Microstructural evolution of SiC powder in molten silicon

SiC_powder/Si_matrix composites represent a new class of microstructurally toughened materials. The interactions between molten silicon and submicronic SiC powder have been considered since it could originate some limitations on the final properties of the material. Experiments putting in interaction a SiC powder and molten Si were performed while heating up to final values ranging between 1450 and 1600?°C for duration up to 8?h. The volume ratio of SiC and silicon was equal to one and SiC particles were freely dispersed within the liquid. X-ray diffraction analyses demonstrated that the apparent crystallites size increase of SiC powder followed a ripening law corresponding to a limitation either by volume diffusion or by dissolution into the liquid. Depending on the relevant mechanism, the activation energy of the crystallites’ growth has been found equal to 357?±?50?kJ?mol^?1 or 441?±?57?kJ?mol^?1. An agglomeration-coarsening process of SiC particles was also identified which promoted a quick formation of larger particles.     

Controlling surface microstructure of calcium phosphate ceramic from random to custom-design

Calcium phosphate ceramics have long been studied as bone graft substitutes due to their similarity with the mineral constitute of bone and teeth, excellent biocompatibility and bioactivity. Chemical composition, macrostructure and surface microstructure are believed to be important for the bone formation within calcium phosphate ceramics. Surface microstructure has shown its crucial role in the osteogenic response of calcium phosphate ceramics; however the presence of surface irregularities and random distribution of surface microstructure in traditional calcium phosphate ceramics make it difficult to explain how surface microstructure play its role in bone formation. In the present study, we evaluated the influence of various starting apatites and sintering temperatures on the surface microstructure of the resulting hydroxyapatite ceramics. In order to minimize the randomness of the surface microstructure, laser ablation was used to generate custom-designed surface microstructures. The resulting hydroxyapatite ceramics with controlled surface microstructures would be helpful to study the role of surface microstructure on bone formation and may provide useful information for further optimization of calcium phosphate ceramics for bone regeneration.     

Facile size-controlled synthesis of well-dispersed spherical amorphous alumina nanoparticles via homogeneous precipitation

Well-dispersed spherical amorphous alumina nanoparticles with a narrow size distribution were obtained by facile homogeneous precipitation and subsequent calcination. In the synthesis, formamide was used as the precipitant, and mixtures of aluminum sulfate and aluminum nitrate with different molar ratios were used as the aluminum sources. The average size of the amorphous alumina nanoparticles was successfully controlled by adjusting the amount of formamide and the sulfate/nitrate molar ratio. The particle size decreased with increasing amount of formamide and decreasing sulfate/nitrate molar ratio. Dispersed spherical amorphous alumina nanoparticles with average sizes of 23, 34, 45, and 57 nm were prepared using 100 mL formamide at sulfate/nitrate molar ratios of 1:9, 2:8, 3:7, and 4:6, respectively.     

Combustion assisted synthesis of hardystonite nanopowder

In this study, single-phase hardystonite (Ca₂ZnSi₂O₆) nanopowder was synthesized by combustion synthesis method from aqueous solution of Calcium nitrate, Zinc nitrate and Tetraethyl orthosilicate. In order to characterize the obtained powders, X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used. The effect of sucrose concentration on the purity and morphology of the obtained hardystonite powder, and its initial annealing temperature was investigated. The optimum sucrose to metal ratio was found to be 2:1. The average particle size and the maximum crystallite size of the produced hardystonite nanopowder were 220 and 40 nm, respectively.     

Effect of uniaxial load on the sintering behaviour of 45S5 Bioglass powder compacts

The effect of a uniaxial compressive load on the sintering behaviour of 45S5 Bioglass^® powder compacts was investigated by means of sinter-forging. In comparison to free sintering, densification kinetics was enhanced and the degree of crystallization was reduced. Significantly lower sintering temperatures, i.e. 610 °C instead of 1050 °C, can be employed to obtain dense Bioglass^® parts when sintering is performed under uniaxial load. The effect of mechanical loading on microstructure (pore density, shape and orientation) is discussed. The results of the investigation are relevant in connection with the development of sintered Bioglass^® substrates for bone replacement devices, where both porosity and crystallinity of the part require careful control and low densification temperatures are sought.     

Sintering and crystallization of plates prepared from coarse glass ceramic frits

The focus of this study was the development of a CAS glass-ceramic (14.0CaO·5.5Al₂O₃.80·5SiO₂) for use in civil engineering as a decorative material. This glass-ceramic system was chosen because it can be used to produce plates with a similar appearance to those of marble and granite materials. Ceramic plates were produced by the Grain Glass Sintering process from frits with a coarse grain size (2.1 mm average particle size). Powder frit samples (5.3-μm average particle size) were prepared and characterized. The results were compared with those obtained with coarse particles. Thermal treatment was performed in one stage, with the heating rate varying from 2 to 25 °C min^?1. Temperatures were defined by differential thermal analysis and heating microscopy. With these results, it was possible to establish correlations among the esthetic effects, the microstructure, and the heat treatment conditions (temperature, heating rate, and holding time). The relative density ranged from 0.92 to 0.96. The formation of wollastonite and pseudowollastonite was observed. An esthetic effect similar to that of natural marble was more apparent in those samples in which the microstructure presented a visual contrast between the bulk and the coarse frit. Lower heating rates intensified the visual contrast and reduced porosity near the surface regions.     

Effect of iron oxide coloring agent on the sintering behavior of dental yttria-stabilized zirconia

To explore the use of yttria-stabilized zirconia (YSZ) for applications in dentistry, the effect of iron oxide coloring agent on the sintering behavior of YSZ is investigated. Through the use of a small amount of iron nitrate, the color of YSZ can be tailored. The iron nitrate starts to decompose to result in iron oxide, then to dissolve into zirconia grains before the shrinkage is even started. The iron solutes enhance the sintering activity of zirconia in terms of the temperatures at the start of shrinkage and at the maximum shrinkage rate. However, the size of zirconia grains is also increased along with Fe content. More monoclinic phase is found in the specimens with higher Fe content. The formation of m-phase is detrimental to both hardness and toughness of zirconia, limiting the amount of coloring agent can be added.     

Effects of fine grains and sintering additives on stereolithography additive manufactured Al2O3 ceramic

Al₂O₃ ceramics are fabricated by stereolithography based additive manufacturing in present reports. To improve the densification and performance of Al₂O₃ ceramic, the introduction of fine grains or sintering additives has been studied by traditional fabrication techniques. However, no research has focused on the effects of adding fine grains and sintering additives on the stereolithography additive manufactured Al₂O₃ ceramic. In this study, both fine grains and sintering additives were added firstly, and then the effects of fine grains and sintering additives on the relative density, microstructure, mechanical properties, and physical properties of the stereolithography additive manufactured Al₂O₃ ceramics were investigated. Finally, defect-free Al₂O₃ ceramic lattice structures with high precise and high compressive strength were manufactured.     

Design and processing of ZnO doped tricalcium phosphate based materials: Influence of β/α polymorph phase assemblage on microstructural evolution

Tricalcium phosphate (TCP) based biomaterials are excellent candidates in hard tissue engineering due to their similarity to the natural bone composition and outstanding properties. The presence of additives such as (Mg²⁺, Zn ²⁺, F⁻, CO₃²⁻ and/or SiO₄⁻) in solid solution in the structure of TCP, affects the stability of its different polymorphs and therefore the properties of TCP based biomaterials. It is known that the incorporation of zinc in TCP in the non-toxic level stimulates bone growth and its mineralization, hence its interest. Nevertheless its effect on phase assemblage and microstructure evolution has not been clearly established. The main purpose of this study was to synthesize TCP and zinc doped monophasic/biphasic α/β-TCP dense biomaterials, by solid-state sintering process, with different ZnO contents and controlled phase proportions and microstructure on the final material. The effect of ZnO content and sintering temperature on phase assemblage, densification and microstructural evolution has been investigated.     

Effects of carbon doping on the microstructural,micro/nano-mechanical,and mesenchymal stromal cells biocompatibility and osteogenic differentiation properties of alumina

It has been demonstrated that carbon (C) doped aluminium oxide (Al₂O₃) nanocomposite (C −0.012 wt%) had greater wear resistance and lower surface grains pull out percentage when compared with monolithic Al₂O₃. In the present study, we investigated the physicochemical, micro- and nanomechanical, cell attachment, in vitro biocompatibility and osteogenic differentiation properties of Al₂O₃ doped carbon (0.012 wt%) nanocomposite (Al₂O₃/C). Data were compared to values obtained for monolithic alumina (Al₂O₃). The calcined Al₂O₃/C nanocomposite was densified using cold isostatic pressing and followed by pressureless sintering. For physicochemical and microstructural characterisation, Energy dispersive X-ray (EDX), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoemission spectrometer (XPS) were used. EDX, XRD peaks and Raman spectroscopy demonstrated correlating to Al₂O₃/C. Surface profiling and contact angle investigations demonstrated highly contoured micro-surface topography. The micro and nano-hardness indicate an improved wear resistance of the Al₂O₃/C when compared with monolithic Al₂O₃. SEM, confocal images and alamar blue reduction assay suggested good cell attachment and proliferation of human bone marrow derived mesenchymal stromal cells (hBMSCs). Osteogenic protein and gene expression indicated Al₂O₃/C had a significant osteogenic potential (p<0.05) when compared with Al₂O₃. In conclusion, our novel Al₂O₃/C nanocomposite had improved mechanical properties. It also supports cell attachment and proliferation which are comparable to Al₂O₃. However, Al₂O₃/C has a significant osteogenic potential than that of Al₂O₃. These findings suggest that Al₂O₃/C nanocomposite is superior to Al₂O₃ and thus has a greater potential for use in orthopaedic applications.     

Structural,mechanical, and in vitro characterization of freeze-cast scaffolds prepared using a sol-gel-derived bioactive glass from the SiO2–CaO–Na2O–P2O5–K2O–MgO system

This work deals with the preparation of freeze-cast scaffolds using a bioactive glass from the SiO₂–CaO–Na₂O–P₂O₅–K₂O–MgO system. This material could be sintered at lower temperatures (650 °C) than other variations of bioactive glasses, which is an important advantage in terms of energy and cost savings. This behavior represents a great advantage in terms of energy and cost savings. The freeze-casting step was conducted using water as a solvent and liquid nitrogen as a coolant. The prepared samples were examined according to their pore structure, thermal behavior, mechanical stability, and bioactivity. The glass transition temperature (T_g), crystallization onset temperature (T_x), and maximum crystallization temperature (T_c) evaluated for this bioactive glass were about 660 °C, 690 °C, and 705 °C. Consequently, the freeze-cast scaffolds could be sintered at 650 °C for 2–8 h, which favored viscous flow sintering without crystallization. Bioactivity assays were conducted by soaking the scaffolds in simulated body fluid for up to 21 days, showing that these materials present a bioactive behavior, inducing hydroxyapatite formation. These materials' mechanical properties and biocompatibility make them promising candidates for use in trabecular bone repair.     

Digital light processing fabrication of mullite component derived from preceramic precusor using photosensitive hydroxysiloxane as the matrix and alumina nanoparticles as the filler

By taking advantage of the low sintering temperatures of the preceramic polymers, stereolithography printed mullite components derived from preceramic polymer precursor containing alumina particles can be sintered at low temperatures. However, due to their high specific surface, nano alumina particles are difficult to be dispersed into the photocurable polysiloxane. Herein, to prepare mullite slurry, a photosensitive hydroxysiloxane was employed as the preceramic polymer matrix while γ-Al₂O₃ nanoparticle was added as the active filler. The introduction of photocurable hydroxysiloxane not only improved the homogeneity and rheological properties of mullite slurry but also shorted the ionic diffusion distance of Si-ion and Al-ion during the sintering process. Therefore, 3D mullite preceramic precursor stereolithography printed from hydroxysiloxane-Al₂O₃ slurry was endowed with a low sintering temperature around 1400 °C. During the sintering process of preceramic precursor, sintering aid AlF₃ can participate in the reaction and further promote the formulation of mullite grains.     

The influence of MgO,Y2O3 and ZrO2 additions on densification and grain growth of submicrometre alumina sintered by SPS and HIP

Spark plasma sintering followed by hot isostatic pressing was applied for preparation of polycrystalline alumina with submicron grain size. The effect of additives known to influence both densification and grain growth of alumina, such as MgO, ZrO₂ and Y₂O₃ on microstructure development was studied. In the reference undoped alumina the SPS resulted in some microstructure refinement in comparison to conventionally sintered materials. Relative density >99% was achieved at temperatures >1200 °C, but high temperatures led to rapid grain growth. Addition of 500 ppm of MgO, ZrO₂ and Y₂O₃ led, under the same sintering conditions, to microstructure refinement, but inhibited densification. Doped materials with mean grain size <400 nm were prepared, but the relative density did not exceed 97.9%. Subsequent hot isostatic pressing (HIP) at 1200 and 1250 °C led to quick attainment of full density followed by rapid grain growth. The temperature of 1250 °C was required for complete densification of Y₂O₃ and ZrO₂-doped polycrystalline alumina by HIP (relative density >99.8%), and resulted in fully dense opaque materials with mean grain size<500 nm.     

Easy manufacturing of 3D ceramic scaffolds by the foam replica technique combined with sol-gel or ceramic slurry

A material, whose composition was based on the SiO₂–CaO–P₂O₅ system, was used to prepare 3D porous ceramic scaffolds made by the foam replication technique linking the sol-gel route or ceramic slurry in barbotine suspension processes. With an appropriate heat treatment, it was possible to obtain a 3D-scaffold containing two crystalline phases or more, and a controlled nano-to microscale microstructure. The obtained scaffolds presented an interconnected and well-defined porous structure. Its pore size was 0.060–600 μm. It had a rough pore wall that could potentially benefit ingrowth and cell adhesion.In terms of hydroxycarbonate apatite formation in simulated body fluid (SBF), the 3D scaffolds’ bioactivity was confirmed after 3 and 14 days of immersion in SBF. This clearly confirms that these new scaffolds are suitable for bone regeneration applications. Processing technology allows 3D scaffolds to be adapted with particular microstructures to increase/decrease their in vitro behavior.     

Densification and grain growth during solid state sintering of LaPO4

This work is devoted to the kinetic study of densification and grain growth of LaPO₄ ceramics. By sintering at a temperature close to 1500 °C, densification rate can reach up to 98% of the theoretical density and grain growth can be controlled in the range 0.6–4 μm. Isothermal shrinkage measurements carried out by dilatometry revealed that densification occurs by lattice diffusion from the grain boundary to the neck. The activation energy for densification (E_D) is evaluated as 480 ± 4 kJ mol⁻¹. Grain growth is governed by lattice diffusion controlled pore drag and the activation energy (E_G) is found to be 603 ± 2 kJ mol⁻¹. The pore mobility is so low that grain growth only occurs for almost fully dense materials.     

Preparation and characterisation of porous composite biomaterials based on silicon nitride and bioglass

The combination of bioinert and bioactive material offers new potentialities in bone tissue engineering. The present paper deals with preparation of novel biomaterial composite based on silicon nitride (Si₃N₄) and bioglass (in amount of 10 and 30 wt%) by free sintering at 980 °C for 1 h in nitrogen atmosphere. The obtained material was characterised by differential thermal analysis (DTA) and X-ray powder diffraction (XRD), porosity and pore size distribution were evaluated by means of mercury intrusion porosimetry (MIP). The bioactivity was examined in vitro with respect to the ability of hydroxyapatite layer formation on the surface of materials as a result of contact with simulated body fluid (SBF). All composites were studied by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) before and after immersion in SBF. The bioglass-free sample was prepared as a reference material to compare the microstructure and bioactivity to the composites.     

Electrophoretic deposition (EPD) of nanohydroxyapatite - nanosilver coatings on Ti13Zr13Nb alloy

Titanium and its alloys are the biomaterials most frequently used in medical engineering, especially as parts of orthopedic and dental implants. The surfaces of titanium and its alloys are usually modified to improve their biocompatibility and bioactivity, for example, in connection with the deposition of hydroxyapatite coatings.The objective of the present research was to elaborate the technology of electrophoretic deposition (EPD) of nanohydroxyapatite (nanoHAp) coatings decorated with silver nanoparticles (nanoAg) and to investigate the mechanical and chemical properties of these coatings as determined by EPD voltage and the presence of nanoAg. The deposition of nanoHAp was carried out at two voltage values, 15 and 30 V. The decoration of nanoHAp coatings with nanoAg was carried out using the EPD process at a voltage value of 60 V and a deposition time of 5 min. The thickness of the undecorated coatings was found to be 2.16 and 5.14 µm for applied EPD voltages of 15- and 30-V, respectively. The release rate of silver nanoparticles into an artificial saliva solution increased with exposure time and EPD voltage. The corrosion current, between 1 and 10 nA/cm², was significantly higher for undecorated nanoHAp coatings and close to that of the substrate for decorated nanoHAp coatings. The hardness of the undecorated nanoHAp coatings obtained at 15 and 30 V of EPD voltage attained 0.2245±0.036 and 0.0661±0.008 GPa, respectively. Resistance to nanoscratching was higher for thicker coatings. The wettability angle was lower for coatings decorated with nanoAg.     

Marine biological waste as a source of hydroxyapatite for bone tissue engineering applications

The demand for bone graft substitutes for orthopedics and dentistry is constantly growing due to the increase of ageing-related diseases. Synthetic hydroxyapatite (HA) is largely used as a bone graft material thanks to its biocompatibility, osteointegration, osteoconductive and osteoinductive properties and similarity to biological apatite, the main mineral component of bones and teeth. Biogenic apatite has gained attention due to its peculiar intrinsic characteristics: multi-doped ion composition and micro- and nano-scale architecture make natural-derived HA particularly promising for biomedical applications.At the same time, the growing interest in green materials is pushing towards the use of more sustainable biomaterials precursors, including re-use materials: marine waste, such as mollusk-shells, shellfish carapaces, cuttlefish bone, and fishbone have become widely studied sources of biogenic HA. Indeed, they are rich in calcium carbonate (CaCO₃), which can be converted into HA by environmentally sustainable processes. This allows the transformation of waste into valuable materials, while paying attention to the issues of sustainability and circular economy.In this review, we listed and discussed the methods to produce HA starting from shell-derived CaCO₃, describing all the steps and synthesis routes proposed for the conversion procedure, with a special focus on the different species of marine shells used. We discussed the use of HA alone or in combination with other materials (natural and synthetic polymers), used to enhance the mechanical and biological properties.We summarized the types of devices obtained by marine-derived HA, including nanorods, particulates and scaffolds and we described their in vitro and in vivo behavior.The up-to-date literature was summarized in tables with a special focus on the in vitro and in vivo biological evaluation of such materials.In conclusion, composite biomaterials based on marine-derived biogenic HA are reported as potential candidates for synthetic bone substitutes highlighting their potential, limitations and future perspectives.     

Properties of calcium phosphates ceramic composites derived from natural materials

In this study, ceramics containing mixed phases of hydroxyapatite/beta-tricalcium phosphate (HA/β-TCP) were fabricated by a solid-state reaction technique. The HA powder was synthesized from cockle shells while the β-TCP powder was synthesized from egg shells. Pure HA and β-TCP fine powders were successfully obtained. The HA and β-TCP were mixed and subjected to a thermal treatment up to 1100 °C. To form the mixed phase ceramics, the resulting powders were sintered at 1350 °C. Effects of HA concentration on the properties of the studied ceramic were investigated. X-ray diffraction analysis revealed that all samples presented multiphase of calcium phosphate compounds. Average grain size of the ceramics decreased with the HA additive content. The 75 wt% HA ceramic showed the maximum hardness value (5.5 GPa) which is high when compared with many calcium phosphate ceramics. In vitro bioactivity test indicated that apatite forming increased with the HA additive content. To increase antibacterial activity, selected ceramics were coated with AgNO₃. Antibacterial test suggested that an Ag compound coating on the ceramics could improve the antibacterial ability of the studied ceramics. In addition, the antibacterial ability for the Ag coated ceramics depended on the porosity of the ceramics.     

Microstructural development of interface layers between co-sintered alumina and spinel compacts

Tests were performed to investigate the microstructure of the interface between alumina and spinel materials after high temperature thermal treatment (1500 °C). The first test involved co-sintering of co-pressed alumina and spinel compacts. Microstructures were investigated by SEM, EDS, WDS and EBSD. A microstructurally distinct layer with columnar grains of up to 40 μm length and 5 μm width was observed after 16 h at 1500 °C. Growth rate of the columnar spinel grains from parent spinel towards alumina follows parabolic kinetics, controlled by a mixed process of O²⁻ ion diffusion and interface reaction. Diffusion couples of spinel and alumina were investigated. Same columnar spinel grains were observed at the interface which grew into alumina during thermal treatment with the same kinetics as in co-sintering experiments. The shape of the phase boundaries between spinel and alumina can be a further indication of the direction of their growth.