Porous β-Ca2SiO4 ceramic scaffolds for bone tissue engineering: In vitro and in vivo characterization

The biodegradable ceramic scaffolds with desirable pore size, porosity and mechanical properties play a crucial role in bone tissue engineering and bone transplantation. A novel porous β-dicalcium silicate (β-Ca₂SiO₄) ceramic scaffold was prepared by sintering the green body consisting of CaCO₃ and SiO₂ at 1300 °C, which generated interconnected pore network with proper pore size of about 300 μm and high compressive strength (28.13±5.37–10.36±0.83 MPa) following the porosity from 53.54±5.37% to 71.44±0.83%. Porous β-Ca₂SiO₄ ceramic scaffolds displayed a good biocompatibility, since human osteoblast-like MG-63 cells and goat bone mesenchymal stem cells (BMSCs) proliferated continuously on the scaffolds after 7 d culture. The porous β-Ca₂SiO₄ ceramic scaffolds revealed well apatite-forming ability when incubated in the simulated body fluid (SBF). According to the histological test, the degradation of porous β-Ca₂SiO₄ ceramic scaffolds and the new bone tissue generation in vivo were observed following 9 weeks implantation in nude mice. These results suggested that the porous β-Ca₂SiO₄ ceramic scaffolds could be potentially applied in bone tissue engineering.     

Increased high temperature strength and oxidation resistance of Al4SiC4 ceramics

Al₄SiC₄ bulk ceramics were synthesized by reaction hot-pressing using Al, graphite powders and polycarbosilane (PCS) as starting materials. The present work confirmed that this process was an effective method for the preparation of Al₄SiC₄ ceramics having high relative density and well-developed plate-like grains. The mechanical, thermal properties and oxidation behaviors of the Al₄SiC₄ ceramics were also investigated. The flexural strength, fracture toughness (K_IC) and Vickers hardness at room temperature were 297.1 ± 22 MPa, 3.98 ± 0.05 MPa m^1/2, 10.6 ± 1.8 GPa, respectively. The high-temperature bending strength showed an increasing trend with increasing test temperatures, with the value of 449.7 ± 26 MPa at 1300 °C. The thermal expansion coefficient was 6.2 × 10⁻⁶ °C⁻¹ in the temperature range from 200 °C to 1450 °C. The isothermal oxidation of Al₄SiC₄ ceramics at 1200–1600 °C for 10–20 h revealed that it had excellent oxidation resistance.     

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.     

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100 °C and 1450 °C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO₂ transformed into monoclinic ZrO₂; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300 °C, and the lowest values were observed at 1200 °C. The highest flexural strength and fracture toughness of ceramics reached 348.27 MPa and 5.23 MPa m^1/2 when sintered at 1450 °C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250 °C, and in this range, a low brittleness index and moderate strength of 0.74–1.19 µm^−1/2 and 46.89–120.15 MPa, respectively, were realized.     

Spark plasma sintering of silicon nitride/barium aluminum silicate composite

Si₃N₄ based composites were successfully sintered by spark plasma sintering using low cost BaCO₃, SiO₂ and Al₂O₃ as additives. Powder mixtures were sintered at 1600–1800 °C for 5 and 10 min. Displacement-temperature-time (DTT) diagrams were used to evaluate the sintering behavior. Shrinkage curve revealed that densification was performed between 1100 and 1700 °C. The specimen sintered at 1700 °C showed the maximum relative density (99.8±0.1%), flexural strength (352±16 MPa), Vickers harness (11±0.1 GPa) and toughness (5.6±0.05 MPa m^1/2).     

Preparation and thermal shock resistance of cordierite-spodumene composite ceramics for solar heat transmission pipeline

Cordierite-spodumene composite ceramics with 5, 10, 15 wt% spodumene used for solar heat transmission pipeline were in-situ prepared via pressureless sintering from kaolin, talc, γ-Al₂O₃ and spodumene. Effects of spodumene on densification, mechanical properties, thermal shock resistance, phase composition and microstructure of the composite ceramics were investigated. The results showed that spodumene used as flux material decreased the sintering temperature greatly by 40–80 °C, and improved densification and mechanical properties of the composite ceramics. Especially, sample A3 with 10 wt% spodumene additive sintered at 1380 °C exhibited the best bending strength and thermal shock resistance. The bending strengths of A3 before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 102.88 MPa and 96.29 MPa, respectively. XRD analysis indicated that the main phases of the samples before 30 thermal shock cycles were α-cordierite, α-quartz and MgAl₂O₄, and plenty of β-spodumene appeared after thermal shock. SEM micrographs illustrated that the submicron β-spodumene grains generated at the grain boundaries after thermal shock improved the thermal shock resistance. It is believed that the cordierite-spodumene composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.     

Facile synthesis of SiOx spheres or dumbbell-shaped β-SiC whiskers on expanded graphite by silicon vapor deposition

A facile method was developed to synthesize SiO_x spheres or dumbbell-shaped β-SiC whiskers on expanded graphite (SiO_x/EG or β-SiC/EG) by silicon vapor deposition without catalyst. With the carbon black atmosphere, the above hybrids were synthesized above 1100 °C in a graphite crucible where silicon powder was placed under the expanded graphite (EG). The growth of SiO_x spheres is controlled by vapor-solid mechanism at 1100 °C and 1200 °C. Namely, the active carbon atoms absorbed SiO (g) and Si (g) to form SiC nuclei. Then, the SiO₂, residual SiO (g) and Si (g) deposited on SiC nuclei to form SiO_x spheres. At 1300 °C and 1400 °C, the same SiO_x spheres formed on EG as well as many dumbbell-shaped β-SiC whiskers. The growth of dumbbell-shaped β-SiC whiskers is controlled by vapor-vapor and vapor-solid mechanism successively. In a word, firstly, the β-SiC whiskers with defects formed via the reaction between Si (g) and CO (g). After that, the SiO₂, residual SiO (g) and residual Si (g) preferentially deposited on defects, then deposited on other parts of β-SiC whiskers to form dumbbell-shaped SiC whiskers.     

Microstructural evolution and dielectric properties of SiO2-doped CaCu3Ti4O12 ceramics

The abnormal grain growth (AGG) behavior of undoped and SiO₂-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. With the addition of 2 wt.% SiO₂, the AGG-triggering temperature decreased from 1100 to 1060 °C, and the temperature for obtaining a uniform and coarse microstructure decreased from 1140 to 1100 °C. The lowering of the AGG temperature by SiO₂ addition was attributed to the formation of a CuO-SiO₂-rich intergranular phase at lower temperature. The apparent dielectric permittivity of coarse SiO₂-doped CCTO ceramics was ∼10 times higher than that of fine SiO₂-doped CCTO ceramics at the frequency of 10³–10⁵ Hz. The doping of SiO₂ to CCTO ceramics provides an efficient route of improving the dielectric properties via grain coarsening. The correlation between the microstructure and apparent permittivity suggests the presence of a barrier layer near the grain boundary.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

Silver cofirable (Ca0.9, Mg0.1)SiO3 microwave dielectric ceramics with Li2CO3–Bi2O3 additive

The effects of Li₂CO₃–Bi₂O₃ (LB) additive on the microstructure, phase formation, microwave dielectric properties and applicability for low-temperature co-fired ceramics (LTCC) technology of (Ca_0.9Mg_0.1)SiO₃ (CMS) ceramics were investigated. The sintering temperature of the CMS ceramics was reduced from 1290 °C to 890 °C by the addition of LB. Secondary phases SiO₂ and Bi₄(SiO₄)₃ were detected when LB content was less than 9 wt%. Low melting point liquid phases were formed when LB content was 11–14 wt%. The Q_f value initially increased with the addition of LB and attained the maximum value for the 9 wt% LB-doped CMS ceramic. When the LB content exceeded 9 wt%, the Q_f value decreased because of the presence of liquid phase and abnormal growth of grains. ?_r of 6.92, Q_f of 27,600 GHz and τ_f of ?43.6 ppm/°C were obtained for 9 wt% LB-doped CMS ceramics sintered at 890 °C for 2 h. Also the ceramics can be well co-fired with Ag electrode.     

Densification and biocompatibility of sintering 3.0 mol% yttria-tetragonal ZrO2 polycrystal ceramics with x wt% Fe2O3 and 5.0 wt% mica powders additive

The densification and biocompatibility of sintered 3.0 mol% yttria-tetragonal zirconia polycrystal (3Y-TZP) ceramics, with X wt% Fe₂O₃ and 5.0 wt% mica powders (denoted by 3Y-TZP: X-5.0 wt% mica) have been studied. When the pellets of 3Y-TZP: X-5.0 wt% mica were sintered at 1300 °C for 1 h, the relative shrinkage increases from 19.20–19.43% with the X increased from 0.3 to 1.0. The relative shrinkage of pellets containing 1.0 wt% Fe₂O₃ (X=1.0) increased from 19.43–19.59% when sintering temperatures were raised from 1300 °C to 1450 °C. X-ray diffraction results show that the pellets of 3Y-TZP: X-5.0 wt% mica sintered at 1400 °C for 1 h only contained single phase of tetragonal ZrO₂ (t-ZrO₂). When the sintering temperature was higher than 1400 °C, the Vickers microhardness was greatest in the pellets with X=0.5. Within pellets with the same Fe₂O₃ content, the dominant wavelength (λ_d) was only slightly different for pellets sintered at 1300 °C and those sintered at 1450 °C. The results of the materials were evaluated in vitro cytotoxicity tests reveals that the powders and sintered pellets are safe materials. The oral mucosa irritation tests did not find erythema or histopathological change including normal epithelium, and was free from leucocyte infiltration, vascular congestion and oedema.     

Graded Si3N4 ceramics with hard surface and tough core by two-step hot pressing

Graded Si₃N₄ ceramics with hard surface and tough core were prepared by two-step hot pressing with the homogenous starting composition. The inner Si₃N₄ layer was firstly hot-pressed at 1800 °C, subsequently covered with Si₃N₄ powders on both sides, and finally hot-pressed at 1600 °C. After two-step hot pressing, the resulting ceramics exhibited a zoned microstructure, differentiated by the phase assemblage of Si₃N₄ and grain size. The outer layers were well bonded to the inner layer. The outer layer exhibited bimodal and fine-grained microstructure, whereas the inner layer exhibited bimodal and coarse-grained microstructure. Vickers hardness of outer and inner layers were 18.1±0.2 GPa and 16.0±0.2 GPa, respectively, and fracture toughness were 4.2±0.1 MPa m^1/2 and 5.5±0.2 MPa m^1/2, respectively.     

The phase stability and thermophysical properties of InFeO3(ZnO)m (m = 2, 3, 4, 5)

The phase stability and thermophysical properties of InFeO₃(ZnO)_m (m = 2, 3, 4, 5) compounds were investigated, which are a general family of homologous layered compounds with general formula InFeO₃(ZnO)_m (m = 1–19). InFeO₃(ZnO)_m (m = 2, 3, 4, 5) ceramics were synthesized using cold pressing followed by solid-state sintering. They revealed an excellent thermal stability after annealing at 1450 °C for 48 h. No phase transformation occurred during heating to 1400 °C. InFeO₃(ZnO)₃ exhibited a thermal conductivity of 1.38 W m⁻¹ K⁻¹ at 1000 °C, which is about 30% lower than that of 8 wt.% yttria stabilized zirconia (8YSZ) thermal barrier coatings. The thermal expansion coefficients (TECs) of InFeO₃(ZnO)m bulk ceramics were in a range of (10.97 ± 0.33) × 10⁻⁶ K⁻¹ to (11.46 ± 0.35) × 10⁻⁶ K⁻¹ at 900 °C, which are comparable to those of 8YSZ ceramics.     

Influence of sintering temperature on dielectric properties and crystal structure of corundum-structured Mg4Ta2O9 ceramics at microwave frequencies

Microwave dielectric properties of corundum-structured Mg₄Ta₂O₉ ceramics were investigated as a function of sintering temperatures by an aqueous sol–gel process. Crystal structure and microstructure were examined by X-ray diffraction (XRD) technique and field emission scanning electron microscopy (FE-SEM). Sintering characteristics and microwave dielectric properties of Mg₄Ta₂O₉ ceramics were studied as a function of sintering temperature from 1250 °C to 1450 °C. With increasing sintering temperature, the density, ε_r and Qf values increased, saturating at 1300 °C with excellent microwave properties of ε_r=11.9, Qf=195,000 GHz and τ_f=?47 ppm/°C. Evaluation of dielectric properties of Mg₄Ta₂O₉ ceramics were also analyzed by means of first principle calculation method and ionic polarizability theory.     

Microwave sintering of CeO2 and Y2O3 co-stabilised ZrO2 from stabiliser-coated nanopowders

Tetragonal ZrO₂ polycrystalline (TZP) composites with 2 wt.% Al₂O₃ and co-stabilised with 1 mol% Y₂O₃ and (4, 6 or 8) mol% CeO₂ were sintered at 1450 °C for 20 min in a single mode 2.45 GHz microwave furnace. For comparison, conventional sintering was performed in air at 1450 °C for 20 min. The starting powder mixture was obtained by a suspension coating technique using yttrium nitrate, cerium nitrate and pure m-ZrO₂ nanopowder. Fully dense material grades were obtained by both sintering methods. The influence of the composition and the sintering methods on the final phase composition and microstructure were investigated by X-ray diffraction and scanning electron microscopy. Finer and more uniform microstructures were observed in the microwave sintered ceramics when compared to the conventionally sintered samples. The fracture toughness increases with decreasing stabiliser content, whereas a reverse relation was found for the Vickers hardness. Comparable toughness and hardness values were obtained for the microwave and conventionally sintered samples.     

Preparation and properties of in-situ mullite whiskers reinforced aluminum chromium phosphate wave-transparent ceramics

In-situ mullite whisker reinforced aluminum chromium phosphate wave-transparent ceramics were designed and prepared. The phase transformation, microstructure, mechanical and electrical properties of the ceramics were investigated, and the mechanisms of in-situ growth and toughening were discussed. Results indicated that the in-situ growth of mullite whisker significantly improved the mechanical properties of the matrix, especially the high temperature flexural strength. The room temperature flexural strength, 1000 °C flexural strength and fracture toughness of the ceramics were 135.60 MPa, 121.71 MPa and 4.52 MPa m^1/2. After sintering at 1500 °C, the optimum properties of ε^'_r, tanδ and microwave transmittance at region 8–12 GHz were <3.6, <0.03 and>80%, respectively. The sinterability of ACP matrix was improved by the in-situ process of high mullization above 1450 °C. Using ACP binder as the raw material can avoid the phase transformation from B-AlPO₄ to T-AlPO₄. The synthesized mullite whiskers played a role in toughening by whiskers fracture, crack deflection and whisker pulling out.     

Li4Mg3Ti2O9: A novel low-loss microwave dielectric ceramic for LTCC applications

Low-loss novel Li₄Mg₃Ti₂O₉ dielectric ceramics with rock-salt structure were prepared by a conventional solid-state route. The crystalline structure, chemical bond properties, infrared spectroscopy and microwave dielectric properties of the abovementioned system were initially investigated. It could be concluded from this work that the extrinsic factors such as sintering temperatures and grain sizes significantly affected the dielectric properties of Li₄Mg₃Ti₂O₉ at lower sintering temperatures, while the intrinsic factors like bond ionicity and lattice energy played a dominant role when the ceramics were densified at 1450 °C. In order to explore the origin of intrinsic characteristics, complex dielectric constants (ε^’ and ε^’’) were calculated by the infrared spectra, which indicated that the absorptions of phonon oscillation predominantly effected the polarization of the ceramics. The Li₄Mg₃Ti₂O₉ ceramics sintered at 1450 °C exhibited excellent properties of ε_r=15.97, Q·f=135,800 GHz and τ_f=−7.06 ppm/°C. In addition, certain amounts of lithium fluoride (LiF) were added to lower the sintering temperatures of matrix. The Li₄Mg₃Ti₂O₉−3 wt% LiF ceramics sintered at 900 °C possessed suitable dielectric properties of ε_r=15.17, Q·f =42,800 GHz and τ_f=−11.30 ppm/°C, which made such materials promising for low temperature co-fired ceramic applications (LTCC).     

Synthesis of β-SiAlON powder by carbothermal reduction–nitridation of zeolites with different compositions

β-SiAlON was synthesized from select zeolite Y compositions with different Si/Al ratios by carbothermal reduction–nitridation (CRN), and the correlation between the starting compositions and products was investigated. The carbon content in all of the zeolite samples was fixed at 1.2 times the required stoichiometric value. Zeolite–carbon mixtures were placed in a carbon boat and fired in a furnace at 1300 °C for 0 min, and 1450 °C for 0, 120 min in a N₂ flow of 0.5 l/min. The main phase in each of the samples fired at 1450 °C for 120 min was determined from XRD results as β-SiAlON. It was also found that the ratio of β-SiAlON to minor phases such as α-Si₃N₄ and Si₂N₂O is typically higher in samples prepared from zeolites rather than from silica–alumina mixtures of the same compositions. This indicates that zeolites are ideal raw materials for the CRN synthesis of high purity β-SiAlONs by CRN with various z values.     

Enhanced nitridation of silicon compacts by Yb2O3 addition

The catalytic effect of ytterbium oxide (Yb₂O₃) on the nitriding reaction of Si compacts was investigated. Si powder mixtures containing Yb₂O₃ were prepared and nitrided in the form of compacts with a multi-step heating schedule over the range of 1200 °C–1450 °C. The nitriding profiles of the powder mixture with increasing temperature indicated that Yb₂O₃ clearly promoted the nitridation of Si compacts at 1200 °C compared with the pure Si compact containing no additives. The critical role of Yb₂O₃ on the nitridation of Si, was elucidated that Yb₂O₃ promotes the loss of initial SiO₂ of the raw Si powder via the measurement of the weight changes at low temperature (1100 °C) and thermogravimetric analysis under N₂ atmosphere. It was also found that the β-ratio of fully nitrided Si was closely related to the intermediate degree of nitridation at 1200 °C and 1300 °C.