200 °C, even for a multicomponent binder system.     

Binder Distribution in Ceramic Greenware During Thermolysis

Capillary forces were shown to influence the distribution of polymer-plasicizer mixtures within ceramic green bodies during binder thermolysis. Isothermal thermogravimetric analysis was performed on tape-cast sheets of an aluminapoly(vinyl butyrall-dibutyl phthalate composite and direct observations were made of the binder distribution and pore growth after partial pyrolysis. This led to the investigation of a model system, an alumina-eicosane composite, by similar experimental techniques. The early stage of binder removal was found to be similar to the drying of particle beds in which capillary forces draw liquid into the smaller pores at the surface. The morphology of the binder distribution produced by these processes dictates which mass-transfer resistances may be controlling in binder burnout. A model is described that determines the length scale over which capillarity acts based on measurable physical parameters of the binder system and the packing of the ceramic particles.     

Influence of the aging time of yttria stabilized zirconia slips on the cracking behavior during drying and green properties of cast tapes

Tape-casting process was used to produce yttria stabilized zirconia (YSZ) substrates in an aqueous system using a low amount of an acrylic latex binder. Concentrated suspensions with different aging times were cast, and the influence of the slip aging time on the drying kinetics and cracking behavior of the tapes were studied. In addition, the effect of the slip aging time on the properties of the resultant green tapes was investigated. The latex particles consolidated by coalescence during the aging time of the slips and resulted in an increase in the smaller pore size of the cast tapes. The pore radius increased with increasing the slip aging time up to 14 days thereby decreasing the capillary pressure in the liquid. Aging times over 14 days did not change the pore radius and consequently the capillary pressure. The capillary tension drove the consolidation; the tapes produced from slips with lower aging times which had higher capillary pressure shrank more, had lower pore volume and consequently higher green density. Cracking was found in tapes prepared from slips with aging times shorter than 14 days; the crack area decreased with increasing the slip aging time. For slip aging time ≥14 days cracking was not observed. Aging before casting up to 14 days reduced cracking in tapes prepared with low amounts of latex; however, the lower capillary pressure resulted in low green density of the cast tapes.     

Effect of particle size distribution on stress development and microstructure of particulate coatings

The role of pigment particle size distribution on stress and microstructure development was studied for coatings prepared from aqueous suspensions of ground calcium carbonate (GCC) and latex binder. Stress development was monitored using a modified beam deflection technique under controlled environment. Microstructure was characterized by scanning electron microscopy (SEM) and cryogenic SEM. For coatings containing only GCC particles and no latex, a wide particle size distribution resulted in a significant particle size gradient in the cross-sectional microstructure and irregular stress development. With latex addition, uniform microstructures were observed in coatings with either wide or narrow GCC particle size distribution. GCC/latex coatings prepared using GCC with a wide particle size distribution developed a higher stress than those prepared using GCC with a similar average particle size but a narrow particle size distribution. The higher stress is related to the particle packing that results in smaller pore sizes and larger capillary pressures that drive compaction. In coatings prepared with the same GCC particles but different latex binders, the stress and cracking behavior of the coating depends on the latex properties.     

Effect of Pore Distribution on Microstructure Development: III, Model Experiments

Model experiments have been conducted on a series of alumina samples in which the microstructures have been tailored to conform to the classical configuratins depicted in the models of final-stage sintering. Simultaneous measurements of sintered density, grain size, pore number density, and pore size distribution were made as a function of sintering time at constant temperature (1850°C). The data supported a model of grain-boundary-diffusion-controlled densification and surface-diffusion-controlled grain growth. An atom flux equation for grain-boundary diffusion transport was deduced from the data. The kinetics analysis highlights the importance of incorporating the number of pores per grain as an independent variable in mechanistic studies of final-stage sintering. The number of pores per unit volume was identified as a critical factor influencing densification kinetics. The effect of pore distribution on microstructure development was simulated for comparison with the data obtained from the model experiments.     

Shrinkage of Tape Cast Products During Binder Burnout

During sintering of tape cast products, anisotropic shrinkage occurs, which can be attributed to an anisotropic green tape structure concerning particle and pore orientation. Little is known about the shrinkage during binder burnout (BBO) and its relation to the microstructure of green tapes including the binder–plasticizer phase. Therefore, the article determines the shrinkage behavior of green tapes derived from alumina powders with different particle shape during binder burnout and prefiring in all spatial directions. The shrinkage after prefiring relative to the green and the debindered states is also discussed. The interrelation between shrinkage behavior and microstructure is investigated in dependence on different process parameters and specifically on the thermal behavior of the binder–plasticizer phase in the green tapes. It is shown that the subtraction of the BBO shrinkage from the total shrinkage results in completely different data for the sintering shrinkage anisotropy in z direction.     

Binder Burnout from Layers of Alumina Ceramics Under Centrifugal Force

The effect of centrifugal force on the delamination of layered green body during binder burnout has been studied in terms of internal gas pressure resulting from gas flow kinetics in porous media. Here, a sheet of nano-particle of γ-alumina was prepared by tape casting using polyvinyl butyral (PVB, binder) and dibutyl phthalate (DBP, plasticizer). Because of the fine pore structure (average pore size of 25 nm), molecular flow kinetics was applied to estimate internal pressure arising from evolved gases. Assuming that delamination is related to internal pressure, the interfacial strength of the layer was estimated. This strength was modified by applying a compressive pressure controlled by a centrifugal force. Because of the increased interfacial strength, delamination was suppressed, even during rapid heating. The compressive pressure required increased proportionally with increasing heating rate, a tendency that agreed with the expectation based on the gas flow kinetics in porous media.     

Partial wick-debinding of low-pressure powder injection-moulded ceramic parts

Al₂O₃-based green bodies were shaped using low-pressure injection moulding. The binder content and the binder distribution during the thermal debinding inside a wicking embedment were analyzed. A distinct trailing front, which separates the binder-lean and binder-rich regions, was observed. This kind of binder distribution forms suddenly, after the moulded piece is heated above the melting point of the binder and is then cooled down. Mechanisms that can explain the observations are presented. The non-uniform binder distribution is explained by a capillary extraction of the binder with two different mobilities, which depend on the size of the pores inside the moulded piece. A sudden loss of binder at the beginning of the debinding process is the result of exudation, caused by a large thermal expansion of the binder as it melts. During cooling, the binder solidifies, which significantly affects the binder distribution due to a contraction of the binder.     

间歇搅拌槽中柠檬酸水溶液吸附传质动力学

柠檬酸水溶液在吸附树脂颗粒中的扩散机理与吸附树脂颗粒表面结构、微孔大小和表面能分布有关,特别是当液相浓度较高时,传质受固相内的扩散速率控制.本文采用Glueckauf和Hill建议的线性推动力近似模型和修正线性推动力近似模型,在液相浓度较高、相平衡关系呈非线性时,得到吸附和解吸阶段模型的解析解和曲线图.     

Effect of Pore Distribution on Microstructure Development: I, Matrix Pores

A model has been developed to describe the effect of the matrix (first-generation) pore distribution on microstructure development in the final stages of sintering. A model of simultaneous densification and grain growth was used to predict the effects of the number of pores per grain and the pore size distribution on microstructure evolution. Increasing the number of pores per grain was predicted to increase the densification rate, the grain growth rate, and the relative densification rate/grain growth rate ratio. Narrowing the pore size distribution was predicted to inhibit grain growth initially and to increase the densification rate indirectly. Overall, the pore distribution was predicted to have a strong influence on microstructure development and sintering kinetics.     

Modelling nucleation in wet granulation

Nucleation is the first stage in any granulation process where binder liquid first comes into contact with the powder. This paper investigates the nucleation process where binder liquid is added to a fine powder with a spray nozzle. The dimensionless spray flux approach of Hapgood et al. (Powder Technol. 141 (2004) 20) is extended to account for nonuniform spray patterns and allow for overlap of nuclei granules rather than spray drops. A dimensionless nuclei distribution function which describes the effects of the design and operating parameters of the nucleation process (binder spray characteristics, the nucleation area ratio between droplets and nuclei and the powder bed velocity) on the fractional surface area coverage of nuclei on a moving powder bed is developed. From this starting point, a Monte Carlo nucleation model that simulates full nuclei size distributions as a function of the design and operating parameters that were implemented in the dimensionless nuclei distribution function is developed. The nucleation model was then used to investigate the effects of the design and operating parameters on the formed nuclei size distributions and to correlate these effects to changes of the dimensionless nuclei distribution function. Model simulations also showed that it is possible to predict nuclei size distributions beyond the drop controlled nucleation regime in Hapgood's nucleation regime map. Qualitative comparison of model simulations and experimental nucleation data showed similar shapes of the nuclei size distributions. In its current form, the nucleation model can replace the nucleation term in one-dimensional population balance models describing wet granulation processes. Implementation of more sophisticated nucleation kinetics can make the model applicable to multi-dimensional population balance models.     

2-D simulation of wick debinding for ceramic parts in close proximity

In wick debinding the binder phase is removed from an injection moulded ceramic part by the capillary action of a wicking powder. The binder is withdrawn in the liquid phase in the form of a front, and fronts originating from parts in close proximity may collide, thereby reducing the potential for the binder to be extracted by the capillarity of the wicking powder. The model presented by Somasundram et al. (2008) is employed here to simulate isothermal debinding of cylindrical parts located in close proximity, using the level-set method to track the progress of binder fronts. Two-dimensional cases were simulated using a commercial finite element solver and the results are compared with preliminary experimental results which show the potential of the model and demand further experimental investigation. A simplified model is also presented which encapsulates the main features of the detailed model for simple geometries.     

Characteristics of poly(ether ether ketone) microporous membranes prepared via thermally induced phase separation (TIPS)

The morphology and bulk properties of microporous membranes based on poly (ether ether ketone) (PEEK) have been investigated as a function of initial casting composition and thermal and mechanical processing history. Membranes were prepared via solid—liquid phase separation of miscible blends of PEEK and polyetherimide (PEI), with subsequent extraction of the PEI diluent. Scanning electron microscopy studies revealed a microporous morphology with two distinct pore size scales corresponding to diluent extraction from interfibrillar and interspherulitic regions, respectively. The membrane structure was sensitive to both initial blend composition and crystallization temperature, with the resulting pore size distribution reflecting the kinetics of phase separation. For membranes prepared with lower initial diluent content or at lower crystallization temperatures, mercury intrusion porosimetry indicated a relatively narrow distribution of fine interfibrillar pores, with an average pore size of approximately 0.04 microns. Membranes prepared at higher diluent content or at higher crystallization temperatures displayed a broad pore distribution, with a sizeable population of coarse, interspherulitic pores (0.1 to 1 μm in size). Uniaxial drawing led to a fibrillated network structure with markedly higher water flux characteristics compared to the as-cast membranes. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2347–2355, 1997     

Fractal Pore Network Simulation on the Drying of Porous Media

Based on the knowledge of fractal geometry, physics of flow through porous media, and transport process principle, a fractal pore network model for the drying process of a natural porous body was established in this article. This model takes various factors into consideration, such as liquid-phase flow, vapor-phase diffusion, temperature gradient, and pore microstructure characteristic. The drying dynamics characteristics of potato slices were obtained by the simulation of a fractal pore network model. The simulation results of the fractal pore network model were contrasted with those of a regular one and the experimental data, respectively. The wet patches were observed on the potato slices during the drying experiments, and it was validated by the drying simulation. The results indicate that the drying kinetics from the fractal pore network model, as well as the distributions of moisture and temperature inside the porous body, are more consistent with that of the drying experiments than that of the regular one, and the throat size distribution in the pore network of the porous media has a notable influence on the drying process.