Sintering Shrinkage Research Articles

Research in the Field of Preparing Molded and Unmolded Refractories Based on High-Alumina HCBS. Part 9. Preparation and Properties of Mixed HCBS Composition: Fuzed Bauxite-Corundum, Quartz Glass, Reactive Alumina. Dilatometric Study of Materials Based on Them

Combined wet comminution is used to prepare HCBS of composite composition, including fuzed bauxite-corundum as a basic material (89%) and very fine fuzed quartz (11%). Specimens are also based on this HCBS with an increased Al2O3 content due to introducing 10 – 30% reactive alumina into their composition. Dilatometric studies are performed making it possible to establish that for materials of all compositions there are four typical nonisothermal heating temperature ranges: normal thermal expansion (up to 900°C), sintering or shrinkage (900 – 1300°C), mullitization or growth (1300 – 1450°C), and sintering (shrinkage) of mullitized material (1450 – 1500°C).

Dilatometric study of anisotropic sintering of alumina/zirconia laminates with controlled fracture behaviour

Al2O3 and ZrO2 monoliths as well as layered Al2O3/ZrO2 composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.

Preparation of micro-porous bioceramic containing silicon-substituted hydroxyapatite and beta-tricalcium phosphate

Dimensional instability caused by sintering shrinkage is an inevitable drawback for conventional processing of hydroxyapatite (HA). A new preparation method for biphasic calcium phosphates was developed to increase micro pores and biodegradation without significant dimensional change. Powder pressed HA discs, under 100MPa, were immersed in a colloidal mixture of tetraethoxysilane (TEOS) and ammonium hydroxide for 10min, followed by drying, and then were sintered at 900°C, 1050°C, and 1200°C, respectively. Comparing with pure HA discs, the newly prepared product sintered up to 1200°C contained silicon substituted HA, beta-tricalcium phosphate, and calcium silicate with better micro-porosity, high specific surface area, less sintering shrinkage and the strength maintained. The cytocompatibility test demonstrated a better viability for D1 mice stem cells cultured on TEOS treated HA for 14days compared to the pure HA. This simple TEOS sol-gel pretreatment has the potential to be applied to any existing manufacturing process of HA scaffold for better control of sintering shrinkage, create micropores, and increase biodegradation.

Sintering behavior of garnet-type Li7La3Zr2O12-Li3BO3 composite solid electrolytes for all-solid-state lithium batteries

In this study, Li3BO3 glass ceramic was utilized as a sintering additive for producing garnet-type Li7La3Zr2O12 solid electrolytes by low temperature sintering. The shrinkage and wetting behaviors of the Li7La3Zr2O12-Li3BO3 composite powder during heating were analyzed by dilatometry. The sintering shrinkage of the Li7La3Zr2O12-Li3BO3 composite powder occurred in two stages and was related to the densification of the composite. It was also closely related to the wetting behavior of the Li3BO3 glass. The sintering of Li7La3Zr2O12-Li3BO3 from around 700°C is driven by the viscous sintering of Li3BO3 glass, while sintering above 850°C is due to particle rearrangement in Li7La3Zr2O12 as well as the solid-state sintering of Li7La3Zr2O12 after the melting of Li3BO3. The density of the Li7La3Zr2O12-8wt% Li3BO3 composite sintered for 8h at 1100°C was 86.4% of the theoretical density, whereas the ionic conductivity was 1.94×10−5Scm−1.

Investigation of Sintering Shrinkage in Binder Jetting Additive Manufacturing Process

Binder jetting process is an additive manufacturing technology that produces objects layer by layer from 3D digital data. After printing process, the secondary post-processing operations, including curing and sintering, are used to solidify the object. The linear dimensional accuracy is one of the most important end-product qualities in manufacturing process. However, it is not easy to control it since the whole process combines several steps and covers different disciplines. Sintering process can affect the shrinkage rate significantly through using different sintering profiles. This project focuses on investigating the effect of sintering parameters on linear dimensional accuracy with stainless steel 316L. The Taguchi method is used to design 9 groups of orthogonal experiments. After that, the results are converted to signal-to-noise (S/N) ratio and using analysis of variance (ANOVA) to analyze. There are 3 sets of recommended sintering parameters to achieve the best dimensional accuracy for each axis and there is also 1 set of recommended sintering parameters which considers dimensional accuracies of all 3 axes. Comparing with the shrinkage rates measured using default sintering profile, the dimensional accuracies are improved.

Ultra low-density mullite foams by reaction sintering of thermo-foamed alumina-silica powder dispersions in molten sucrose

Ultra low-density mullite foams are prepared by thermo-foaming followed by reaction sintering of alumina-silica powder dispersions in molten sucrose. The foaming & setting time, foam rise, sintering shrinkage, porosity, cell size and compressive strength are studied as a function of ceramic powder loading, foaming temperature and magnesium nitrate (blowing agent and setting agent) concentration. Phase pure mullite is produced by reaction sintering at 1600°C. The mullite foams produced without magnesium nitrate have porous struts and cell walls due to improper densification. The magnesium nitrate drastically decreases the foaming & setting time and increases the foam rise and cell interconnectivity. The MgO produced from the magnesium nitrate assists the densification of the mullite as evidenced from the non-porous struts and cell walls at higher magnesium nitrate concentrations. The maximum porosity of 94.92 and 96.28vol.% achieved without and with magnesium nitrate, respectively, is the highest reported for mullite foams.

Binder-Jet Printing of Fine Stainless Steel Powder with Varied Final Density

Binder jetting is an additive manufacturing process that produces weak porous parts that are strengthened through sintering and/or infiltration. This article reports on two different methods of preparing fine 316 stainless steel powder and their impact on the final sintered density and dimensions relative to direct printing into <22 micron powder . The first method uses agglomerates of fine powder. In the second, nylon 12 powders are mixed with the steel powder as a fugitive space holder to increase porosity. Sintered density and sintering shrinkage of agglomerate material are shown to vary with the density of the spread powder bed. Nevertheless, with added nylon, the shrinkage correlates with the shrinkage of the base steel powder, whereas the density depends on the quantity of the nylon. Thus, it is possible to create varied sintered density with compatible shrinkage levels—a key step toward creating binder-jetting systems with spatially controlled porosity.

Preparation of mullite based ceramics from clay–kaolin waste mixtures

Mullite based ceramics were prepared by reaction sintering of mixtures containing kaolin clay and kaolin waste. Phase composition, apparent density, apparent porosity, and modulus of rupture were investigated as a function of sintering temperature and kaolin waste content (varied from zero to 50wt%). The sintering shrinkage behavior was monitored by dilatometry. The results showed that samples derived from formulations containing kaolin waste were composed of acicular mullite and glass phases at 1500°C. Ceramic bodies prepared with kaolin waste (25 or 50wt%) showed better physico-mechanical properties (for sintering up to 1400°C) than the ones of pure kaolin clay. The evidence seems to suggest that this behavior can be due to a liquid phase assisted sintering mechanism associated with a content of mica ranging from 13.3 to 22.4wt%.

Porosity and cell size control in alumina foam preparation by thermo-foaming of powder dispersions in molten sucrose

The foaming characteristics of alumina powder dispersions in molten sucrose have been studied as a function of alumina powder to sucrose weight ratio (WA/S) and foaming temperature. The increase in foaming temperature significantly decreases the foaming and foam setting time and increases the foam volume due to an increase in the rate of –OH condensation as well as a decrease in the viscosity of the dispersion. Nevertheless, the foam collapses beyond a critical foaming temperature, which depends on the WA/S. The sintering shrinkage depends mainly on the WA/S and marginally on the foaming temperature. The porosity (83.4–94.6 vol.%) and cell size (0.55–1.6 mm) increase with an increase in foaming temperature (120–170 °C) and a decrease in WA/S (0.8–1.6). The drastic decrease in compressive strength and modulus beyond a WA/S of 1.2 is due to the pores generated on the cell walls and struts as a result of particle agglomeration. Gibson and Ashby plots show large deviation with respect to the model constants ‘C’ and ‘n’, especially at higher alumina powder to sucrose weight ratios.

Low temperature co-fired LiZrZn ferrites with LBBS glass

LiZrZn ferrite materials (Li0.47Zr0.11Zn0.27Fe2.15O4) were synthesized at 900 °C with different contents of LBBS (Li2O-B2O3-Bi2O3-SiO2) glass. We characterized and analyzed the variation in phase formation, structure, sintering shrinkage percentage and magnetic properties of samples along with the content of LBBS glass. A spinel phase could be formed at 900 °C with LBBS glass, which allowed them to be co-fired with silver in LTCF application. The saturation magnetization M s increased first with increase of LBBS glass, and then decreased. The coercivity H c decreased first from 51.1 to 44 Oe, and then increased to 47 Oe. The real part of magnetic permeability increased from about 21 to 29 in a range frequency of 1 MHz ~10 MHz, and magnetic loss kept a low value of about 0.05.

Fabrication and characterization of bone china using synthetic bone powder as raw materials

The synthetic bone powder was studied as a raw materials for bone china, completely replacing natural bone ash raw materials. The physical and thermal properties of samples obtained by the two bone powders were tested and comparatively studied. Performance tests included pyroplastic deformation, flexural strength, bulk density, sintering shrinkage, water absorption, transmittance, thermal expansion coefficient and the thermal shock resistance. The phase composition and morphology were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicated that using synthetic bone powder could shorten the preparation time, reduce the sintering temperature and result in high-quality bone china. The pyroplastic deformation decreased from 9.14% to 7.92%, the three-point flexural strength increased from 123MPa to 191MPa, the light transmittance (at a 2-mm thickness) increased from 6.7% to 11.2%, the thermal expansion coefficient decreased from 8.24×10−6°C−1 to 7.69×10−6°C−1, and the thermal shock resistance increased from 140°C to 180°C. A continuous interface layer without cracks was produced by using the synthetic bone powder.

Anode-supported solid oxide fuel cells based on Sm0.2Ce0.8O1.9 electrolyte fabricated by a phase-inversion and drop-coating process

The performance of anode-supported solid oxide cells is often limited by gas transport in the thick anode substrate. In this study, anode-supported solid oxide fuel cells with a cell configuration of Ni-Sm0.2Ce0.8O1.9 (SDC)|SDC|Sm0.5Sr0.5CoO3 (SSC)-SDC are fabricated by a combination of phase-inversion tape-casting, direct drop-coating on the green anode, and co-firing. The porous anode substrate is composed of a thin skin layer, a thick finger-like porous layer, and a thin sponge-like porous layer. The skin layer with fine pores offers extended three-phase boundaries (TPBs) at the electrolyte/anode interface while the finger-like porous layer/support provides pore channels for efficient mass transport. Dense SDC electrolyte membranes with a thickness of ∼15 μm are obtained at a relatively low co-firing temperature (1400 °C) due to the large sintering shrinkage of the green anodes. Peak power densities of 0.608 Wcm−2 at 600 °C and 0.789 Wcm−2 at 650 °C have been demonstrated using humidified (3 vol. % water vapor) H2 as fuel and ambient air as oxidant, suggesting that the triple-layered anode effectively decreases the anode polarization resistance and thus enhances the cell performance.

Fabrication and properties of porous silicon nitride wave-transparent ceramics via gel-casting and pressureless sintering

In order to fabricate a radome material with small volume shrinkage and not only good mechanical properties but also excellent dielectric properties, gel-casting and pressureless sintering were adopted to prepare porous silicon nitride (Si3N4) ceramics. By adjusting the solid loading and the additives’ (Y2O3/Al2O3) content, the drying and sintering shrinkage of ceramics could be controlled under 5% and 15%, respectively. Through decreasing temperature and prolonging holding time properly, the flexural strength of porous Si3N4 with few sintering aids could get close to that of ceramics with more additives. Both the flexural strength and the logarithmic dielectric constant showed a linear relation with the porosity of ceramics. Typically, the porous Si3N4 sintered at 1650°C for 3h with only 3wt% sintering additives displayed relatively good performance, attaining a porosity of 39.28%, a flexural strength of 146.0MPa, a fracture toughness of 1.44MPam1/2 and a calculated dielectric constant of 4.03 at a resonant frequency of 14GHz.

Effects of Pore-Forming Agent on Characterization of NiO/YSZ Porous Anode for SOFC

A comparative study has been carried out on the effects of pore-forming agents on the microstructural characteristics and properties including porosity, microstructures, linear sintering shrinkage and mechanical strength of NiO/yttria stabilized zirconia (YSZ). Different pore-forming agents (soluble starch, graphite and activated carbon) were incorporated into NiO/YSZ at various mass mixing ratios (5%, 10% and 15%). The experimental results show that under the condition of same content, sintered samples using activated carbon as pore-forming agent had a higher porosity than that with identical content of the other two kinds of pore-forming agents. Sintered samples with 10.0 wt% activated carbon had a porosity of 35%. Section morphologies show that pores distributed uniformly and connected very well in the anode with activated carbon as pore-forming agent. The mechanical strength of these sintered samples with carbon as pore-forming agent was also higher. With the activated carbon particle size decreasing (from 100 mesh to 320 mesh), the open porosity of porous NiO/YSZ was reduced (from 31.7% to 30.9%). It may be concluded that The activated carbon is the better choice as pore-forming agent for NiO/YSZ anode.

Acicular porous mullite from diatom frustules

We report for the first time using diatom frustules, the porous silica shells of diatoms, as Si precursor for synthesis of porous mullite ceramics with fibrous pore morphology. The porous mullite ceramics prepared by mould pressing method without using any porogen and binder showed high open porosity (up to 60%) due to the highly porous nature of diatom frustules and near zero sintering shrinkage of mullite whiskers. The original circular pores of diatom frustules were turned into fibrous pores of mullite ceramics. The porous ceramics showed flexural strength of 38.6MPa at the porosity of 55.5% when sintered at 1600°C for 3h.

Preparation and properties of AlN ceramic suspension for non-aqueous gel casting

A non-aqueous gel casting process based on the mixed solvent (ethanol and polyethylene glycol) and low-toxicity N,N-dimethylacrylamide (DMAA) was developed for an aluminum nitride (AlN) ceramic. In the present work, rheological properties of non-aqueous concentrated AlN suspensions were investigated in the presence of mixed solvent, dispersant, milling time, monomer and solid loading, in order to screen for the most suitable experimental conditions to obtain a good rheological behavior for gel casting. The results showed that the 50vol% slurry with 0.2wt% dispersant concentration, 2h milling time, 6wt% -monomer content, and a solvent ratio of 3:1, can meet the requirements for the casting process of AlN ceramic slurries. After being dried at 100°C for 1h, the optimum bulk density and maximum flexural strength of the AlN green bodies were as high as 1.97g/cm3 and 18.68MPa, respectively. SEM photographs revealed that the green body had a relative uniform microstructure when the solid loading was 50vol%. The shrinkage and deformation of shaped sintered bodies prepared through gel casting were small after sintering. The sintering shrinkage, apparent porosity, bulk density and flexural strength were 14.8%, 0.22%, 3.21g/cm3 and 310MPa, respectively.

粉体加工プロセスの計算機援用設計と新材料開発

Numerical methods for simulating deformation behavior of powder compacts have been developed aiming to establish the computer-aided design (CAD) for powder processing. The computational analysis of the change in microstructure or internal stress may be useful especially when new materials are fabricated by powder processing. In this review paper, our approach to CAD mainly for sintering process and its application to material design are explained. First, the finite element analysis (FEA) of sintering process at the component level is introduced. The density distribution as well as the distortion due to gravity and friction in a sintered component is demonstrated as an example of analytical results. The warping and the internal stress generation in a powder layer obtained from FEA are also shown. Secondly, a combined phase-field/discrete-element method is presented as a particle level modelling of sintering, where both grain boundary migration and sintering shrinkage are simultaneously computed, and the influence of several factors on the sintering behavior can be clarified. This method may be used to improve the constitutive equation for FEA, or to predict directly the microstructural evolution in micro-scale parts.

３Ｄプリンターと高速遠心成形法を組み合わせたディーゼルノズルの製造

Diesel nozzle tips with multiple tapered spray holes were fabricated by P/M processing (high-speed centrifugal compaction) combined with resin cores made by three-dimensional printer. The nozzle tips with three tapered spray holes (inwardly larger) were obtained by the process, as net-shape formation. There was no obvious deformation during sintering shrinkage, therefore core shapes were precisely copied inside of the tips, forming tapered (8, or 16 deg.) spray holes in densely sintered products. Spray observation of the nozzles revealed that the highly tapered holes improved spray penetration and spray cone angle.

Low-temperature sintering and microstructure control of mullite–Mo composites

Dense mullite–Mo (45vol%) composites with homogeneous microstructure have been obtained by plasma activated sintering of a mixture of Mo and mullite precursors at a relatively low temperature (1350°C) and a pressure of 30MPa. The mullite precursor was synthesized by a sol–gel process followed by a heat-treatment at 1000°C. The influence of different mullite precursors on the densification behavior and the microstructure of mullite–Mo composites has been studied. The precursor powder heat-treated at 1000°C with only Si–Al spinel but no mullite phase shows an excellent sintering activity. Following the sintering shrinkage curves, a two-stage sintering process is designed to enhance the composite densification for further reducing the sintering temperature. The study reveals that viscous flow sintering of amorphous SiO2 at low temperatures effectively enhances the densification. Moreover, microstructure of these composites can be controlled by selecting different precursors and sintering temperatures.

Densification and sintering shrinkage of Zinc Reclaimed WC-Co

Zinc Reclaimed WC-Co powder exhibits faceted WC crystal shape while virgin WC-Co powder can have rounded WC particles. During sintering, the WC grains in Zinc Reclaimed WC-Co powder remain faceted while virgin WC-Co retains its rounded WC shape until the later stages of sintering. The effect of the shape of WC grains in Zinc Reclaimed WC-Co on its densification behavior is not fully understood. The effect of WC crystal shape on the densification of Zinc Reclaimed WC-Co during sintering is investigated. The shrinkage of Zinc Reclaimed WC-Co is measured by dilatometry and found to lag behind the shrinkage of virgin WC-Co after the first densification stage during solid state sintering. There is a greater lag in the shrinkage of coarse grained Zinc Reclaimed WC-Co than fine grained Zinc Reclaimed WC-Co relative to the corresponding virgin WC-Co. The role of packing density on the densification of Zinc Reclaimed WC-Co during solid state sintering is discussed.