LAS glass–ceramic scaffolds by three-dimensional printing

Highly porous (>60% open porosity) glass–ceramic scaffolds with remarkable mechanical properties (compression strength of ～15 MPa) were produced by indirect 3D printing. Precursor glass powders were printed into 3D ordered structures and then heat treated to sinter and develop crystalline phases. The final glass–ceramic contained a β-spodumene solid solution together with a secondary phase of lithium disilicate.The precision of the printed geometry and the density of the struts in the scaffold depended on several processing parameters (e.g. powder size and flowability, layer thickness) and were improved by increasing the binder saturation and drying time. Two types of powders with different particle size distribution (PSD) and flowability were used. Powders with a larger PSD, could be processed within a wider range of printing parameters due to their good flowability; however, the printing precision and the struts density were lower compared to the scaffolds printed using the powder in a smaller average PSD.     

Preparation and characterization of vancomycin releasing PHBV coated 45S5 Bioglass®-based glass–ceramic scaffolds for bone tissue engineering

Porous 45S5 Bioglass^®-based glass–ceramic scaffolds with high porosity (96%) and interconnected pore structure (average pore size 300 μm) were prepared by foam replication method. In order to improve the mechanical properties and to incorporate a drug release function, the scaffolds were coated with a drug loaded solution, consisting of PHBV and vancomycin. The mechanical properties of the scaffolds were significantly improved by the PHBV coating. The bioactivity of scaffolds upon immersion in SBF was maintained in PHBV coated scaffolds although the formation of hydroxyapatite was slightly retarded by the presence of the coating. The encapsulated drug in coated scaffolds was released in a sustained manner (99.9% in 6 days) as compared to the rapid release (99.5% in 3 days) of drug directly adsorbed on the uncoated scaffolds. The obtained drug loaded and bioactive composite scaffolds represent promising candidates for bone tissue engineering applications.     

New insights into the crystallization process of sol-gel–derived 45S5 bioactive glass

In this work the influence of thermal treatment conditions on crystallization of a sol-gel-derived 45S5 bioactive glass was evaluated using DSC, XRD, TEM, EDX, and X-ray nanocomputed tomography (nano-CT). Temperature and time of the thermal treatment strongly influence the composition of the crystalline phases. At the onset of the glass transition temperature (600°C), combeite crystallizes as the main phase along with a calcium silicate-phosphate phase, which decomposes into rhenanite from 2 hours of thermal treatment at this temperature. At the crystallization temperature (700°C), combeite remains as the main crystalline phase. Additionally, Na₂Ca₂Si₂O₇ crystalline phase is formed. Our results provide a basic platform for tailoring the crystalline phases by controlling the nucleation and growth of crystalline phases via thermal treatments. Different morphologies (round particles, stacked layers, toothpick-like, and long features) were discerned by TEM as a function of temperature and time of treatment. It is the first time that bioactive glass is investigated by nano-CT at laboratory scale. This novel technique enables the 3D visualization of features in the nanometer range, giving clear information about the volumetric distribution of phases in the sample.     

Effect of silicon carbide whisker reinforcement on CaO–ZrO2–SiO2 glass–ceramic system

Abstract

An industrial frit formulated in the new CaO–ZrO₂–SiO₂ glass–ceramic system was studied as a matrix for whisker reinforced composites. The frit was ball milled in acetone and wet ultrasonically mixed with 5, 10, 20, and 30 vol.-% SiC whiskers in order to overcome whisker agglomeration and obtain intimate mixing of the two phases. The samples were hot pressed at 14 MPa in graphite dies, using a N₂ atmosphere, for 2 h at 1280°C. In order to investigate the effect of whiskers as a reinforcement, flexural strength as well as crack configuration and propagation were taken into consideration. Whisker orientation perpendicular to the hot pressing direction was found by SEM observation, and no carbon layer at the whisker/matrix interface was detected by EPMA. Further characterisation of the specimens involved physical (density, elastic modulus) and microstructural properties (XRD, SEM, TEM). The result of glass devitrification was inter locked wollastonite crystals.     

Microstructure and properties of CaO–ZrO2–SiO2 glass–ceramics prepared by sintering

For the development of a new wear resistant and chemically stable glass-ceramic glaze, the CaO–ZrO₂–SiO₂ system was studied. Compositions consisting of CaO, ZrO₂, and SiO₂ were used for frit, which formed a glass-ceramic under a single stage heat treatment in electric furnace. In the sintered glass-ceramic, wollastonite (CaSiO₃) and calcium zirconium silicate (Ca₂ZrSi₄O₁₂) were crystalline phases composed of surface and internal crystals in the microstructure. The internal crystal formed with nuclei having a composition of Ca_1.2Si_4.3Zr_0.2O₈. The CaO–ZrO₂–SiO₂ system showed good properties in wear and chemical resistance because the Ca₂ZrSi₄O₁₂ crystals positively affected physical and mechanical properties.     

Lightweight glass–ceramic tiles from the sintering of mining tailings

Modern thermally-insulating building façades comprise lightweight structural panels, in turn mostly composed of porcelain stoneware with engineered porosity. Sintered glass–ceramics may represent a valid alternative, mainly considering layered articles, with a dense surface layer on a highly porous body that could be manufactured by double pressing. In this paper we present a low cost route to lightweight tiles, developed starting from mining tailings, such as waste from the mining of boron-rich minerals and basalt rock, and recycled glasses, such as common soda-lime glass and pharmaceutical borosilicate glass. A highly porous body was obtained by direct sintering of mixtures of mining tailings and soda-lime glass; despite the homogeneity of porosity and the formation of new crystal phases (at only 1000 °C), favorable to good mechanical properties, the water absorption remained far above the limits (>2 wt%). The water absorption was minimized by introduction of a dense glaze, associated to the firing of mixtures coated by a thin layer of recycled borosilicate glass powders; both color and shrinkage were optimized by the mixing of borosilicate glass with powders of zircon mineral and vitrified boron waste/basalt/soda-lime mixture.     

Far from equilibrium ultrafast high-temperature sintering of ZrO2–SiO2 nanocrystalline glass–ceramics

Ultrafast high-temperature sintering (UHS) is a novel sintering technique with ultrashort firing cycles (e.g., a few tens of seconds). The feasibility of UHS has been validated on several ceramics and metals; however, its potential in consolidating glass–ceramics has not yet been demonstrated. In this work, an optimized carbon-free UHS was utilized to prepare ZrO₂–SiO₂ nanocrystalline glass–ceramics (NCGCs). The phase composition, grain size, densification behavior, and microstructures of NCGCs prepared by UHS were investigated and compared with those of samples sintered by pressureless sintering. Results showed that NCGCs with a high relative density (～95%) can be obtained within ～50 s discharge time by UHS. The UHS processing not only hindered the formation of ZrSiO₄ and cristobalite but also enhanced the stabilization of t-ZrO₂. Meanwhile, owing to the ultrashort firing cycles, the UHS technology allowed the NCGCs to be consolidated in a far from equilibrium state. The NCGCs showed a microstructure of spherical monocrystalline ZrO₂ nanocrystallites embedded in an amorphous SiO₂ matrix.     

High emissivity MoSi2–ZrO2–borosilicate glass multiphase coating with SiB6 addition for fibrous ZrO2 ceramic

To develop a high emissivity coating on the low thermal conductivity ZrO₂ ceramic insulation for reusable thermal protective system, the MoSi₂–ZrO₂–borosilicate glass multiphase coatings with SiB₆ addition were designed and prepared with slurry dipping and subsequent sintering method. The influence of SiB₆ content on the microstructure, radiative property and thermal shock behavior of the coatings has been investigated. The coating prepared with SiB₆ included the top dense glass layer, the surface porous coating layer and the interfacial transition layer, forming a gradient structure and exhibiting superior compatibility and adherence with the substrate. The emissivity of the coating with 3 wt% SiB₆ addition was up to 0.8 in the range of 0.3–2.5 μm and 0.85 in the range of 0.8–2.5 μm at room temperature, and the “V-shaped grooves” surface roughness morphology had a positive effect on the emissivity. The MZB-3S coating showed excellent thermal shock resistance with only 1.81% weight loss after 10 thermal cycles between 1773 K and room temperature, which was attributed to the synergistic effect of porous gradient structure, self-sealing property of oxidized SiB₆ and the match of thermal expansion coefficient between the coating and substrate. Thus, the high emissivity MoSi₂–ZrO₂–borosilicate glass coating with high temperature resistance presented a promising potential for application in thermal insulation materials.     

Influences of powder morphology on the densification and microstructure of a ZrO2-based nanocrystalline glass–ceramic

Morphology is an important characteristic of raw powder utilized for ceramic sintering but the role of powder morphology is mostly overlooked. In this study, two types of ZrO₂–SiO₂ powder with different morphologies (fiber and particle) were synthesized by blow spinning and sol–gel method, respectively, followed by direct current electric field-assisted hot pressing (FAHP) to obtain nanocrystalline glass–ceramics (NCGCs). Results showed that the two as-synthesized powders had different pyrolysis behaviors. The two types of as-synthesized powders were amorphous and tetragonal-ZrO₂ nanocrystallites first formed after calcination at 800°C. During FAHP, the particle powder can be densified at a lower temperature than that of the fiber powder, due to the facts that the particle powder showed higher specific surface area and higher densification driving force. The fiber powder was predominately densified by fiber fusion and plastic deformation, whereas the particle powder was densified via particle fusion. Both the two types of powder can be fully densified to obtain ZrO₂–SiO₂ NCGCs at 1230°C for 4 min. Tetragonal-ZrO₂ nanocrystallites in the NCGCs with particles as raw powder showed higher stability than those in the NCGCs with fibers as raw powder.     

Two-step sinter-crystallization of K2O–CaO–P2O5–SiO2 (45S5-K) bioactive glass

Bioactive glasses and glass-ceramics (GCs) effectively regenerate bone tissue, however most GCs show improved mechanical properties. In this work, we developed and tested a rarely studied bioactive glass composition (24.4K₂O-26.9CaO-46.1SiO₂-2.6P₂O₅ mol%, identified as 45S5-K) with different particle sizes and heating rates to obtain a sintered GC that combines good fracture strength, low elastic modulus, and bioactivity. We analyzed the influence of the sintering processing conditions in the elastic modulus, Vickers microhardness, density, and crystal phase formation in the GC. The best GC shows improved properties compared with its parent glass. This glass achieves a good densification degree with a two-step viscous flow sintering approach and the resulting GC shows as high bioactivity as that of the standard 45S5 Bioglass®. Furthermore, the GC elastic modulus (56 GPa) is relatively low, minimizing stress shielding. Therefore, we unveiled the glass sintering behavior with concurrent crystallization of this complex bioactive glass composition and developed a potential GC for bone regeneration.     

Synthesis,characterization and biocompatibility evaluation of sol–gel derived bioactive glass scaffolds prepared by freeze casting method

In this study, a new class of bioactive glass scaffolds was prepared through freeze casting method for bone tissue engineering applications. After analyzing the structural characteristics of the scaffolds, in vitro biological evaluations were assessed through monitoring alkaline phosphatase (AP) activity of osteoblast cells and soaking in simulated body fluid (SBF) for different time intervals. It was shown that the scaffolds consisted of bioactive glass plates with interconnected pores between them, aligned along the ice growth direction. The ability of the scaffolds for supporting the growth of human fetal osteoblastic cells (hFOB 1.19) was approved. Moreover, inductively coupled plasma-atomic emission spectrometry (ICP-AES) showed meaningful compositional changes of calcium, phosphorus and silicon in SBF solution, indicating the apatite forming ability of the scaffolds. The present investigation revealed that freeze casting could be an effective method for the preparation of highly bioactive scaffolds. In addition, the scaffolds proved to be highly compatible for the proposed works in vivo.     

Carbothermal production of ZrB2–ZrO2 ceramic powders from ZrO2–B2O3/B system by high-energy ball milling and annealing assisted process

Zirconium diboride (ZrB₂)-zirconium dioxide (ZrO₂) ceramic powders were prepared by comparing two different boron sources as boron oxide (B₂O₃) and elemental boron (B). The production method was high-energy ball milling and subsequent annealing of powder blends containing stoichiometric amounts of ZrO₂, B₂O₃/B powders in the presence of graphite as a reductant. The effects of milling duration (0, 2 and 6 h), annealing duration (6 and 12 h) and annealing temperature (1200–1400 °C) on the formation and microstructure of ceramic powders were investigated. Phase, thermal and microstructural characterizations of the milled and annealed powders were performed by X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and transmission electron microscope (TEM). The formation of ZrB₂ starts after milling for 2 h and annealing at 1300 °C if B₂O₃ is used as boron source and after milling for 2 h and annealing at 1200 °C if B is used as boron source.     

Liquid-phase sintering of ZrO2-based nanocrystalline glass–ceramics achieved by multielement co-doping

Liquid-phase sintering (LPS) is an effective pathway to assist the densification of ceramics. However, it has seldom been used to densify glass–ceramics. In the present study, a multielement co-doping strategy has been utilized to achieve LPS of a ZrO₂–SiO₂ nanocrystalline glass–ceramic. Compared with undoped samples densified by solid-state sintering, doping of equimolar Al, Y, and Ca promoted the densification of the glass–ceramic at lower temperatures with a faster densification rate. Ternary doping enhanced coarsening of ZrO₂ nanocrystallites during sintering and annealing. The distribution of dopants was carefully observed with X-ray energy-dispersive spectrometry technique in scanning electron transmission microscopy mode. Results showed that the three dopants showed different distribution behaviors. After sintering, Y dopants were predominately distributed in ZrO₂ nanocrystallites, whereas parts of Al and Ca dopants were distributed in ZrO₂ nanocrystallites and part of them co-segregated at the ZrO₂/SiO₂ heterointerfaces. Meanwhile, the segregation of Ca dopant at some intergranular films among ZrO₂ nanocrystallites was observed. Redistribution of dopants did not occur during annealing.     

Preparation of Si–C–N–Fe magnetic ceramic derived from iron-modified polysilazane

In this paper, Si–C–N–Fe magnetoceramics were obtained by pyrolysis of iron-modified polysilazane (PFSZ) precursors which were synthesized by using polysilazane (PSZ) and iron (III) acetylacetonate (Fe(acac)₃) as starting materials. The as-synthesized PFSZ precursors were characterized by Fourier transform infrared spectroscopy (FT-IR) and gel permeation chromatography. The polymer-to-ceramic conversion of the PFSZ was studied by FT-IR and thermal gravimetric analysis. It is found that the ceramic yield of the PFSZ precursor is ca. 25% higher than that of the original PSZ. The crystallization behavior, microstructures and magnetic properties of the PFSZ-derived Si–C–N–Fe magnetoceramics were studied by techniques such as X-ray diffraction, transmission electron microscopy and vibrating sample magnetometer. The results indicate that the formed α-Fe nanoparticles are uniformly dispersed in amorphous Si–C–N(O) matrix, leading to the soft magnetization of the resultant Si–C–N–Fe ceramics. Moreover, the iron content and the magnetic properties of the Si–C–N–Fe ceramic could be easily controlled by the amount of Fe(acac)₃ in the precursor.     

Additive manufacturing of Ca–Mg silicate scaffolds supported by flame-synthesized glass microspheres

Novel manufacturing techniques such as additive manufacturing also referred to as 3D printing hold a critical role in the preparation of novel bioactive three-dimensional glass-ceramic scaffolds. The present paper focuses on the use of Ca–Mg silicates microspheres (Ca₂MgSi₂O₇, i.e. 40 mol% CaO, 20% MgO and 40% SiO₂) for the fabrication of 3D structures by additive manufacturing. In the first step, the crystallization of the åkermanite system was avoided, by feeding nearly fully crystallized precursor powders prepared by conventional melt quenching into oxygen-methane (O₂/CH₄) torch, and solid glass microspheres (SGMs) with diameters bellow 63 μm were prepared. In the second step, the crystallization was utilized to control the viscous flow of SGMs during firing of reticulated scaffolds, obtained by digital light processing (DLP) of the SGMs suspended in a photocurable acrylate binder. The spheroidal shape facilitated a high solid content, up to 77 wt% of the SGMs in the suspension. After burn-out of the organic binder, a fast sintering treatment at 950 °C, for 30 min, led to scaffolds preserving the macro-porosity from 3D printing model (diamond cell lattice) but with well densified struts. The crystallization of 3D scaffolds during the sintering process led to 3D structures with adequate strength-to-density ratio.     

Densification of glass powders belonging to the CaO–ZrO2–SiO2 system by microwave heating

Densification and microstructural changes of two glassy compositions belonging to the wollastonite and zirconia stability fields in the ternary CaO–ZrO₂–SiO₂ system were studied in a 2.45 GHz multimode microwave cavity. The effect of microwaves is to lower the sintering and devitrification temperature with stronger influence for high zirconia content composition. Correlation was found between dielectric properties and heating rate, showing lower interaction temperature for high zirconia content composition which starts to absorb microwave energy at about 400°C compared to 800°C for the low-zirconia one. Sintering and crystallization processes evolved in complex ways during heat treatment so that the two final glass-ceramic materials exhibit different microstructures, crystalline phases and mechanical properties.     

Processing of Foturan® glass ceramic substrates for micro-solid oxide fuel cells

The microfabrication of Foturan^® glass ceramic as a potential substrate material for micro-solid oxide fuel cells (micro-SOFC) was investigated. Foturan^® was etched in 10% aqueous hydrofluoric (HF) acid solution at 25 °C with a linear rate of 22 ± 1.7 μm/min to create structures with an aspect ratio of 1:1 in 500 μm-thick Foturan^® substrates for micro-SOFCs. The concentration of the HF etchant was found to influence the etching rate, whereas the UV-exposure time creating nuclei in the glass for subsequent crystallization of the amorphous Foturan^® material had no significant influence on the etching rates. The surface roughness of the crystallized Foturan^® was determined by the crystallite size in the order of 10–15 μm. Free-standing micro-SOFC membranes consisting of a thin film Pt cathode, an yttria-stabilized-zirconia electrolyte and a Pt anode were released by HF etching of the Foturan^® substrate. An open-circuit voltage of 0.57 V and a maximum power density of 209 mW/cm² at 550 °C were achieved.     

Development of a new glass–ceramic by means of controlled vitrification and crystallisation of inorganic wastes from urban incineration

This paper reports the results of a study of the feasibility of recycling the solid residues from domiciliary waste incineration by producing a glass-ceramic. The major components of the raw material (TIRME F+L), which was from a Spanish domiciliary incinerator, were CaO, SiO₂ and Al₂O₃ but nucleating agents, such as TiO₂, P₂O₅, and Fe₂O₃ were also present in reasonable amounts. It was found that a relatively stable glass with suitable viscosity could be obtained by mixing 65 wt% TIRME F+L with 35 wt% glass cullet. The heat treatment required to crystallise the glass produced from this mixture, designated TIR65, was nucleation at 560°C for 35 min followed by crystal growth at 100°C for 120 min. The resulting glass-ceramic contained a number of crystalline phases, the most stable being clinoenstatite (MgSiO₃), or perhaps a pyroxenic phase which incorporates Ca, Mg and Al in its composition, and åkermanite (Ca₂MgSi₂O₇). The microstructure contained both fibre-like and dendritic crystals. The mechanical properties were acceptable for applications such as tiles for the building industry. ©     

Praseodymium-doped cubic Ca–ZrO2 ceramic stain

The cubic Ca–ZrO₂ structure has been used as host for preparing a yellow ceramic stain using praseodymium as dopant. Samples with Ca_xPr_0.1Zr_0.9−xO₂ (X=0.14, 0.17, 0.2) compositions have been prepared by the ceramic method (CE) and by several gel processing techniques: the colloidal method (CG), the gelatine method (GE), the citrate method (CI) and a polymeric route (PG). Fired samples have been evaluated as ceramic pigments following enameling with the powders in ceramic glazes. The results show that a yellow ceramic pigment is obtained in all samples without significative differences among the different methods and compositions. This yellow ceramic pigment has been identified as a Pr–(Ca–ZrO₂) solid solution.