Porous Hydroxyapatite Scaffolds Coated With Bioactive Apatite–Wollastonite Glass–Ceramics

The objective of this study was to fabricate porous hydroxyapatite (HA) scaffolds coated with bioactive A/W glass–ceramics and to examine their mechanical and biological properties. Firstly, the HA scaffolds were prepared by the polymeric sponge replication method, and then A/W glasses were coated on the surface of the struts. All of the scaffolds had a highly porous structure with well-interconnected pores. It was observed that the bioactive glass coating markedly increased the strength of the HA scaffolds. This enhancement was attributed to the formation of a dense and strong coating layer on the weak HA struts. The in vitro bioactivities of the scaffolds were markedly improved by the coatings. When the coated scaffolds were soaked in a simulated body fluid (SBF), the bone-like apatite crystals were well mineralized on their surfaces. Osteoblast-like cells (MC3T3) adhered, spread, and grew well on the porous scaffolds. The cells placed on the glass-coated HA scaffold showed a higher proliferation rate and alkaline phosphatase (ALP) activity than those on the pure HA scaffold. These results demonstrate that the bioactive glass coating is effective in improving the strength and bioactivity of the porous HA scaffolds.     

Barium/Lead-Rich High Permittivity Glass–Ceramics for Capacitor Applications

Dielectric properties of glass–ceramics containing barium/lead-based sodium niobates and barium titanate-based silicates were evaluated for capacitor applications. The glasses were formed by melt-rolling the respective constituents which were then crystallized by reheating them at higher temperatures. Crystallization schedules were formulated based on differential thermal analysis results. X-ray diffractometer patterns indicated that the samples were highly crystallized. Microstructure and microchemistry of samples were studied by transmission electron micropscopy. Perovskite, tungsten–bronze and fresnoite phases developed during crystallization have a strong effect on the resulted dielectric properties with permittivities ranging from 20 to 700. Resistivity measurements were done to study conduction mechanisms in samples and the resistivity values for glass–ceramics were found to be between 10¹¹ and 10¹³Ω·cm at 150°C. MnO₂ additions were made to improve the electrical resistivity of glass–ceramics.     

Sintered Glass–Ceramics and Glass–Ceramic Matrix Composites from CRT Panel Glass

Sintering with simultaneous crystallization of powdered glass represents an interesting processing route for glass–ceramics, especially originating from wastes. Highly dense glass–ceramic samples may be obtained from a simple and short treatment at a relatively low temperature. In addition, glass–ceramic matrix composites may be obtained by mixing glass with suitable reinforcements. In this work sintered nepheline glass–ceramics, based on panel glass from cathode ray tubes, are illustrated. A limited addition of Al₂O₃ platelets caused a significant improvement in the mechanical properties (elastic modulus, bending strength, microhardness, fracture toughness), already remarkable for the un-reinforced glass–ceramic, compared with traditional nepheline glass–ceramics.     

Colloidal Processing of Glass–Ceramics for Laminated Object Manufacturing

Green tapes of Li₂O–ZrO₂–SiO₂–Al₂O₃ (LZSA) parent glass were produced by aqueous tape casting as the starting material for the laminated object manufacturing (LOM) process. The rheological behavior of the powder suspensions in aqueous media, as well as the mechanical properties of the cast tapes, was evaluated. According to ξ potential measurements, the LZSA glass powder particles showed acid surface characteristics and an IEP of around 4 when in aqueous media. The critical volume fraction of solids was about 72 wt% (27 vol%), which hindered the processability of more concentrated slurries. The glass particles also showed an anisometric profile, which contributed to an increase in the interactions between particles during flow. Therefore, the suspensions could not be processed at high solids loadings. Aqueous-based glass suspensions were also characterized by shear thickening after the addition of dispersants. Three slurry compositions were formulated, suitable green tapes were cast, and tapes were successfully laminated by LOM to a gear wheel geometry. A higher tensile strength of the green tapes corresponded to a higher tensile strength of the laminates. Thermal treatment was then applied to the laminates: pyrolysis at 525°C, sintering at 700°C for 1 h, and crystallization at 850°C for 30 min. A 20% volumetric shrinkage was observed, but no surface flaws or inhomogeneous areas were detected. The sintered part maintained the curved edges and internal profile after heat treatment.     

Preparation of Whisker β-Spodumene Glass–Ceramics

β-spodumene glass–ceramics with a whisker-like microstructure were prepared from the following materials (in wt%): 64.5 SiO₂, 18.0 Al₂O₃, 4.2 Li₂O, 4 ZrO₂, and 8 MgF₂. Scanning electron microscope (SEM) analysis showed that phase separation in the base glass leads to the formation of a primary crystal phase of MgF₂ that promotes the formation of spherical β-spodumene. Whisker spodumene crystals surrounded by spherical crystals are observed at 720°C after 1 h, and the whisker crystals grow at the cost of spherical crystals with increasing temperature and time. The flexural strengths of the glass–ceramics reach a maximum of 228 MPa after heat treatment at 850°C for 1 h.     

Effect of Aluminum Phosphate Additions on the Crystallization and Bioactivity of Fluorrichterite Glass–Ceramics for Biomedical Applications

This study evaluated the effect of aluminum phosphate (AlPO₄) additions on the crystallization behavior and bioactivity of potassium fluorrichterite glasses and glass–ceramics. Four glass compositions with increasing amounts of AlPO₄ were melted. Mica and fluorrichterite were present in all glass–ceramics after crystallization. The microstructure consisted of platelet-shaped mica and prismatic fluorrichterite crystals. After incubation in simulated body fluid, a layer consisting of spherical-nanosized particles was present on the surface of the glasses. X-ray diffraction revealed the presence of hydroxyapatite after incubation of the glass and glass–ceramic powders. The addition of AlPO₄ promoted mica nucleation and bioactivity in K-fluorrichterite glass–ceramics.     

Applicability of Optical Scattering Model to β-Quartz Solid Solution Glass–Ceramics with Nanoscale Crystalline Phase

The applicability of Rayleigh-Gans and quasi-continuum scattering models to β-quartz solid solution (s.s.) glass–ceramics was studied. We evaluated the validity of these models using two specimens with different microstructures. The refractive indices of the crystalline and glass phases of the specimens were calculated by a method utilizing the transmittance change of the specimens due to structural relaxation. It was found that the quasi-continuum model is applicable to the glass–ceramics for estimating scattering intensity and its microstructure dependence. We demonstrated that the optical transmittance of the glass–ceramics can be simulated as a function of the microstructure on the basis of this model. The result of this study provides a quantitative approach for controlling the optical transmittance of β-quartz s.s. glass–ceramics.     

Pressureless Sintering of Apatite/Wollastonite–Phlogopite Glass–Ceramics

Apatite/wollastonite (A/W)—phlogopite (Ph) glass-ceramics of various compositions were prepared by means of pressureless sintering of two frit mixtures. Sintering conditions were studied by differential thermal analysis (DTA) and hot-stage microscopy. The sintered samples were characterized by X-ray diffraction. The results showed that by addition of a frit of phlogopite composition to a frit of A/W composition, the crystallization temperature decreases which results in suppressing the sintering process. The 50–50 frits mixture could be sintered to 0.90 relative density after 1 h heating at 1100°C. The type and sequence of formation of crystalline phases were also modified. Whereas the A/W glass–ceramic crystallized to apatite and wollastonite, the crystallization products of 50–50 frits mixture were fluoro-apatite, wollastonite, diopside, and mellilite.     

Carbon Dioxide Sensor Mechanism of Porous Hydroxyapatite Ceramics

Hydroxyapatite and Cl⁻ -containing hydroxyapatite powders are prepared and characterized. Reversible substitution of CO^2-₃ for OH⁻ at the surface is presumed to be responsible for the sensor function. The role of Cl⁻, which is necessary to realize the sensor function and is incorporated during soaking treatment, is considered as follows. It may reduce the strain caused by the incorporation of CO^2-₃ (which is larger than OH⁻), and, hence, promote the reversible substitution reaction. This behavior is presumed because Cl⁻ -containing hydroxyapatite samples exhibit sensor characteristics typical of CO₂ without any treatment.     

Microstructural Evolution in Some Silicate Glass–Ceramics: A Review

Just as the microstructures in glass–ceramics encompass the range from nanocrystalline transparent materials to microcrystalline tough materials, so the paths of microstructural evolution in glass–ceramics vary widely. Evolution can proceed in numerous ways, their genesis being a perturbation of some type, including the surface nucleation used in glass frit processing, crystallization of the primary phase or phases upon distinct crystalline nuclei, and nucleation promoted by nano- or microscale amorphous phase separation in the parent glass. Examples of the crystallization history of several glass–ceramic materials are described, with emphasis on how their microstructural evolution influences their ultimate physical and optical properties.     

Deterministic Microgrinding, Lapping, and Polishing of Glass–Ceramics

We report a series of microgrinding and polishing experiments on glass–ceramics. Microgrinding includes deterministic microgrinding (two-body abrasion at fixed infeed rate) and loose abrasive lapping (three-body abrasion at fixed pressure). We correlate material mechanical properties (Young's modulus, hardness, fracture toughness) and chemical properties (chemical susceptibility or mass loss under chemical attack) with the quality of the resulting surface (surface microroughness and surface grinding-induced residual stresses). We compare deterministic microgrinding (at fixed infeed) and loose abrasive microgrinding (at fixed pressure) in terms of material removal rates and resulting surface quality.     

Cellular Anorthite Glass–Ceramics: Synthesis, Microstructure and Properties

Highly permeable cellular anorthite glass ceramics with porosity of ∼95% were obtained by a simple replication technique using near-stoichiometric glass powders and a polymeric foam as sacrificial template material. Impacts of sintering conditions and additions of minor constituents, respectively, on microstructural evolution and resulting macroscopic properties of the derived foams were investigated by X-ray diffraction, scanning electron microscopy (SEM), differential thermal analysis and X-ray microcomputer tomography. Apparent activation energies of crystallization and the Avrami coefficient were estimated from nonisothermal crystallization experiments to evaluate the impact of titania and zirconia, respectively, as potential nucleation agents. Consistent with SEM analyses, it was found that crystal growth occurs in two dimensions. While TiO₂ primarily acts on the viscosity of the precursor glass and, thus, on the process of sintering by viscous flow, ZrO₂ is found to exhibit at least some nucleation efficiency. As compared with sintering of ceramic powders and solid-state reactions in general, the glass ceramic route enables significant reduction in sintering time and temperature.     

Preparation of Machinable Fluoramphibole Glass–Ceramics from Soda-Lime Glass and Fluormica

A novel route, directly mixing fluormica crystals with recycled soda-lime glass powder and then sintering, is proposed to fabricate machinable fluoramphibole glass–ceramics. The effect of fluormica addition on the sinterability, reactive crystallization behavior, strength, and machinability of the material was investigated. The relative densities of the glass–ceramics decreased with increasing fluormica content. An interaction between fluormica crystals and glass powder occurred during the sintering process. Diopside was formed in the glass–ceramics with 20 wt% fluormica, and fluorrichterite was formed in the glass–ceramics with more than 30 wt% fluormica. Machinability and strength of the glass–ceramics were improved with increasing crystalline phase content.     

Porous PZT Ceramics with Aligned Pore Channels for Energy Harvesting Applications

In this study, aligned porous lead zirconate titanate (PZT) ceramics with high pyroelectric figures‐of‐merit were successfully manufactured by freeze casting using water‐based suspensions. The introduction of aligned pores was demonstrated to have a strong influence on the resultant porous ceramics, in terms of mechanical, dielectric, and pyroelectric properties. As the level of porosity was increased, the relative permittivity decreased, whereas the Curie temperature and dielectric loss increased. The aligned porous structure exhibited improvement in the compressive strength ranging from 19 to 35 MPa, leading to easier handling, better processability and wider applications for such type of porous material. Both types of pyroelectric harvesting figures‐of‐merit (F_E and F′_E) of the PZT ceramics with a porosity level of 25–45 vol% increased in all porous ceramics, for example, from 11.41 to 12.43 pJ/m³/K² and 1.94 to 6.57 pm³/J, respectively, at 25°C, which were shown to be higher than the dense PZT counterpart.     

Effects of V and Mn Colorants on the Crystallization Behavior and Optical Properties of Ce-Doped Li-Disilicate Glass–Ceramics

The critical cooling rate and fluorescence properties of lithium (Li) disilicate glasses and glass–ceramics, doped with 2.0 wt% CeO₂ and with up to 0.7 wt% V₂O₅ and 0.3 wt% MnO₂ added as colorants, were investigated. The critical cooling rates, R _c, of glass melts were determined using differential thermal analysis and were found to be dependent on the relative concentrations of V₂O₅ and MnO₂, decreasing from 25±3° to 16±3°C/min. Annealed glasses were heat treated first to 670°C, and then to 850°C to form Li metasilicate and Li disilicate glass–ceramics, respectively. The fluorescence intensities of the Ce-doped glasses and glass–ceramics decrease by a factor of 100 with the addition of the transition metal oxides. This optical quenching effect is explained by the association of the Ce³⁺ ions with the transition metal ions in the residual glassy phase of the glass–ceramics.     

Near‐Net‐Shaped Porous Ceramics for Potential Sound Absorption Applications at High Temperatures

We present an interesting processing route for obtaining alumina/mullite‐based ceramics with controlled porosity and airflow resistance leading to promising microstructures for application as sound absorbers. The use of ceramic materials aims for potential applications where high temperatures or corrosive atmospheres are predominant, e.g., in combustion chambers of gas turbines. For the production of the porous ceramics we combined freeze gelation and sacrificial templating processes to produce near‐net‐shaped parts with low shrinkage (<3%) based on environmental‐friendly and low cost conditions. The obtained microstructure presents a bimodal pore size distribution, with small pores derived from the freeze gelation process (~30 μm) connecting large pores (2–5 mm diameter) originated from the expanded polystyrene template particles. These connections, called “windows” in this study, show a significant impact on the sound absorption properties, allowing the pressure diffusion effect to take place, resulting in a significant improvement of the sound absorption coefficient. By varying the template particle content and the slurry solid content, it is possible to control the sound absorption behavior at different frequencies of the open‐celled ceramics. These ceramics feature a high open porosity, from 77% to 82%, combined with sufficient compressive strength ranging from 0.27 to 0.68 MPa and sound absorption coefficients of 0.30–0.99, representing a highly promising combination of properties for noise control and reduction at corrosive environments and high temperatures.     

Fabrication of a Porous Bioactive Glass–Ceramic Using Room-Temperature Freeze Casting

The room-temperature freeze-casting method was used to fabricate porous bioactive glass–ceramics. In this method, a glass/camphene slurry prepared at 60°C was cast into a mold at 20°C, resulting in the production of a rigid green body that was comprised of three-dimensional dendritic camphene networks surrounded by highly concentrated glass powder walls. After the sublimation of camphene, the samples were sintered for 3 h at elevated temperatures ranging from 700° to 1100°C. As the sintering temperature was increased to 1000°C, the densification of the glass–ceramic wall was remarkably enhanced, while its highly porous structure was preserved. The sample sintered at 1000°C showed a high porosity of 53% and pore channels with a size of several tens of micrometers, as well as dense glass–ceramic walls. In addition, the fabricated samples effectively induced the deposition of apatite on their surfaces when immersed in simulated body fluid, implying that they are very bioactive.     

Temperature–Time–Mechanical Properties of Glass–Ceramics Produced from Coal Fly Ash

Physical and mechanical properties of glass–ceramics fabricated from thermal power plant fly ash were analyzed and compared with suggest a temperature–time–mechanical (T–T–M) diagram. Coal fly ash with SiO₂–Al₂O₃–MgO–CaO as major components and TiO₂ as a nuclear agent were used to develop glass–ceramic materials which were heat treated at 900°–1050°C for 0.5–4 h for crystallization. It was verified that the high aspect ratio of unknown crystallines in the microstructure contributed high hardness, strength, fracture toughness, and wear resistance. These results are correlated with heat treatment conditions and microstructure and a T–T–M properties (hardness, strength, elastic constant, toughness, and wear rate) diagram on glass–ceramics produced from coal fly ash is proposed.     

Importance of Crystallization Hierarchies in Microstructural Evolution of Silicate Glass–Ceramics

Quench studies of in-house melted glasses were used to study the microstructural evolution in Macor-type and Corning Code 9606-type glass–ceramics to elucidate the microstructures of the commercial products. They reveal that phase separation initiates crystallization in both systems which then proceeds via formation of several phases at different scales of size, the first phases to form being those containing the most rapidly diffusing species. This study highlights the concept of crystallization hierarchies whereby crystals form, sometimes simultaneously, sometimes sequentially, on heating glasses, but at different length scales . This is illustrated by the simultaneous nucleation of μ-cordierite nanocrystals and ∼0.2 μm MgAl₂Ti₃O₁₀ rosettes in Corning 9606-type compositions and of ∼0.3 μm chondrodite and 3–4 μm fluorophlogopite laths in Macor-type compositions.     

Self-Constrained Sintering of a Multilayer Low-Temperature-Cofired Glass–Ceramics/Alumina Laminate

A self-constrained low-temperature-cofired ceramic system with a multilayer structure of a low-temperature cofirable CaO–B₂O₃–SiO₂ glass (CBSG) tape and a pure alumina tape has been developed. Because the densification temperature range required for the CBSG layer is much lower than that needed for pure alumina, its lateral shrinkage is inhibited by the presence of a nondensified alumina layer during cofiring. Porosity in the alumina layer is later filled by the capillary infiltration of free glass in the densified CBSG. This leads the linear shrinkage of the multilayer CBSG/alumina laminate to take place only in the Z direction with little shrinkage in the X – Y directions, which exhibits unique characteristics of self-constrained densification.