Binder burnout-material-process interaction during sintering of MnZn-ferrites

The binder burnout process step for MnZn-ferrite materials has been investigated in order to explain the origin of the binder burnout cracks that occur during the sintering of MnZn-ferrites. The binder burnout reaction extends to temperature regions where oxidation of the ferrite material (i.e. Mn²⁺≧Mn³⁺≧Mn⁴⁺) occurs simultaneously. Under such conditions, the oxidation of the binder is favoured relative to that of the ferrite until the binder burn out oxidation has been completed. As a consequence, the ferrite oxidation occurs at higher temperatures and at higher reaction rates. The associated material shrinkage rates are therefore also increased, enhancing defect formation and eventual crack development within the products. When ferrite products of large dimensions are considered, the developed temperature gradients across the body provide a position dependency of these reactions, which results in the development of high stresses and ultimately in product cracking.     

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.     

Space-resolved study of binder burnout process in dry pressed ZnO ceramics by neutron imaging

A novel non-destructive space-resolved method for studying binder burnout is presented. Debinding of dry pressed zinc oxide green bodies was analyzed by neutron imaging. Neutrons in contrast to X-rays allow penetrating samples with large dimensions up to 80?mm and at the same time detecting variations in the organics content as small as 0.1?wt%. We used neutron radiography (2D) and tomography (3D) to investigate with a spatial resolution in the order of 100?μm the distribution of binder during burnout under three different conditions. First a reference case with green bodies placed on a plate in a lab furnace. Second, a configuration with a reduced volume of air with green bodies enclosed in a sagger and third a green body placed on an improved supporting structure. The method is extendable to other particulate materials and green bodies shaped by processes involving organic binder such as casting, extrusion and injection.     

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.     

Determination of Binder Decomposition Kinetics for Specifying Heating Parameters in Binder Burnout Cycles

The decomposition kinetics of poly(vinyl butyral) binder from barium titanate multilayer ceramic capacitors with platinum metal electrodes were analyzed by thermogravimetric analysis as a function of the heating rate. The activation energy and pre-exponential factor for the decomposition kinetics were determined from two types of integral equations, from the Redhead method, and from the variation in heating rate method. The accuracy of the kinetic parameters determined from these methods was then evaluated for describing the observed rate of binder decomposition. Although the individual models yielded very different kinetic parameters, all were capable of describing the experimental data within ±15% accuracy. The kinetic parameters were then used in a coupled transport and kinetic model for describing the buildup of pressure within the ceramic green body as a function of the heating cycle. A methodology based on calculus of variations was also developed to predict the minimum duration for the binder burnout cycle.     

Relationship between mechanical properties of ceramic green body and structures of photo-cured acrylate polymer for ceramic 3D printing based on photo polymerization

3D printing technologies using photo polymerization of photo-curable monomer and oligomers as an organic binder have been studied to fabricate ceramics with a complicated shape. For minimizing distortion of ceramics during the manufacturing process and fast 3D printing of ceramic green body, high mechanical strength of ceramic green body and fast photo polymerization of the photo-curable resin are required, respectively. In this study, the ceramic green bodies with various compositions were fabricated by photo radical polymerization of the slurry-type resin composed of fused silica bead, and tri- or (and) mono-functional acrylate. Photo radical polymerization behaviors of mono-, tri-functional acrylate monomer, and blend of two monomers were analyzed by photo-DSC and FT-IR measurements. The structure analysis of photo-cured polymers made by each monomer was performed by thermo-mechanical analysis. Through mixing of mono- and tri-functional acrylate monomer, we confirmed that polymerization rate more increased compared with those of only mono-, tri-functional monomer. Unreacted vinyl groups in the polymers prepared with blend of two monomers decreased by an addition of mono-functional monomer in tri-functional monomer. The polymers prepared with the blend showed higher storage modulus and broader viscoelastic behavior compared to those fabricated with tri-functional monomer. Thus, to achieve high fracture strength of the green body, we verified that the photo-cured polymer in the green body should have high crosslinking density and low free volume based on reduction of unreacted vinyl groups in the polymer. Additionally, through analysis of cross-sectional SEM of the green body, we confirmed that acrylate monomer should include proper content in the slurry-type resin to maximize the fracture strength of the body regardless of the type of the used acrylate monomer. This was because low content of acrylate monomer in the slurry-type resin makes it difficult to form neckings composed with photo-cured polymers between the silica beads.     

Thermoplastic Green Machining for the Fabrication of a Piezoelectric Ceramic/Polymer Composite with 2-2 Connectivity

A piezoelectric ceramic/polymer composite with 2-2 connectivity was fabricated by thermoplastic green machining. A thermoplastic body, consisting of 60 vol% lead zirconate titanate ceramic particles (PZT) and 40 vol% thermoplastic binders, was computer numeric controlled-machined, creating periodic channels in the green PZT body. Following thermal treatment (binder burnout and sintering), a 25 vol% array of 147 μm thin PZT slabs with an aspect ratio of seven separated by 442 μm channels was fabricated. The channels were infiltrated with epoxy resin, in order to fabricate the PZT/epoxy composite with 2-2 connectivity. This novel process was evaluated in terms of the machinability and sinterability of the thermoplastic PZT compound. Also, the electromechanical properties of the PZT/epoxy composite were measured.     

Determination of rapid heating cycles for binder removal from open pore green ceramic components

Abstract

Rapid heating cycles have been determined for the thermal removal of binder from open pore green ceramic components. The samples are multilayer green bodies with barium titanate as the dielectric, and the binder consists of poly(vinyl butyral) and dioctyl phthalate. The kinetics of binder decomposition, the gas permeability of the green body and the conditions at failure of the green body were determined from a combination of experiments and modelling. These results were then used with an algorithm based on variational calculus to develop successful rapid heating cycles without causing failure of the component.     

Improved Compaction in Multilayer Capacitor Fabrication

The most common defects found in multi-layer ceramic capacitors are derived from residual porosity formed when solvents and binders are released from a ceramic green body. Without a well-controlled compaction technique, defects between sheets in the stacked body are often present, leading to lamination problems. This paper suggests alternative approaches, using compaction applied before and/or after the process of binder burnout with the intention of reducing the number of stacking-generated defects and the volume of voids formed after binder burn-out. Barium titanate tapes and stacked multilayer electroded discs have been investigated and characterised by microstructural examination and density measurement. The resulting properties are described, and the behaviour of the multi-layers discussed in terms of the microstructure and processing procedures. It is shown that the porosity content of sintered samples compacted at 300 MPa is approximately half that of samples compacted at 30 MPa when both are sintered at 1300°C for 2 h. A further improvement of densification can be achieved by a postcompaction treatment after binder burn out.     

A process control algorithm for reaction-diffusion minimum time heating cycles for binder removal from green bodies

Minimum time heating cycles have been simulated for binder removal from ceramic green bodies where the product of binder decomposition exits the green body via diffusion. The model consists of the reaction-diffusion partial differential equation, ordinary differential equations describing the reaction kinetics and a heating function, and algebraic equations, one of which imposes a constraint on concentration or pressure to avoid failure of the green body. Two solution approaches were compared: an earlier approximate method based on the pseudo-steady state assumption combined with a variational calculus algorithm and a new approach based on the finite element method combined with a process control algorithm. The agreement between the two solution strategies reinforces the validity of the pseudo-steady state approximation and the utility of the process control methodology. The latter, which was also applied to problems in which no approximate solution was obtainable, is thus a general method for obtaining minimum time heating cycles.     

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.     

New Freeze-Casting Technique for Ceramics with Sublimable Vehicles

A new ceramic freeze-casting technique capable of manufacturing near room temperature with a sublimable vehicle was accomplished. Fluid-concentrated slurries of Al₂O₃ powder in molten camphene (C₁₀H₁₆) were prepared at 55°C. These slurries were quickly solidified (frozen) at room temperature to yield rigid solid green bodies, followed by frozen camphene removal by sublimation (freeze-drying) with negligible shrinkage. Sintering without any special binder burnout process yielded sintered bodies with over 98% theoretical density. The proposed advantages include (1) elimination of extremely cold temperatures, (2) elimination of troublesome binder burnout process, and (3) fast manufacturing cycle due to quick solidification.     

Effect of Decomposition Kinetics and Failure Criteria on Binder-Removal Cycles From Three-Dimensional Porous Green Bodies

A previously developed algorithm can be used to predict the minimum time for the thermal removal of binder from porous green ceramic bodies. The algorithm combines a variational statement on internal pressure, which is generated by binder decomposition, with a transport model that treats the convective flow of binder decomposition products in a porous medium. The minimum time heating cycles depend on a number of coupled transport, kinetic, and dimensional parameters. The predicted minimum time heating cycles depend on the decomposition kinetics (activation energy, preexponential factor, and decomposition mechanism) and on the temperature and pressure at which failure occurs in the green body.     

Pyrolysis of polyvinyl butyral (PVB) binder in thermoelectric green tapes

The pyrolysis of polyvinyl butyral (PVB) binder and other organic additives in thermoelectric green tapes, are analysed through differential thermal analysis (DTA), thermogravimetric analysis (TGA) and published results of fourier transformer infrared spectroscopy (FTIR). Based on these analyses the optimum balance of binder degradation mechanism, heating rate, burnout temperature and burnout atmosphere were determined. The maximum upper temperature at which pyrolysis can take place in an oxidising atmosphere, was imposed at 450°C, to avoid the risk of oxidising the thermoelectric material above this temperature, which could degrade its thermoelectric properties. Thermoelectric cast green tapes made with PVB formulation were found to leave char residue after pyrolysis at 450°C, estimated to be almost 20% of the total PVB content in the tape. Different pyrolysis atmospheres of air, argon, CO₂ and Ar/H₂O were used to minimise the char content. The best pyrolysis for the PVB was obtained with the use of CO₂ atmosphere at 450°C with a hold-out time of 5 h, which reduced the char residue to only 1%. Even with this small percentage, the char residue was in the form of a very fine black powder (soot) which covered the thermoelectric material powder in the tape, preventing its densification at the later stages of the sintering process. It was therefore concluded that the PVB system was not a suitable binder candidate to be used in the fabrication of thermoelectric generator by the tape casting method.     

In-situ investigation on crack-initiation and deformation of Al2O3 green bodies during dewaxing process by combined OCT-TG-FTIR and TMA

The dewaxing process is used to remove an organic binder from the ceramic green bodies before sintering, which occasionally generates cracks. The crack formation behavior depends on various factors including softening and decomposition of the organic binder, generation of gases, and strength degradation of the green body thereby. Herein, this correlation was investigated to elucidate the crack formation behavior during the dewaxing process using two types of Al₂O₃ green bodies; one is added with polyvinyl butyral (PVB) and stearic acid (SA) and the other is with paraffin. The internal structures of Al₂O₃ green bodies during dewaxing were observed using optical coherence tomography (OCT), and the generated gases were analyzed simultaneously using a thermogravimetric (TG) analyzer and Fourier transform infrared (FTIR) spectroscopy (TG-FTIR). The mechanical properties of the green bodies were investigated at RT–600 °C using a thermomechanical analyzer (TMA). The weight change occurred in both the green bodies with formation of gases depending on the type of the binder. In the OCT studies, cracks were observed with substantial deformation in the PVB/SA-added green body during the dewaxing, whereas no cracks were seen in the paraffin-added one. The TMA investigation showed that the paraffin-added sample possessed higher strength and better structural stability than the PVB/SA-added one throughout the dewaxing, leading to the crack-free green body of the former. Therefore, the crack-initiation and deformation behaviors of the green bodies were significantly affected by the type of the binder used. The combination of the in-situ observations using the combined OCT-TG-FTIR system and the mechanical properties measurement using TMA was found to be effective in verifying the structural stability of the green bodies during the dewaxing.     

Laser-Sintered Barium Titanate

Laser sintering of alkoxy-derived ultrafine BaTiO₃ powders was investigated. The temperature increases of the sample with laser irradiation were measured with a thermocouple. It was found that laser irradiation could generate enough heat to sinter ceramics. A slurry was prepared by mixing an alkoxy-derived BaTiO₃ powder, binder additives, solvent, and plasticizer. The slurry was tape cast and dried to give a green sheet. The green sheet was laser sintered and was then characterized by SEM, XRD, and density measurements. The effect of burnout before laser irradiation and the characteristic microstructure of laser-sintered BaTiO₃ are described.     

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.     

Extraction of binders from green ceramic bodies by supercritical fluid: influence of the porosity

Organic additives can be extracted from extruded or injection-moulded ceramic parts by using supercritical fluids leading to defect free green parts in a short time. A model previously defined, that predicts kinetics of extraction of solid binder molecules by solubilization and diffusion of solubilized species has been modified with a corrective term taking into account the capillary migration of a liquid organic phase according to the microstructure (pore size distribution, tortuosity) of the green sample and to the characteristics of the liquid phase (viscosity, surface tension). An analytical expression of this corrective term is proposed, for a paraffin binder and for three alkanes which are representative of its molecular distribution. The corrective term is related to the length scale over which the liquid binder flow occurs and to its linear velocity. The modified model allows the prediction of the kinetics of extraction of a given paraffin binder, by using the effective diffusion coefficient of the binder, determined with large pore size green samples and a capillary factor, calculated from binder and microstructure characteristics of the green part.     

Reaction-Controlled Binder Burnout of Ceramic Multilayer Capacitors

Reaction-controlled binder burnout of ceramic multilayer capacitors (CMCs) is conducted in a series of small-scale experiments. the burnout process is followed by monitoring the weight of the CMCs. The observed maximum weight loss rate, obtained with a conventional linear heating process, is used to design a weight-time program for a reaction-controlled process. In this process the actual CMC weight is controlled to closely follow the weight program by means of continuous control with the batch temperature. The advantages of this procedure are discussed. The results of the present study, although being restricted to oxidative burnout of a special product, are thought to be of some use for other ceramic burnout situations. The relevant parts of controller design are discussed together with some comments on scale-up.     

Studies in the polarographic and coulometric behaviour of aromatic nitro compounds—III. nitrosobenzene in ethanol,acetone and dioxan

The green nitrosobenzene monomer is reduced polarographically to phenylhydroxylamine in the pH range 4—9. Though this reduction is known to be a two-electron process, coulometry invariably gives a lower value of n because of the reaction of unreacted nitrosobenzene and the phenylhydroxylamine formed. The green monomer is attacked by mercury in acid medium. In alkaline medium, the green monomer undergoes a change that follows first-order kinetics with respect to nitrosobenzene. The rate of the transformation depends on the solvent. It decreases in the order acetone > ethanol > dioxan.