Microstructure stabilization of a novel glass/YSZ composite coating material by adding alumina particles

Glass-based composite coating materials incorporating particles of alumina or YSZ were prepared by reaction sintering. It was revealed that phase evolution played a key role on thermal expansion behavior of the composite coating materials. Both precipitating of t-ZrO₂ crystals and adding YSZ inclusions could raise CTEs of the glass-based matrix, while the formation of zircon produced the reverse effect. Especially, alumina additives retarded the crystallization of the base glass and reduced reaction rates between YSZ and the glass matrix remarkably. Thus, the Al₂O₃/YSZ/glass tri-composites could serve as an environmental barrier coating for intermetallics and superalloys because of the stabilized microstructure.     

Preparation of biphasic hydroxyapatite/ β-tricalcium phosphate foam using the replication technique

Biphasic hydroxyapatite/β-tricalcium phosphate foams were prepared using the replication technique starting from a precipitated hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: HAP) powder, and sodium glycerophosphate (GP). The effect of the grinding time, solid loading, dispersant amount, and etching, replication, and sintering processes were investigated. The SEM, OEM and FTIR analyses proved that the surface of the polyurethane template must be treated with NaOH solution to make it more hydrophilic prior to the coating process. With a solid loading of 40 wt-%, the slurries prepared from the precipitated hydroxyapatite presented a shear thinning behavior, which was useful for the coating process. The SEM analysis of the foams showed that the optimum number of coating layers to obtain foam with an identical structure with the template was limited to three. The use of GP and the optimized preparation parameters helped to decrease the consolidation temperature of the ceramic foams to 1000 °C. The XRD and FTIR analyses of the prepared foams showed that the thermal treatment of the GP and the HAP mixture led to a partial decomposition of the HAP to tricalcium phosphate. The fitting of the XRD patterns and the obtained lattice parameters proved that the decomposition was accompanied by the insertion of sodium from GP toward the lattice of tricalcium phosphate and the formation of Na-β-tricalcium. The results of the SEM analysis, the pore size distribution and the mechanical strength showed that the presence of the Na-β-tricalcium reduced the pore size distribution from 500-2700 to 100–1700 μm, decreased slightly the total porosity from 80 vol-% to 70 vol-%; and improved the mechanical strength of the obtained foam from 1.56 MPa to 2.60 MPa.     

Influence of pores arrangement on stability of photonic structures during sintering

Discrete Element Method (DEM) has been used for numerical investigation of sintering-induced structural deformations occurring in inverse opal photonic structures. The influence of the initial arrangement of template particles on the stability of highly porous inverse opal α-Al₂O₃ structures has been analyzed. The material transport, densification, as well as formation of defects and cracks have been compared for various case studies. Three different stages of defects formation have been distinguished starting with local defects ending with intrapore cracks. The results show that the packing of the template particles defined during the template self-assembly process play a crucial role in the later structural deformation upon thermal exposure. The simulation results are in very good agreement with experimental data obtained from SEM images and previous studies by ptychographic X-ray tomography.     

Hydroxyapatite/bioactive glass functionally graded materials (FGM) for bone tissue engineering

In this work, HA/bioactive glass Functionally Graded Materials (FGMs) are obtained for the first time by means of Spark Plasma Sintering (SPS). Two series of highly dense 5 layered products, namely FGMS1 and FGMS2, are prepared under optimized SPS conditions, i.e. 1000 °C/2 min/16 MPa and 800 °C/2 min/50 MPa, respectively, using a die with varying cross section.Results arising from XRD, SEM, mechanical and biological characterization in SBF, evidence that lower temperature and higher-pressure levels used for FGMS2 samples provide better materials in terms of microstructure, compactness, hardness, elastic modulus and in vitro bioactivity. Indeed, a fully sintered and crack-free microstructure with no crystallisation at the top layer (100% bioactive glass) is correspondingly produced.The obtainment of such FGMs is quite promising, since it permits to vary the relative volume fractions of the two constituents and, consequently, tailor the biological response for specific clinical applications.     

Influence of processing conditions on the ionic conductivity of holmium zirconate (Ho2Zr2O7)

Nanopowders of holmium zirconate (Ho₂Zr₂O₇) synthesised through carbon neutral sol-gel method were pressed into pellets and individually sintered for 2 h in a single step sintering (SSS) process from 1100 °C to 1500 °C at 100 °C interval and in a two step sintering (TSS) process at (I) −1500 °C for 5 min followed by (II) - 1300 °C for 96 h. Relative density of each of the sintered pellet was determined using the Archimedes’ technique and the theoretical density was calculated from crystal structure data. Grain size was obtained from SEM micrographs using ImageJ. Pellets processed by TSS have been found to be denser (98 %) with less grain growth (1.29 μm) as compared to the pellets processed using SSS process. Ionic conductivity of Ho₂Zr₂O₇ pellets sintered by two different processes was measured using ac impedance spectroscopy technique over the temperature range of 350 °C–750 °C in the frequency range of 100 mHz–100 MHz for both heating and cooling cycles. The temperature dependence of bulk (2.67⨯10⁻³ Scm⁻¹) and grain boundary (2.50⨯10⁻³ Scm⁻¹) conductivities of Ho₂Zr₂O₇ prepared by TSS process are greater than those processed by SSS process suggesting the strong influence of processing conditions and grain size. Results of this study, indicates that the TSS is the preferable route for processing the holmium zirconate as it can be sintered to exceptionally high densities at lower temperature, exhibits less grain growth and enhanced ionic conductivity compared with the samples processed by SSS process. Hence, holmium zirconate can be considered as a promising new oxide ion conducting solid electrolyte for intermediate temperature SOFC applications between 350 °C and 750 °C temperature range.     

烧结机卸料装置耐磨性改善

烧结机导料箱、单辊破碎机和卸料漏斗承载着烧结热矿的导料、破碎与输送,是烧结生产的重要组成部分。高温、重载的工况环境决定着机尾卸料装置必须具有抗磨损、耐高温的特性。针对邯钢1^#435m^2烧结机机尾关键部件耐磨工作面寿命短,更换不方便等问题,分析磨损机理与失效原因,提出改进抗磨工作面结构,升级耐磨材料,优化焊接工艺等措施。通过技术方案的实施,使烧结机导料箱、单辊破碎机和卸料漏斗等卸料装置维持了较高的抗磨损性能,达到了预期的改造效果。     

Role of MgO on densification and mechanical properties in spark plasma sintered Os0.9Re0.1B2 ceramic

To promote the densification and therefore the mechanical properties of boride-based ceramics, MgO was added as sintering aid into Os_0.9Re_0.1B₂ powders for densification by using spark plasma sintering (SPS). The Os_0.9Re_0.1B₂ powders were synthesized by mechanochemical method from powder mixture of Os, Re and amorphous B. The role of MgO on densification, phase composition, microstructure and mechanical properties (hardness, fracture toughness and wear behavior) were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS), micro indentation and ball-on-disk tribometer. The results show that, with the introduction of MgO as sintering aid, the relative density of the Os_0.9Re_0.1B₂ ceramic samples increased. When the MgO content reached 9 wt%, the as-sintered sample is almost fully dense. No obvious regularity was found from the samples with the addition of different content of MgO. Vickers hardness values of the samples with 0, 3 wt% and 9 wt% MgO are found to be very close with each other within the experimental error (~30 GPa), while the sample with the addition of 6 wt% MgO exhibits the highest hardness of ~35 GPa. The fracture toughness of the samples is decreased slightly with the addition of MgO. The friction coefficient and wear rate of the sample with the addition of 6 wt% MgO was also found to be the lowest among all samples, which indicate best wear resistance. As a whole, with the addition content of 6 wt% MgO, the Os_0.9Re_0.1B₂ ceramic sample performs relatively excellent mechanical properties among four groups of samples.     

Mullitization kinetics from kaolinite incorporating nanometric Ba and Ca fluorides

The work presents the kinetic effect of nanometric BaF₂ and CaF₂ particles on kaolinite to mullite transformation. The kinetics were evaluated from dilatometric data using two different non-isothermal procedures: conversional model-fitting method and diffusional sintering analysis. From experimental data, the activation energy of mullite formation calculated from sintering (942 kJ/mol) and from conversional method (910 kJ/mol) were in good agreement with those values reported by other authors (mean value 1030 kJ/mol). After incorporation of 3 mol% of nanometric BaF₂ and CaF₂ in kaolinite and applying both analytical procedures, lower activation energies for mullite formation were obtained, assigning to the transformation the value of 635 kJ/mol for kaolinite/BaF₂ and 428 kJ/mol for kaolinite/CaF₂ composites.     

Microwave assisted-in situ synthesis of porous titanium/calcium phosphate composites and their in vitro apatite-forming capability

Microwave irradiation has been proven to be an effective heating source in synthetic chemistry, and can accelerate the reaction rate, provide more uniform heating and help in developing better synthetic routes for the fabrication of bone-grafting implant materials. In this study, a new technique, which comprises microwave heating and powder metallurgy for in situ synthesis of Ti/CaP composites by using Ti powders, calcium carbonate (CaCO₃) powders and dicalcium phosphate dihydrate (CaHPO₄·2H₂O) powders, has been developed. Three different compositions of Ti:CaCO₃:CaHPO₄·2H₂O powdered mixture were employed to investigate the effect of the starting atomic ratio of the CaCO₃ to CaHPO₄·2H₂O on the phase, microstructural formation and compressive properties of the microwave synthesized composites. When the starting atomic ratio reaches 1.67, composites containing mainly alpha-titanium (α-Ti), hydroxyapatite (HA), beta-tricalcium phosphate (β-TCP) and calcium titanate (CaTiO₃) with porosity of 26%, pore size up to 152 μm, compressive strength of 212 MPa and compressive modulus of 12 GPa were formed. The in vitro apatite-forming capability of the composite was evaluated by immersing the composite into a simulated body fluid (SBF) for up to 14 days. The results showed that biodissolution occurred, followed by apatite precipitation after immersion in the SBF, suggesting that the composites are suitable for bone implant applications as apatite is an essential intermediate layer for bone cells attachment. The quantity and size of the apatite globules increased over the immersion time. After 14 days of immersion, the composite surface was fully covered by an apatite layer with a Ca/P atomic ratio approximately of 1.68, which is similar to the bone-like apatite appearing in human hard tissue. The results suggested that the microwave assisted-in situ synthesis technique can be used as an alternative to traditional powder metallurgy for the fabrication of Ti/CaP biocomposites.