Preparation of glass by sintering

Preparation of glass articles by sintering of glass and amorphous powders is considered. The classification of sintering processes includes: (a) sintering of premelted and pulverized glasses; (b) sintering of premelted chemically treated glasses; (c) sintering without melting; (d) sintering with melting. The most interesting class of processes is sintering without, or simultaneously with, melting and it includes preparation of preforms for optical fibres and the sol-gel route for glass preparation. This last route is reviewed in detail. Two main versions are considered: preparation of gels by hydrolysis and polymerization of alkoxides, and sintering of amorphous colloidal powder compacts. The sol-gel processes represent an embryo of a new technology for the production of high-melting glasses (including quartz glass) at relatively low temperatures.     

Sintering and crystallization of volume- and surface-modified cordierite glass powders

The sintering and crystallization behaviour of a series of cordierite-type glass powders with varying substitution of Na2O for MgO was compared with that of glass powders subjected to an Mg2+2Na+ ion-exchange treatment. The ion-exchange modifies only the properties of a 3–4 μm thick surface layer on the glass grains. Whereas the bulk substitution results in a decrease in the glass transition temperature, Tg, the surface modification shows only a small influence on the overall Tg of the ion-exchanged glass. Both glass powder series show, however, a temperature decrease of the start of sintering with increasing Na2O content which demonstrates that only the viscosity of a surface layer is decisive for the start of sintering. The sintering range is enlarged to higher temperatures through the suppression of the crystallization of β-quartz solid solution. The surface-modified glass powders show strong cordierite crystallization at constant temperatures around 1000 °C, whereas in the bulk-modified glass powders, the temperature of cordierite crystallization increases and the total amount of cordierite decreases with increasing Na2O content. This revised version was published online in November 2006 with corrections to the Cover Date.     

On sintering of tiny glass beads in oscillating diametric compressions

Sintering of tiny spherical glass beads induced by oscillating diametric compression at room temperature is considered. The amorphous glass beads are modeled as isotropic elasto-plastic solid with bilinear stress-strain constitutive relationship, which is implemented in a finite element code. The coupled thermal and mechanical response of the glass beads under cyclic compressions is investigated. Our numerical simulations show that the local temperatures and stresses near the contact points can significantly be raised during adiabatic compression cycles. The temperature rise is more pronounced for contact between two beads with different sizes or for contact with more neighboring beads. There exist temperature and stress gradients from the interiors toward the contact points. These findings provide a feasible explanation for the sintering found in recent triaxial compression experiments with tiny glass beads under room temperature and low pressure.     

Fabrication of transparent silica glass by powder sintering

Transparent silica glasses were obtained by sintering a green compact (fabricated by slip-casting methods for high-purity silica glass powder) in diverse atmospheres. The relationships between sintering atmosphere and sintering temperature that result in transparent, sintered silica glass were shown. The results indicate that there are four forming phases for each sintering atmosphere and temperature: (1) nontransparent glass resulting from an overabundance of pores (2) crystal, such as cristobalite or β-quartz, (3) moganite, and (4) transparent glass. Optimum sintering temperature for fabricating transparent silica glass was above 1673 K in a high-vacuum (10^–4 Pa: p(O₂) = 10^–14) atmosphere. We investigate the fabrication of transparent and hydroxyl-free silica glass by a powder-sintered method. After studying the effect of sintering schedule on residual [OH^–] concentration for transparent, sintered silica glasses, we sintered a green compact prepared by silica powders with a mean particle size of 1.6 µm, first heating it to 1523 K for dehydration and then to 1873 K for densification. This typical fabricated condition resulted in a transparent, sintered silica glass with <1 ppm [OH^–] concentration.     

The kinetics of surface induced sinter crystallization and the formation of glass-ceramic materials

A thorough analysis is given of a process which is of great importance for the formation of many present day glass ceramic materials: sinter-crystallization. In the first part of the paper the problems determining surface induced nucleation of glasses are analyzed, emphasis being given to the influence of elastic strains and surface contamination by active substrates. The second stage of the analysis is centred on the dependence of crystal growth and overall crystallization kinetics on the mean size of an ensemble of sintering glass grains. Here a formalism is derived, connecting overall crystallization with the mean size of the crystallizing system of glass particles. In the third part the interdependence sintering – crystallization is investigated. Several cases of this interrelation are analyzed in details for different mechanisms of growth of nuclei, athermally formed on the grain surface.     

Sintering and crystallization of a glass powder in the MgO-Al2O3-SiO2-ZrO2 system

The sintering and crystallization behaviour was studied of a glass powder in the MgO-Al₂O₃-SiO₂-ZrO₂ system in which the main crystal phases to form are clino-enstatite (MgSiO₃) and cubic zirconia (c-ZrO₂). During isothermal, atmospheric sintering of the glass powder, a fine dispersion of c-ZrO₂ particles, 50–100 nm diameter, was observed to form, but this did not appear to inhibit the sintering process. Nucleation of the main crystal phase, clino-enstatite, occurred both within the original glass powder particles and at the former particle surfaces, but the rate of crystallization was greater at the former particle surfaces. The c-ZrO₂ precipitates are thought to act as nucleation sites for the crystallization of the clino-enstatite. Relative densities of up to 98% were attainable during sintering, and were reached at a stage where a significant degree of crystal phase development had already taken place, proving that completion of sintering prior to the commencement of crystallization is not always a pre-requisite for the attainment of high final densities. In the material studied, the large volume contraction ( 11%) on crystallization and the possible release of dissolved gases led to a decrease in relative density as crystallization proceeded. The relative density after complete crystallization was found to be 94%±1%, irrespective of the temperature and duration of the initial sintering stage of heat-treatment, and it appeared that most of the residual porosity was a result of the volume contraction on crystallization rather than poor densification during sintering.     

Fabrication of transparent silica glass by powder sintering

Transparent silica glasses were obtained by sintering a green compact (fabricated by slip-casting methods for high-purity silica glass powder) in diverse atmospheres. The relationships between sintering atmosphere and sintering temperature that result in transparent, sintered silica glass were shown. The results indicate that there are four forming phases for each sintering atmosphere and temperature: (1) nontransparent glass resulting from an overabundance of pores (2) crystal, such as cristobalite or β-quartz, (3) moganite, and (4) transparent glass. Optimum sintering temperature for fabricating transparent silica glass was above 1673 K in a high-vacuum (10⁻⁴ Pa: p(O₂)=10⁻¹⁴) atmosphere. We investigate the fabrication of transparent and hydroxyl-free silica glass by a powder-sintered method. After studying the effect of sintering schedule on residual [OH⁻] concentration for transparent, sintered silica glasses, we sintered a green compact prepared by silica powders with a mean particle size of 1.6 μm, first heating it to 1523 K for dehydration and then to 1873 K for densification. This typical fabricated condition resulted in a transparent, sintered silica glass with <1 ppm [OH⁻] concentration.     

Investigation of impact characteristics of tempered vehicle and vessel glasses

Glass is widely used at various engineering applications, especially in vehicle and vessel industry due to the reasons such as transparency, hardness at the room temperature and its strength against impact. One of the processes to be applied to glass is tempering. This process is applied to glass with the aim of providing 2–5 times more impact resistance. Additionally, if tempered glass is fractured it goes to pieces without sharp edges. These pieces are too small to injure people around them. Thus, nowadays side and rear glasses of vehicles and vessels are tempered glasses. In this study, tempered and untempered vehicle glasses were tested by using Instron Dynatup 9250 HV Impact Apparatus in the Mechanical Engineering Department of Pamukkale University, impact was applied to 145 samples with an average of 5.3 (771) times and their impact strength values were determined. In the first stage of this study, magnitudes of impact energies causing damages at tempered and untempered vehicle and vessel glasses with a thickness of 4 mm and 6 mm at +20 °C, were determined. In the second stage, impact experiments were performed at the environment temperatures of –20 °C and +40 °C apart from the room temperature, because the glasses to be produced will be used in different climate zones. In this way, a useful source of knowledge, especially for developing tempered vehicle and vessel glasses, will be constituted by determining the impact strength characteristics of these glasses.     

Surface patterning of glass via electrostatic imprinting using a platinum mold

Electrostatic imprinting is a highly suitable process for patterning large area and high efficiency glasses because it enables glass patterning at low temperatures with low pressures. Because high DC voltage bias is applied to the mold and glass during the thermal imprinting, the mold materials should have electrical conductivity, appropriate glass adhesion properties, and excellent thermal and electrochemical stability. In this study, thin Pt/Ni molds were fabricated via Si micromachining and electroforming techniques and were then used in the electrostatic imprint process in order to evaluate their feasibility as molds. Under the investigated process conditions, the pattern transfer to glass was accomplished without noticeable degradation of the mold. Furthermore, the process parameter effects on replication fidelity and potential defects were investigated.     

Monolithic glass formation by chemical polymerization

Historically, glasses have been formed by solidification of oxide melts from elevated temperatures. Recently it has been demonstrated that monolithic oxide glasses can be formed by chemical polymerization at low temperatures. By the use of this technique, high-temperature reactions such as crystallization, phase separation, etc., which restrict glass formation in certain systems and regions, can be largely avoided. Thus, the technique not only permits investigation of glass structure from a fundamentally different point of view, but also allows the formation of new glasses which can not be formed by thermal means.In this work, the nature of inorganic network formation by chemical polymerization is described. The method of preparing polymerizable active species in a soluble state and general considerations that must be observed to prevent incoherent self-condensation of glass-forming species during hydrolysis and polymerization are discussed. Certain characteristic properties of materials formed by chemical polymerization are also included.     

Fabrication of porous bioactive glass particles by one step sintering

In this study, we reported a facile method to prepare porous bioactive glass microparticles. Porous particles were synthesized by sintering hollow bioactive glass microspheres obtained using a sol-gel co-template technology. The results showed that porous bioactive glass particles possessed a narrow particle size distribution, a relatively porous surface morphology and a hollow structure. It is worth to say that the resulting microparticles present an amorphous structure although the sintering temperature was improved compared to hollow microspheres. The presence of macropore on the shell may provide an efficient method to carry drugs in the hollow cores. Considering the high deposit rate of nanoscale apatite for bioactive glass materials, the porous microparticles should have potential applications in drug and bioactive molecules delivery, in addition to bone tissue regeneration.     

Functionally graded bioactive glass coating on magnesia partially stabilized zirconia (Mg-PSZ) for enhanced biocompatibility

The coating of magnesia partially stabilized zirconia (Mg-PSZ) with a bioactive glass was investigated for enhancing the bioactivity and bone-bonding ability of Mg-PSZ orthopedic implants. Individual coatings of three different bioactive glasses were prepared by depositing a concentrated suspension of the glass particles on Mg-PSZ substrates, followed by sintering at temperatures between 750 °C and 850 °C. Two silicate-based glass compositions (designated 13–93 and 6P68), and a borosilicate glass composition (H12) were investigated. The microstructure and adhesive strength of the coatings were characterized, and the in vitro bioactivity of the glasses was compared by measuring their conversion kinetics to hydroxyapatite in an aqueous phosphate solution at 37 °C. The 6P68 glass provided the highest adhesive strength (40 ± 2 MPa) but showed very limited bioactivity, whereas the H12 glass had lower adhesive strength (18 ± 2 MPa) but the highest bioactivity. A functionally graded coating, consisting of a 6P68 interfacial layer and an H12 surface layer, was developed to provide a coating with high adhesive strength coupled with rapid in vitro bioactivity.     

New insights into the crystallization of cordierite from a stoichiometric glass by in situ high-temperature SEM

The crystallization of cordierite from stoichiometric glass is observed in situ in the scanning electron microscope. A heating stage was, therefore, operated at 1000 and 1100 °C. The surface crystallization of well-separated crystals was recorded. Surprisingly, at the late stage of crystallization, the formation of cracks as well as a squeezing out of a low viscous phase took place. To our knowledge, this has not been reported before and gives rise to new considerations of the crystallization mechanism of cordierite from glasses.     

Influence of SrO substitution for CaO on the properties of bioactive glass S53P4

Commercial melt-quenched bioactive glasses consist of the oxides of silicon, phosphorus, calcium and sodium. Doping of the glasses with oxides of some other elements is known to affect their capability to support hydroxyapatite formation and thus bone tissue healing but also to modify their high temperature processing parameters. In the present study, the influence of gradual substitution of SrO for CaO on the properties of the bioactive glass S53P4 was studied. Thermal analysis and hot stage microscopy were utilized to measure the thermal properties of the glasses. The in vitro bioactivity and solubility was measured by immersing the glasses in simulated body fluid for 6 h to 1 week. The formation of silica rich and hydroxyapatite layers was assessed from FTIR spectra analysis and SEM images of the glass surface. Increasing substitution of SrO for CaO decreased all characteristic temperatures and led to a slightly stronger glass network. The initial glass dissolution rate increased with SrO content. Hydroxyapatite layer was formed on all glasses but on the SrO containing glasses the layer was thinner and contained also strontium. The results suggest that substituting SrO for CaO in S53P4 glass retards the bioactivity. However, substitution greater than 10 mol% allow for precipitation of a strontium substituted hydroxyapatite layer.     

Near net-shape fabrication of hydroxyapatite glass composites

Near net-shape fabrication of hydroxyapatite (HA) glass composites has been attempted by infiltrating a glass into porous HA performs. Main efforts were put to develop glasses that are chemically compatible with HA at elevated temperatures. After extensive investigations in the phosphate and borosilicate systems, glasses of (50-55)SiO₂-(20-25)B₂O₃-(10-20)Li₂O-(0-6)CaO (wt%) composition were successfully developed. The glass shows good chemical compatibility with HA at elevated temperatures. Dense HA/glass composites can be fabricated at 850–950 C by the melt infiltration process. Investigations demonstrated a good near net-shape capability of the process, where the linear shrinkage induced by the infiltration process is less than 0.1%. Preliminary mechanical tests showed that the fracture strength and toughness of the infiltrated HA/glass composite are comparable with dense HA.     

Examining porous bio-active glass as a potential osteo-odonto-keratoprosthetic skirt material

Bio-active glass has been developed for use as a bone substitute with strong osteo-inductive capacity and the ability to form strong bonds with soft and hard tissue. The ability of this material to enhance tissue in-growth suggests its potential use as a substitute for the dental laminate of an osteo-odonto-keratoprosthesis. A preliminary in vitro investigation of porous bio-active glass as an OOKP skirt material was carried out. Porous glass structures were manufactured from bio-active glasses 1-98 and 28-04 containing varying oxide formulation (1-98, 28-04) and particle size range (250–315 μm for 1-98 and 28-04a, 315–500 μm for 28-04b). Dissolution of the porous glass structure and its effect on pH was measured. Structural 2D and 3D analysis of porous structures were performed. Cell culture experiments were carried out to study keratocyte adhesion and the inflammatory response induced by the porous glass materials. The dissolution results suggested that the porous structure made out of 1-98 dissolves faster than the structures made from glass 28-04. pH experiments showed that the dissolution of the porous glass increased the pH of the surrounding solution. The cell culture results showed that keratocytes adhered onto the surface of each of the porous glass structures, but cell adhesion and spreading was greatest for the 98a bio-glass. Cytokine production by all porous glass samples was similar to that of the negative control indicating that the glasses do not induce a cytokine driven inflammatory response. Cell culture results support the potential use of synthetic porous bio-glass as an OOKP skirt material in terms of limited inflammatory potential and capacity to induce and support tissue ingrowth.     

Deformation, stress relaxation, and crystallization of lithium silicate glass fibers below the glass transition temperature

The deformation and crystallization of Li₂O·2SiO₂ and Li₂O·1.6SiO₂ glass fibers subjected to a bending stress were measured as a function of time over the temperature range 50 to 150°C below the glass transition temperature (T _g). The glass fibers can be permanently deformed at temperatures about 100°C below T _g, and they crystallize significantly at temperatures close to, but below T _g, about 150°C lower than the onset temperature for crystallization for these glasses in the no-stress condition. The crystallization was found to occur only on the surface of the glass fibers with no detectable difference in the extent of crystallization in tensile and compressive stress regions. The relaxation mechanism for fiber deformation can be best described by a stretched exponential (Kohlrausch-Williams- Watt (KWW) approximation), rather than a single exponential model.The activation energy for stress relaxation, E _s, for the glass fibers ranges between 175 and 195 kJ/mol, which is considerably smaller than the activation energy for viscous flow, E (400 kJ/mol) near T _g for these glasses at normal, stress-free condition. It is suspected that a viscosity relaxation mechanism could be responsible for permanent deformation and crystallization of the glass fibers below T _g.     

Polymeric crystallization and condensation of calcium polyphosphate glass

Calcium polyphosphate (Ca(PO₃)₂)_n (CPP) is under investigation as a resorbable bone biomaterial. Sintering CPP glass particles in humid air produces a porous, interconnected, degradable, crystalline construct that is suitable for connective tissue engineering applications. Porous CPP constructs sintered at 585 °C dissolved in water more rapidly than those sintered at 950 °C. FTIR, ³¹P NMR, powder XRD, and density data for CPP glass and fully crystalline fibers were compared with data for the as-sintered and partially dissolved constructs sintered at 585 or 950 °C. The results suggest that condensation continues during sintering, and CPP glass crystallizes in a process analogous to the crystallization of linear organic polymers. During sintering, water vapor caused hydrolytic degradation of the surface polyphosphate chains, forming a surface layer with different dissolution properties than the particle interior. Thus, sintering CPP glass results in a heterogeneous crystalline product that impacts the dissolution rate of CPP as a degradable biomaterial.     

The stability of mother glass for porous glass-ceramics of the TiO2-SiO2 system

The stability of the glasses of TiO₂-SiO₂-Al₂O₃-B₂O₃-CaO-MgO and TiO₂-SiO₂-Al₂O₃-P₂O₅-CaO-MgO systems during formation has been investigated as a basic study on the preparation of porous glass-ceramics. The main factor affecting the stability of these glasses which are used as mother glasses of porous glass-ceramics was CaO content. The stability was remarkably improved by increasing the CaO content.The preparation of porous glass-ceramics with an appropriate amount of pore volume was possible from a glass containing CaO up to 30 mol%. The DTA trace of the glasses showed two distinct exothermic peaks in the ranges 30–130° C and 140–300° C above glass transition temperatures. The porous glass-ceramics of TiO₂-SiO₂ system containing more than 60 mol% TiO₂ and having a surface area larger than 400m²g^–1, a pore volume of 0.3–0.5 ml g^–1, and average pore radius between 1 and 20 nm were fabricated.