Viscous Flow Sintering Research Articles

Preparation of translucent YAG glass/ceramic at temperatures below 900 °C

The work presented here deals with the preparation of bulk yttrium aluminium garnet (YAG) glass-ceramics and YAG ceramics from glass microspheres with a YAG composition. Sol-gel prepared YAG powder was fed into a high temperature methane-oxygen flame where the particles melted and glass microspheres, with a YAG composition, were formed. Viscous flow sintering of the microspheres was then performed to prepare bulk YAG glass-ceramics or ceramics in a hot press.Rapid crystallization of YAG glass was traced during hot pressing through a change in the heating rate slope due to release of latent (crystallization) heat. This allowed control of crystallization and enabled preparation of YAG-based materials with different amounts of residual glass. YAG ceramic with relative density of 94.2 % was prepared at 891 °C without isothermal heating; additionally, YAG glass-ceramic reached relative density > 99 % at temperature 815 °C without isothermal heating.

Ceria-Based Dual-Phase Membranes for High-Temperature Carbon Dioxide Separation: Effect of Iron Doping and Pore Generation with MgO Template.

Dual-phase membranes for high-temperature carbon dioxide separation have emerged as promising technology to mitigate anthropogenic greenhouse gases emissions, especially as a pre- and post-combustion separation technique in coal burning power plants. To implement these membranes industrially, the carbon dioxide permeability must be improved. In this study, Ce0.8Sm0.2O2−δ (SDC) and Ce0.8Sm0.19Fe0.01O2−δ (FSDC) ceramic powders were used to form the skeleton in dual-phase membranes. The use of MgO as an environmentally friendly pore generator allows control over the membrane porosity and microstructure in order to compare the effect of the membrane’s ceramic phase. The ceramic powders and the resulting membranes were characterized using ICP-OES, HSM, gravimetric analysis, SEM/EDX, and XRD, and the carbon dioxide flux density was quantified using a high-temperature membrane permeation setup. The carbon dioxide permeability slightly increases with the addition of iron in the FSDC membranes compared to the SDC membranes mainly due to the reported scavenging effect of iron with the siliceous impurities, with an additional potential contribution of an increased crystallite size due to viscous flow sintering. The increased permeability of the FSDC system and the proper microstructure control by MgO can be further extended to optimize carbon dioxide permeability in this membrane system.

Four-dimensional imaging and quantification of viscous flow sintering within a 3D printed bioactive glass scaffold using synchrotron X-ray tomography

Bioglass® was the first material to form a stable chemical bond with human tissue. Since its discovery, a key goal was to produce three-dimensional (3D) porous scaffolds which can host and guide tissue repair, in particular, regeneration of long bone defects resulting from trauma or disease. Producing 3D scaffolds from bioactive glasses is challenging because of crystallization events that occur while the glass particles densify at high temperatures. Bioactive glasses such as the 13–93 composition can be sintered by viscous flow sintering at temperatures above the glass transition onset (Tg) and below the crystallization temperature (Tc). There is, however, very little literature on viscous flow sintering of bioactive glasses, and none of which focuses on the viscous flow sintering of glass scaffolds in four dimensions (4D) (3D + time). Here, high-resolution synchrotron-sourced X-ray computed tomography (sCT) was used to capture and quantify viscous flow sintering of an additively manufactured bioactive glass scaffold in 4D. In situ sCT allowed the simultaneous quantification of individual particle (local) structural changes and the scaffold's (global) dimensional changes during the sintering cycle. Densification, calculated as change in surface area, occurred in three distinct stages, confirming classical sintering theory. Importantly, our observations show for the first time that the local and global contributions to densification are significantly different at each of these stages: local sintering dominates stages 1 and 2, while global sintering is more prevalent in stage 3. During stage 1, small particles coalesced to larger particles because of their higher driving force for viscous flow at lower temperatures, while large angular particles became less faceted (angular regions had a local small radius of curvature). A transition in the rate of sintering was then observed in which significant viscous flow occurred, resulting in large reduction of surface area, total strut volume, and interparticle porosity because the majority of the printed particles coalesced to become continuous struts (stage 2). Transition from stage 2 to stage 3 was distinctly obvious when interparticle pores became isolated and closed, while the sintering rate significantly reduced. During stage 3, at the local scale, isolated pores either became more spherical or reduced in size and disappeared depending on their initial morphology. During stage 3, sintering of the scaffolds continued at the strut level, with interstrut porosity reducing, while globally the strut diameter increased in size, suggesting overall shrinkage of the scaffold with the flow of material via the strut contacts.This study provides novel insights into viscous flow in a complex non-idealized construct, where, locally, particles are not spherical and are of a range of sizes, leading to a random distribution of interparticle porosity, while globally, predesigned porosity between the struts exists to allow the construct to support tissue growth. This is the first time that the three stages of densification have been captured at the local and global scales simultaneously. The insights provided here should accelerate the development of 3D bioactive glass scaffolds.

Comparative Analysis of Wollastonite‐Diopside Glass‐Ceramic Structures Fabricated via Stereo‐Lithography

One key feature of digital light processing consists of the high flexibility in the design of highly porous reticulated scaffolds. In the present paper, the authors report recent experiences concerning two glasses, both converted into wollastonite‐diopside (CaSiO3–CaMgSi2O6) glass‐ceramics by viscous flow sintering of fine powders (<45 μm) with concurrent crystallization. Glass particles are homogeneously dispersed in a photosensitive polymer, selectively hardened, layer‐by‐layer, by exposure to visible light, forming diamond cell lattices. The printing accuracy is slightly influenced by the glass chemistry, whereas the shape maintenance, after firing at 1100 °C, is observed for both compositions. The newly developed scaffolds are compared to other glass‐ceramic scaffolds, previously developed on the basis of the starting glass: the adoption of a new design does not determine any substantial degradation of mechanical properties.

Predicting Viscosity and Surface Tension at High Temperature of Porcelain Stoneware Bodies: A Methodological Approach.

The shear viscosity and the glass-vapor surface tension at high temperature are crucial to understand the viscous flow sintering kinetics of porcelain stoneware. Moreover, the pyroplastic deformation depends on the viscosity of the whole body, which is made up of a suspension of crystals dispersed in the melt. The existing fundamental theoretical background, along with semi-empirical constitutive laws for viscous flow sintering and glass densification, can be exploited through different approaches to estimate the physical properties at high temperatures starting from amount and chemical composition of the melt. In this work, a comprehensive attempt to predict the properties of the liquid phase is proposed by means of a detailed overview of existing models for viscosity and surface tension of glasses and melts at high temperature. The chemical composition of the vitreous phase and its physical properties at high temperature are estimated through an experimental approach based on the qualitative and quantitative chemical and phase analyses (by Rietveld refinement of X-ray powder diffraction patterns) of different porcelain-like materials. Repercussions on the firing behavior of ceramic bodies, are discussed. Comparative examples are provided for porcelain stoneware tiles, vitreous china and porcelain bodies, disclosing differences in composition and properties but a common sintering mechanism.

Phosphate glass matrix composites incorporating trisilanol phenyl polyhedral oligomeric silsesquioxane prepared by viscous flow sintering method with enhanced benefits

The effect of mixing and sintering processes to prepare tin fluorophosphate glass (Pglass) matrix composites incorporating trisilanol phenyl polyhedral oligomeric silsesquioxane (TSP-POSS) was investigated by comparing manual and suspension mixing, one-step and stepwise sintering processes to explore the structure dynamics and physical properties in the composites as a function of the different process conditions used. Energy Dispersive X-ray analysis confirmed optimal homogeneous dispersion of the TSP-POSS molecules in the composites prepared by the suspension method. The observed increase of glass transition temperature and the reduction of non-bridging bonds in the composites are believed to be the reason for the effective dispersion of TSP-POSS molecules in the composites. The chemical reaction between the TSP-POSS and Pglass was strongly influenced by the mixing/dispersion and sintering processes investigated. 13C cross polarized magic angle spinning (CP MAS) solid state nuclear magnetic resonance (NMR) spectroscopy confirmed the chemical stability of the TSP-POSS during the sintering process at elevated temperatures. In addition, a chemical reaction between the TSP-POSS and Pglass was evidenced by 29Si CP MAS NMR analysis. This study will provide a better fundamental understanding of the effective dispersion mechanism of the TSP-POSS molecules in the Pglass matrix that will facilitate tailoring the physicochemical properties of the composites with addition of various small concentrations of TSP-POSS for a number of applications where the pure Pglass is not applicable due to the intrinsic properties of the Pglass.

Bioactive glass-ceramic scaffolds by additive manufacturing and sinter-crystallization of fine glass powders

Abstract

Bioactive Glass-Ceramic Foam Scaffolds from 'Inorganic Gel Casting' and Sinter-Crystallization.

Highly porous bioactive glass-ceramic scaffolds were effectively fabricated by an inorganic gel casting technique, based on alkali activation and gelification, followed by viscous flow sintering. Glass powders, already known to yield a bioactive sintered glass-ceramic (CEL2) were dispersed in an alkaline solution, with partial dissolution of glass powders. The obtained glass suspensions underwent progressive hardening, by curing at low temperature (40 °C), owing to the formation of a C–S–H (calcium silicate hydrate) gel. As successful direct foaming was achieved by vigorous mechanical stirring of gelified suspensions, comprising also a surfactant. The developed cellular structures were later heat-treated at 900–1000 °C, to form CEL2 glass-ceramic foams, featuring an abundant total porosity (from 60% to 80%) and well-interconnected macro- and micro-sized cells. The developed foams possessed a compressive strength from 2.5 to 5 MPa, which is in the range of human trabecular bone strength. Therefore, CEL2 glass-ceramics can be proposed for bone substitutions.

Grain boundary scavenging through reactive sintering of strontium and iron in samarium doped ceria electrolyte for ITSOFC applications

Lowering the operational temperature of ceria based electrolytes is important for commercialization of solid oxide fuel cells. Here, we report the effect of iron, strontium and iron/strontium co-addition in samarium (20 mol %) doped ceria (SDC) with respect to sintering, microstructural modifications and ionic conductivity. The nanoparticle synthesized through glycine-nitrate combustion method was sintered at 1000, 1200 and 1300 °C. An increase in density was observed in iron doped SDC due to viscous flow sintering while co-addition of strontium and iron lead to the reactive sintering at 1300 °C by forming a eutectic liquid phase which resulted in the density of 99%. The presence of iron and strontium generated a compensatory tensile strain in the lattice and an enhancement in oxygen vacancy concentration which resulted in higher ionic conductivity. Co-addition of strontium and iron in SDC, not only increases the density but also reduces the activation energy for grain boundary conductivity.

Viscous flow flash sintering of porous silica glass

For the first-time compacts of porous glass particles (95wt% SiO2, 2.3% Na2O, 1.6% Al2O3) exhibiting macro-, meso- and micro-pores were densified by flash sintering, using DC electric field in the range 1000–3000Vcm−1. The results point out the applicability of this sintering technology to glasses characterized by viscous flow sintering mechanisms. Excluding the anodic region, the specimens resulted well densified using a current limit of 2mAmm−2 and a dwelling time of 30s. The obtained microstructure at the anode and at the cathode side is asymmetric, the former being characterized by the formation of Na-enriched region, the latter by the local formation of large pores (hundreds of microns). The mechanism, which triggers the flash event, appears to be associated to dielectric breakdown.

Sintering behavior of Ba/Sr celsian precursor obtained from zeolite‐A by ion‐exchange method

Abstract(Ba, Sr)‐exchanged zeolite A with composition Ba0.74Sr0.22Na0.04Al2Si2O8 was prepared by cation exchange; a mild thermal treatment converts into an amorphous phase. Successive crystallization and sintering behavior was studied by XRD, DTA, and thermodilatometric analysis. The results point out the activation of viscous flow sintering mechanisms between 900°C and 1050°C. The densification process starts when the amorphous phase reaches its glass transition temperature (897°C) and finishes when the material crystallizes forming hexacelsian. The application of an external pressure in such temperature range allows to achieve an almost complete densification, the material transforming at 1300°C into dense monoclinic celsian much below the typical processing temperature.

Initial stage sintering of polymer particles – Experiments and modelling of size-, temperature- and time-dependent contacts

The early-stage sintering of thin layers of micron-sized polystyrene (PS) particles, at sintering temperatures near and above the glass transition temperature Tg (~ 100°C), is studied utilizing 3D tomography, nanoindentation and confocal microscopy. Our experimental results confirm the existence of a critical particle radius (r crit ~ 1 μm) below which surface forces need to be considered as additional driving force, on top of the usual surfacetension driven viscous flow sintering mechanism. Both sintering kinetics and mechanical properties of particles smaller than r crit are dominated by contact deformation due to surface forces, so that sintering of larger particles is generally characterized by viscous flow. Consequently, smaller particles require shorter sintering. These experimental observations are supported by discrete particle simulations that are based on analytical models: for small particles, if only viscous sintering is considered, the model under-predicts the neck radius during early stage sintering, which confirms the need for an additional driving mechanism like elastic-plastic repulsion and surface forces that are both added to the DEM model.

Fabrication of yttria-doped barium zirconate electrolyte with sub-micrometer thickness via low temperature viscous flow sintering

Viscous flow sintering strategy has been adopted to obtain dense sub-micrometer thin yttria-doped barium zirconate (BZY) films fabricated via chemical solution deposition method. Viscous flow sintering was carried out at the temperatures between 475 and 575°C for 24h to allow densification in amorphous phase so as to avoid coarsening. The sintered films were finally heat treated at 950°C for 2h to eliminate the pores generated due to the removal of residual carbon which remains after low temperature viscous flow sintering. Dense, single-phase perovskite BZY films with thickness between 325 and 375nm were obtained without the presence of barium carbonate, which is a common issue associated with soft chemical synthesis of BZY. In addition to this, this process does not require sintering aid.

Effect of P2O5on the Nonisothermal Sinter‐Crystallization Process of a Lithium Aluminum Silicate Glass

Dense (~98.5%), lithium aluminum silicate glass‐ceramics were obtained via the sinter‐crystallization of glass particle compacts at relatively low temperatures, that is, 790–875°C. The effect of P2O5on the glass‐ceramics' sinter‐crystallization behavior was evaluated. We found that P2O5does not modify the surface crystallization mechanism but instead delays the crystallization kinetics, which facilitates viscous flow sintering. Our glass‐ceramics had virgilite (LixAlxSi3‐xO6; 0.5 < x < 1), a crystal size <1 μm, and a linear thermal expansion coefficient of 2.1 × 10−6°C−1in the temperature range 40–500°C. The overall heat treatment to obtain theseGCs was quite short, at ~25 min.

Effects of Alumina on Cristobalite Crystallization and Properties of Silica-Based Ceramic Cores

In this work, the influences of alumina addition on cristobalite crystallization and properties of injection molded silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the samples, and the XRD result indicated that the addition of alumina promoted crystallization of fused silica during sintering at 1180–1220 °C and thus increases the amount of cristobalite. The increased amount of cristobalite as well as alumina addition led to much more thermal dilation due to their higher coefficients of thermal expansion than that of fused silica. The flexural strengths at room temperature and 1500 °C were tested, and it was shown that alumina addition could not affect room temperature strength, but decreased the flexural strength at 1500 °C. In addition, deflection resistance during heating to high temperatures was investigated, and the result indicated that alumina addition speeded up high temperature softening of the samples. XRD and scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) analysis suggested that this softening behavior was related with viscous flow sintering which could be accelerated by the reaction of alumina and silica with a product of mullite.

Two different techniques used in the production of foam structures: 3D printing and glass foaming

This paper reports on the preparation of cellular materials by different techniques. Bioactive and resorbable scaffolds based on biphasic calcium phosphate were produced by 3D-printing using extrudable pastes through fine nozzles, according to a pre-defined spatial arrangement considered suitable for bone regeneration and tissue engineering applications. Milled powders of cathode ray glass tubes and egg shell wastes were mixed and compacted to produce recycled glass foams via viscous flow sintering and thermal decomposition of the egg shell component that played the role of foaming agent.

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.

Preparation and characterization of novel foamed porous glass-ceramics

Foamed glass-ceramics without using foaming agent have been synthesized in a novel glass system of SrO-CaO-Al2O3-TiO2-B2O3-SiO2-P2O5-MxOy (where M=Ba, Mg, La, Ce and Ni) by a simple process of powder sintering. The glass and glass-ceramics are characterized by dilatometry, differential scanning calorimetry (DSC), heating stage microscopy (HSM), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), optical microscopy and Fourier transformed infrared spectroscopy (FTIR). All the glasses formed are amorphous and the glass transition temperature and dilatometric softening temperature of these glasses are found to be in the range 673–678°C and 706–728°C respectively. The glasses are highly stable as indicated by the DSC evaluated glass stability parameters of the range 195–240°C. Quantitative sintering study of glass powder compacts revealed swelling in the samples with NiO and CeO2 corresponding to a geometry change of 75 and 108% around 900°C respectively. With reference to this finding the glass powder compacts are heated to 900°C and the foamed glass-ceramics are obtained. Characteristic crystalline silicate phases have been identified in the XRD studies and their microstructures are recorded by FESEM. Optical microscope study of the foamed samples revealed formation of bigger foamed cavity with residual pores in samples with NiO and CeO2 in comparison to samples with BaO, MgO and La2O3. The mean pore diameters of the samples with NiO and CeO2 are determined to be 43 and 32μm, and their respective porosities are 2.34 and 1.82cm3/g respectively. Thus NiO and CeO2 are found to be very effective to obtain foamed glass-ceramics without using foaming agent by the viscous flow sintering of fine glass powder compacts along with the reduction of the respective polyvalent ions.

Recycling of pre-stabilized municipal waste incinerator fly ash and soda-lime glass into sintered glass-ceramics

A recent method, based on the use of amorphous silica, allows for the inertization of MSWI fly ash, avoiding expensive vitrification. The present paper demonstrates that glass-ceramics, i.e. the most established products from waste-derived glasses, are feasible by direct, inexpensive viscous flow sintering of pre-stabilized fly ash mixed with clay and recycled soda-lime glass. The sintering treatment did not compromise the chemical stabilization of fly ash, as confirmed by leaching test and by cell culture studies (with mouse embryonic fibroblasts used to assess possible cytotoxicity), applied on sintered glass-ceramics. Optimized glass-ceramic tiles, processed at 1050 °C for 30 min, not only featured attractive aesthetic appearance and low water absorption (<2%), but also exhibited a remarkable specific strength (∼3.6 MPa0.5 cm3/g).

Role of iron addition on grain boundary conductivity of pure and samarium doped cerium oxide

The present paper reports the effect of iron doping (0.5, 1.5 mol%) on the densification and electrical properties of cerium oxide (CeO2) and 20 mol% samarium-doped cerium oxide (SDC) electrolytes for intermediate temperature solid oxide fuel cell (ITSOFC) applications. A single-step solution combustion method was used for doping and the resultant powder was compacted into green pellets and subsequently sintered at 1200 °C. X-ray diffraction (XRD) studies indicated the presence of a cubic fluorite CeO2 structure without the formation of a secondary phase and the stoichiometry was confirmed by X-ray fluorescence spectroscopy. In the as-compacted green pellets, the XRD peak position shifted to lower or higher angles depending on the ionic radii of the dopants due to lattice level mixing. Addition of iron resulted in smaller crystallite sizes ( 40 nm) after sintering. Densification was found to be higher (95%) in iron-doped samples than in bare samples (<90%) due to viscous flow sintering. Upon sintering the calculated strain value showed a lower value due to the segregation of iron from the lattice. Raman spectroscopic studies indicate that sintering marginally modifies the oxygen vacancy concentration in the SDC system, and found it to be higher than in CeO2. Addition of iron into the SDC improved the grain boundary conductivity 1.8 fold, but only a minor change was noticed for CeO2. The activation energy for the grain boundary conductivity was found to be lower for 1.5 mol% (1.06 eV) iron-doped SDC than for pure SDC (1.24 eV). Our results indicate that lattice level mixing of iron in SDC improves the density at relatively lower sintering temperatures and scavenges the grain boundary impurities, thereby increasing the grain boundary conductivity.