Production and study of in vitro behaviour of monolithic α-tricalcium phosphate based ceramics in the system Ca3(PO4)2–Ca2SiO4

In this work a new kind of α-tricalcium phosphate (α-Ca₃(PO₄)₂) doped with dicalcium silicate (Ca₂SiO₄) ceramic materials, with compositions lying in the field of the Ca₃(PO₄)₂ solid solution in the system Ca₃(PO₄)₂–Ca₂SiO₄, were obtained. The properties of the sintered ceramics were discussed in detail as well as some in vitro relevant properties for bone repairing. Crystalline α-Ca₃(PO₄)₂ solid solution (α-TCPss) was the only phase in the ceramics containing from 1 wt% to 4 wt% of Ca₂SiO₄. The release of ionic Si in simulated body fluid increased with the content of Ca₂SiO₄ and favoured α-Ca₃(PO₄)₂ surface transformation. In addition, cell attachment test showed that the α-TCPss supported the mesenchymal stem cells adhesion and spreading, and the cells established close contact with the ceramics after 24 h of culture. According to the results, the investigated α-TCPss ceramics possesses good bioactivity, biocompatibility and mechanical properties, and might be a promising bone implant material.     

Pore structure control of Si3N4 ceramics based on particle-stabilized foams

Porous Si₃N₄ ceramics with open, closed pores and nest-like structures were prepared by direct foaming method, and the stability of bubbles and the microstructures of sintered Si₃N₄ foam ceramics were investigated. The bubbles produced by short-chain amphiphiles have higher stability as compared with that produced by long-chain surfactants. Si₃N₄ ceramic foams using short-chain amphiphiles are particle-stabilized one, porous Si₃N₄ ceramics with open and closed pores can be easily prepared with this method, and the nest-like microstructure in Si₃N₄ foam ceramics is achieved at high gas-pressure sintering conditions. The decrease of flexural strength due to the increase of porosity can be weakened by decreasing pore size.     

Structural investigations on P2O5-CaO-Na2O-K2O: SrO bioactive glass ceramics

In the present work, glasses of a particular composition (60-x) P₂O₅-20CaO-17Na₂O-3K₂O: xSrO (0.5≤x≤1.5) mol% were synthesized using conventional melt quenching technique. Further, samples were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Differential Thermal Analyses (DTA) techniques and Fourier Transform Infrared (FT-IR) spectra. In vitro bioactivity was evaluated by soaking glass ceramic powders in SBF solution for 7 and 15 days. XRD patterns of glass ceramics have clearly confirmed the formation of various crystalline phases K₂Sr(PO₃)₄, α-Ca₂P₂O₇, Ca₂Sr(PO₄)₂, Ca₅(PO₄)₃(OH) and Ca₃(PO₄)₂. Random spreading of uneven sized micro crystals with distinct boundaries in the glass matrix have been observed from SEM pictures. DTA scans revealed an increase in the content of SrO with heating rate causes the glass transition (T_g) and crystallization temperatures (T_c) towards lower side, that confirms the decrease in rigidity of glass network. FT-IR spectra showed that there is an increase in the degree of structural disorder and the formation of a crystalline hydroxyapatite layer with soaking time. From the analyses of all the above results, it can be concluded that the sample doped with 1.5 mol% of strontium is found to exhibit high bioactivity.     

Fabrication and properties of Si3N4 bioscaffolds with orderly–interconnected big pore channels and well–distributed small pores

This paper focuses on investigating the technical potential for fabricating porous ceramic bioscaffolds for the repair of osseous defects from trauma or disease by inverse replication of three–dimensional (3–D) printed polymer template. Si₃N₄ ceramics with pore structure comprising orderly–interconnected big pore channels and well–distributed small pores are successfully fabricated by a technique combining 3–D printing, vacuum suction filtration and oxidation sintering. The Si₃N₄ ceramics fabricated from the Si₃N₄ powder with addition of 10?wt% talcum by sintering at 1250?°C for 2?h have little deformation, uniform microstructure, low linear shrinkage of 4.1%, high open porosity of 58.2%, relatively high compression strength of 6.4?MPa, orderly–interconnected big pore channels and well–distributed small pores, which are promising bioscaffold in the field of bone tissue engineering.     

Fabrication and properties of CaSiO3/ Sr3(PO4)2 composite scaffold based on extrusion deposition

In recent years, 3D printing technology has been increasingly used to fabricate porous bone scaffolds for treating bone tissue defects. Calcium silicate (CS) is a bioceramic material with broad application prospects, but the characteristics of poor sintering performance and fast degradation have limited its further application. In this paper, porous CS scaffolds with different proportions of strontium phosphate (Sr₃(PO₄)₂) were formed by pneumatic extrusion deposition. Experiments showed that the Sr element replaced the Ca element in CaSiO₃, which altered the crystal structure of CaSiO₃, changed its physical and chemical properties, and improved the sintering property of CS ceramics. At the same time, the substituted Ca element formed Ca₃(PO₄)₂. After mixing Ca₃(PO₄)₂ and CaSiO₃, the grain of CaSiO₃ was refined and the sintering property was improved. Because of this dual role, the Sr element improved the sintering property of CaSiO₃ ceramics and delayed the degradation of CS ceramics. Moreover, cell experiments showed that the addition of the Sr element had a positive effect on cell proliferation and differentiation.     

Microstructure and Mechanical Properties of Lu2O3-Doped Porous Silicon Nitride Ceramics Using Phenolic Resin as Pore-Forming Agent

The joint process consisting of pressureless sintering and chemical vapor infiltration (CVI) was developed to prepare porous Si₃N₄ ceramics with controlled microstructure. Lu₂O₃ and phenolic resin acted as sintering aid and pore-forming agent, respectively. The 5 wt% Lu₂O₃-doped ceramics using 12–57 vol% phenolic resin attained a porosity ranging from 46% to 53%. With increasing the resin content, the average pore size increased from 1 to 2 μm. The porous ceramic infiltrated with CVI Si₃N₄ had an improved microstructure. The decreased pore size and porosity led to an increase in flexural strength, and the densified surface led to an improved surface hardness.     

Synthesis of Nb2O5 and Ta2O5 nanopowders by means of an endo-templating method

Finding efficient templates for the nanostructuring of materials is a key point. Here, the niobium (V) - and tantalum (V) oxide ceramics nanopowders have been synthesized by a hard-templating approach by using the tricalcium phosphate biomaterial (Ca₃(PO₄)₂.xH₂O) as template agent. The oxide ceramics were investigated by X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, nitrogen physisorption and Scanning Electron Microscopy (SEM). It was observed that the surface properties (specific surface areas, pore volumes) of the Nb₂O₅ and Ta₂O₅ powders were strongly dependent of the amount of the Ca₃(PO₄)₂.xH₂O template previously used in the preparation of the [oxide ceramics/template] composites. For instance, with a Ca₃(PO₄)₂.xH₂O/ceramic salt weight ratio ranging from 0 to 1, the specific surface areas of Nb₂O₅ and Ta₂O₅ were gradually enhanced of 48–166 m²/g and 5–84 m²/g, respectively. The pore volumes were increased as well. The use of the eco-friendly tricalcium phosphate material (Ca₃(PO₄)₂.xH₂O) as template in the hard-templating approach may be suitable and efficient in the aim of synthesizing materials with enhanced surface properties.     

Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering

Photocurable gray-colored Si₃N₄ ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si₃N₄ ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, D_p (the attenuation length) and E_c (the critical energy dose) of Si₃N₄ ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm², respectively. Si₃N₄ ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si₃N₄ ceramics with 3.28 g/cm³ sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si₃N₄ ceramics are ~14.63 GPa and ~5.82 MPa m^1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si₃N₄ ceramics.     

ZrO2-phosphates porous ceramic obtained via SPS-RS “in situ” technique: Bacteria test assessment

The work presents an original way to obtain porous and mechanically strong (to 560 MPa) ZrO₂ ceramics containing calcium phosphates, HAP (Ca₁₀(PO₄)₆(OH)₂) and TCP (Ca₃(PO₄)₂), using SPS-RS technique. “In situ” formation of calcium phosphate phases (15 and 50 wt%) has been observed in the bulk of ZrO₂ arising from solid-phase interaction of CaO and CaHPO4 mixture at SPS conditions (900–1300 °C). Temperature regime for “in situ” interaction was optimized in accordance with HAP and TCP stability investigations under conditions of oxidative annealing and SPS heating without ZrO₂. Addition of a poreforming agent (carbon template) has been demonstrated to enhance the porosity of the ZrO₂-phosphate ceramics. Effect of the poreformer amount (2, 5, 10 and 15 wt%) on the structural and mechanical properties of ceramics has been studied by the means of mercury porosimetry, low-temperature nitrogen sorption, microscopy, and other analytical techniques. Microbiological tests were performed to assess the efficiency of bacterial film formation on the samples of composite ceramics depending on the calcium phosphate content. Morphology of the biofilms and the relative surface they occupied on the ceramics were investigated using electron microscopy and image processing method based on local binary pattern (LBP) classifier. Suggested SPS-RS method provides porous and mechanically strong ZrO₂-phosphate composite ceramics containing biocompatible components HAP and TCP, which can be prospective for bone-ceramic implants for bone tissue recovery.     

Low‐Temperature Preparation of β‐Si3N4 Porous Ceramics with a Small Amount of Li2O–Y2O3

Porous β‐Si₃N₄ ceramics are sintered at 1600°C in N₂ and postheat treated at 1500°C under vacuum using Li₂O and Y₂O₃ as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li₂O. The addition of Y₂O₃ is advantageous for the formation of high aspect ratio β‐Si₃N₄ grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β‐Si₃N₄ porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li₂O and 0.33 wt% Y₂O₃, and it is suitable for high‐temperature applications.     

Low-temperature preparation of porous SiC ceramics using phosphoric acid as a pore-forming agent and a binder

Porous SiC ceramics combine the properties of both SiC ceramics and porous materials. Herein, we design a facile method via pressureless sintering at relatively low temperatures for the synthesis of porous SiC ceramics. In the synthesis process, phosphoric acid was used as the sintering additive that reacted with SiO₂ on the surface of SiC to form phosphates. The formed phosphates acted as a binder to connect the SiC particles. At a fixed temperature, the phosphates were partially decomposed and released a large amount of gas. This changed the pore structure of the ceramics and greatly improved their porosity. Finally, we obtained the porous SiC ceramics with high porosity and high strength. We investigate the effects of H₃PO₄ content on the phase composition, microstructure, porosity, mechanical properties and thermal expansion coefficient of the prepared porous SiC ceramics. It was shown that at the sintering temperature of 1200 °C, the highest porosity of the samples can reach 70.42% when the H₃PO₄ content is 25 wt%, and their bending strength reaches 36.11 MPa at room temperature when the H₃PO₄ content is 15 wt%. In addition, the porous SiC ceramics show good high-temperature stability with a bending strength of 42.05 MPa at 1000 °C and the thermal expansion coefficient of 3.966 × 10⁻⁶/°C.     

Fabrication,microstructure, mechanical and luminescence properties of transparent Yb3+-doped Sr5(PO4)3F nanostructured ceramics

The Sr₅(PO₄)₃F (S-FAP) crystal material is regarded as one of the most ideal optical materials for diode pumping owing to its huge absorption and emission cross sections and long fluorescence lifespan. In this investigation, S-FAP powders with varying Yb concentrations (0.1–5%) were produced using the coprecipitation method. Then a variety of S-FAP transparent ceramics with varying Yb content were fabricated using hot-pressing sintering. The crystalline phase structure of hexagonal Sr₅(PO₄)₃F was verified by XRD analysis of the precursor powder and the final ceramics. According to the powder SEM, the average grain size and the long axial-radial ratio of powders are decreasing as the Yb³⁺ concentration increases. Thermal-etched surface SEM reveals nanostructured S-FAP transparent ceramics with an average grain size of less than 200 nm were synthesized. The highest transmittances of the 3% ceramics at 500 and 1100 nm wavelengths are 51% and 79.78%, respectively. The ceramic cross-sectional SEM demonstrated that porosity is the primary scattering source influencing the enhancement of optical characteristics. The absorption, emission, and fluorescence lifetimes of S-FAP transparent ceramics with varying Yb concentrations were tested and discussed, and the absorption and emission cross sections corresponding to the major peak were reported. Some physical parameters of this set of ceramic samples were shown, including thermal diffusivity, specific heat capacity, and thermal diffusivity data, as well as micro-hardness.     

Preparation of high-strength Si3N4 antenna window using selective laser sintering

In this study, a high-strength silicon nitride (Si₃N₄) antenna window was successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding before final sintering. The effects of CIP after debinding and sintering aids on the bulk density, total porosity, bending strength and microstructure of Si₃N₄ ceramics were examined. The results show that the bending strength of SLS Si₃N₄ ceramics can be greatly improved by adding sintering aids between Si₃N₄ granules and by CIP after debinding. Optimal performance of ceramics is obtained by CIP after debinding and the use of inter-granule sintering aids. The porosity, bulk density, and bending strength are 18.7%, 3.11 g/cm³, and 685 MPa, respectively. Eliminating the pores by the CIP after debinding and by inter-granule sintering aids promotes the growth of rod-like β-Si₃N₄, which lock with each other contribute to the strengthening of Si₃N₄ ceramics.     

Bioactive fluorapatite-containing glass ceramics

Fluorapatite-containing glass ceramics were synthesized on the basis of the glass-forming system SiO₂–Al₂O₃–P₂O₅–CaO–CaF₂. The introduction of phosphorus and fluorine containing materials, as well as the specific regime of heat treatment of the glasses gave glass ceramic materials with crystalline phases of the apatite group—fluorapatite (Ca₁₀(PO₄)₆F₂), apatite (Ca₃(PO₄)₂), vitlokite (Ca₉P₆O₂₄), etc. The X-ray phase analysis showed that the main phase in all the glass ceramic samples was fluorapatite. The phase composition, structure and some of the basic properties of the glass ceramic samples were determined.     

SiAlON–Al2O3 ceramics as potential biomaterials

When used as implants, Al₂O₃ is unable of directly achieving good chemical bonding with soft and hard tissues. To overcome this problem, SiAlON–Al₂O₃ ceramics were prepared in this study by direct nitridation. Phase composition, porosity, bulk density, and compression strengths were examined, and biological properties were evaluated by cell culture on ceramic surface. Major phase of SiAlON–Al₂O₃ ceramics was identified as Si₄Al₂O₂N₆, formed by reaction of Si, Al and Al₂O₃ under nitrogen atmosphere at high temperature. As Al₂O₃ content increased, porosity and compressive strength decreased. Therefore, Si₄Al₂O₂N₆ phase could improve sintering, leading to formation of composites with better properties. The porosity and compression strength were found suitable for requirement of biomaterials. Cell culture experiments showed that cells could proliferate and survival well on ceramic surface, indicating good biocompatibility of Si₄Al₂O₂N₆ phase in SiAlON–Al₂O₃ ceramics. Overall, these data look promising and might provide novel strategies for development of future SiAlON–Al₂O₃ bioceramics.     

Fabrication of textured B4C ceramics with oriented tubal pores by strong magnetic field-assisted colloidal processing

Highly structure-controlled B₄C ceramics were prepared via strong magnetic field-assisted slip casting of a slurry, containing B₄C base particles and pore-forming agents with a fiber shape. To achieve gas release at a lower porosity for maintaining its mechanical strength, these B₄C ceramics had a structure in which a large number of oriented tubal pores were dispersed in a crystallographically-aligned and dense B₄C matrix phase. The B₄C microstructure, such as structuration and orientation degree distributions of the B₄C grains and tubal pores, was characterized by SEM observation, EBSD analysis, and X-ray CT. Among the investigations, it was found that the oxidic impurities, as an inhibitor of sintering, which existed on the B₄C surface, can be removed by ethylation and azeotropy due to an ethanol treatment followed by vacuum drying. Thus, an ethanol treatment of a green compact before sintering was significantly effective for the fabrication of the B₄C ceramics, including the microstructure that coexisted with a dense matrix phase with tubal pores. The resultant ceramic specimens showed the remarkable three-point bending strength of 459?554 MPa, which is two times higher when compared to conventional B₄C pellets with a similar porosity.     

Calcium Cl/OH-apatite,Cl/OH-apatite/Al2O3 and Ca3(PO4)2 fibre nonwovens: Potential ceramic components for osteosynthesis

Polycrystalline calcium phosphate ((Cl/OH)Ap = Ca₅(PO₄)₃(OH/Cl); TCP = Ca₃(PO₄)₂) fibres were prepared from aqueous solutions of calcium chloride and phosphoric acid using poly(ethylene oxide) (PEO) as spinning aid. Generation of nonwoven materials was accomplished via rotary jet spinning. Polycrystalline (Cl/OH)Ap fibres 10–25 μm in diameter were obtained with 37% ceramic yield by pyrolysis of the green fibres followed by sintering at 1150 °C in air. X-ray diffraction (XRD) analysis provided evidence for apatite formation starting at 650 °C while (Cl/OH)Ap ceramic fibres were obtained at 1100 °C via transformation through intermediate dicalcium dichloride hydrogen phosphate (Ca₂Cl₂(HPO₄)) and calcium pyrophosphate (Ca₂P₂O₇) phases. A glass-forming Al-based additive was applied to enhance the mechanical properties of the Cl/OH)Ap ceramic fibres and indeed resulted in the formation of (Cl/OH)Ap/Al₂O₃ fibres with improved mechanical stability. Finally, TCP, (Cl/OH)Ap and (Cl/OH)Ap/Al₂O₃ fibres were subjected to seeding with mesenchymal stem cells. Negligible cytotoxicity is observed.     

Effects of AlN inorganic binder on the properties of porous Si3N4 ceramics prepared by selective laser sintering

Porous Si₃N₄ ceramics are widely used in the aerospace field due to its lightweight, high-strength, and high wave transmission. Traditional manufacturing methods are difficult to fabricate complex structural and functional ceramic parts. In this paper, selective laser sintering (SLS) technology was applied to prepare porous Si₃N₄ ceramics using AlN as an inorganic binder. And the effects of AlN content on the properties of the obtained ceramic samples were explored. As the AlN content increased, nano-Al₂O₃ and nano-SiO₂ formed the eutectic liquid phase, enhancing the sintering densification and phase transformation of Si₃N₄ poly-hollow microspheres (PHMs). The island-like partial densification structures in Si₃N₄ green bodies increased. During the high-temperature sintering, the eutectic liquid phase partially transformed into the mullite phase or reacted with AlN and Si₃N₄ to form the Sialon phase. With the increase of AlN content, the fracture mode of Si₃N₄ ceramics changed from fracturing along PHMs to fracturing across PHMs. The bonding depth between PHMs increased and the connection between the grains was tighter, so the Si₃N₄ ceramics became denser. With the increase of AlN addition, the total porosity of the porous Si₃N₄ ceramics tended to decrease and the flexural strength gradually increased. When AlN content was 20 wt%, the total porosity and the flexural strength were 33.6% and 23.9 MPa, respectively. The addition of AlN inorganic binder was carried out to develop a novel way to prepare high-performance porous Si₃N₄ ceramics by SLS.     

Microstructural design of ceramics for bone regeneration

Dense tricalcium phosphate, Ca₃(PO₄)₂ (TCP) – diopside, CaMg(SiO₃)₂, composites present better mechanical properties than single phase TCP. In this work, it is investigated whether the mechanical behaviour improvement by diopside is maintained in porous scaffolds.The processing parameters to obtain cylinders with ≈ 50% of aligned pores of elliptical cross sections with major axis up to 100 μm by freeze casting and sintering were established. Pore channels were introduced in the green specimens by laser ablation. After sintering, the diameter of the cross sections of the channels was ≈ 700 μm. The ceramic composite microstructure was constituted by a substructure of small diopside particles (2 −7 μm) and dense β-TCP zones of larger dimensions (up to 40 μm). Strength values, determined by diametral compression (DCDT) (≈ 2.5–4 MPa) are in the range of strength of cancellous bone. Diopside presents transgranular fracture, hindering crack propagation from the β-TCP areas and the pores, as occurred in the dense materials.     

Fabrication and characterization of porous ZrO2 with a high volume fraction of fine closed pores

A novel technique for the fabrication of porous ZrO₂ with a high volume fraction of fine closed pores was investigated. A partially stabilized ZrO₂ (3 mol% Y₂O₃; Y-PSZ) body, with a 97–99% relative density and containing a small amount of impurities, exhibited a large volume expansion related to the formation of closed pores after heating at 1700 °C for 10 min in N₂. These closed pores seemed to mainly form due to the vaporization of hydroxyl apatite: Ca₁₀(OH)₂(PO₄)₆ as an impurity and superplasticity of the ZrO₂ during heating. Porous ZrO₂ with approximately 24.6% closed pores (total porosity: 26.7%) was successfully fabricated by the addition of 1 mass% SiO₂, 1 mass% TiO₂, and 1 mass% hydroxyl apatite. The closed pore size and morphology of the resultant porous ZrO₂ bodies were investigated, and the formation mechanism of the closed pores was examined on the basis of chemical thermodynamics.