Material removal and surface damage in EDM of Ti3SiC2 ceramic

Material removal and surface damage of Ti₃SiC₂ ceramic during electrical discharge machining (EDM) were investigated. Melting and decomposition were found to be the main material removal mechanisms during the machining process. Material removal rate was enhanced acceleratively with increasing discharge current, i_e, working voltage, u_i, but increased deceleratively with pulse duration, t_e. Microcracks in the surface and loose grains in the subsurface resulted from thermal shock were confirmed, and the surface damage in Ti₃SiC₂ ceramic led to a degradation of both strength and reliability.     

Synthesis of single-phase Ti3SiC2 powder

In this study, free 2Ti/2Si/3TiC powder mixture was heated at high temperatures in vacuum, in order to reveal the possibility for the synthesis of high Ti₃SiC₂ content powder. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the evaluation of phase identities and the morphology of the powder after different treatments. Results showed that almost single phase Ti₃SiC₂ powder (99.3 wt.%) can be synthesized by heat treatment with free 2Ti/2Si/3TiC powders in vacuum at 1210°C for about 3 h. The nucleation and growth of Ti₃SiC₂ within TiC particles was observed. The typical appearance of the formed Ti₃SiC₂ is equiaxed with particle size of 2–4 μm. Effects of temperature and heating time on the morphology and the particle sizes of the synthesized Ti₃SiC₂ powders are not obvious.     

Preparation of SiC reinforced Ti3SiC2-base composite and its biocompatibility evaluation

Soft Ti₃SiC₂ ceramic is a new class of potential biomaterial for orthopaedic applications and dental implants. Introducing SiC into Ti₃SiC₂ can compensate for the relatively low hardness of the Ti₃SiC₂ implants. In this study, SiC reinforced Ti₃SiC₂-base composites were fabricated by reactive sintering of powder compacts. Mechanical properties and phase constitution of the prepared samples were studied. Biocompatibility of the composite was evaluated by implanting the material onto the shaft surface of the femur of a rabbit, while, biostability was studied by immersing the material into human blood plasma and also into a simulated body fluid (Hank's) for prolonged time.     

CVD synthesis of Al2O3 nanotubular structures using a powder source

Alumina (Al₂O₃) 1D nanotubular structures were synthesized by chemical vapor deposition (CVD) using aluminum (Al) and Al₂O₃ powder sources at a temperature of 1300 °C and a pressure of 100 Pa. At present, no research has been published regarding the synthesis of α-phase Al₂O₃ nanotubes using only a powder source. In this work, we attempted to grow α-phase Al₂O₃ nanotubes without catalysts. As-deposited nanotubular structures had an irregular and ragged surface morphology that was related to the boundary layer thickness. Moreover, complementary nanotubular structures can be obtained via an annealing process. A model to describe the irregular nanotubular structure formation was suggested, and the effect of the boundary layer thickness was demonstrated in our experimental conditions.     

Effect of liquid reaction on the synthesis of Ti3SiC2 powder

Synthesis of Ti₃SiC₂ powder was carried out by heat treating powder mixtures of Si, TiC and coarse Ti (−150 μm) in a temperature range of 1000–1400 °C. The phase content of Ti₃SiC₂ in the synthesized powder was improved to 99% when heat treated at 1400 °C for 4 h. Ti–Si liquid reaction was found to occur above the binary eutectic temperature, and this liquid reaction is believed to have assisted the synthesis reaction of Ti₃SiC₂.     

Oxidation behavior and kinetics of in situ (TiB2 + TiC)/Ti3SiC2 composites in air

The isothermal oxidation behavior of in situ (TiB₂ + TiC)/Ti₃SiC₂ composite ceramics with different TiB₂ content has been investigated at 900-1200 °C in air for exposure times up to 20 h by means of X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. The oxidation of (TiB₂ + TiC)/Ti₃SiC₂ composites follows a parabolic rate law. With the increase in TiB₂ content, the oxidation weight gain, thickness of the oxidation scale, and parabolic rate constant decrease dramatically, which suggests that the incorporation of TiB₂ greatly improves the oxidation resistance of the composites. With the increase in oxidation temperature, the enhancement effect becomes more pronounced. Due to the incorporation of TiB₂, the oxidation scale of (TiB₂ + TiC)/Ti₃SiC₂ composites is generally composed of an outer layer of coarse-grained TiO₂ and an inner layer of amorphous boron silicate and fine-grained TiO₂. Only the dense inner layer formed on the surface acts as a diffusion barrier, retarding the inward diffusion of O, and consequently contributing to the improved oxidation resistance of the (TiB₂ + TiC)/Ti₃SiC₂ composites.     

Effect of Ti5Si3 on wear properties of Ti3Si(Al)C2

Near-fully dense Ti₃Si(Al)C₂/Ti₅Si₃ composites were synthesized by in situ hot pressing/solid–liquid reaction process under a pressure of 30 MPa in a flowing Ar atmosphere at 1580 °C for 60 min. Compared to monolithic Ti₃Si(Al)C₂, Ti₃Si(Al)C₂/Ti₅Si₃ composites exhibit higher hardness and improved wear resistance, but a slight loss in flexural strength (about 26% lower than Ti₃Si(Al)C₂ matrix). In addition, Ti₃Si(Al)C₂/Ti₅Si₃ composites maintain a high fracture toughness (K_IC = 5.69–6.79 MPa m^1/2). The Ti₃Si(Al)C₂/30 vol.%Ti₅Si₃ composite shows the highest Vickers hardness (68% higher than that of Ti₃Si(Al)C₂) and best wear resistance (the wear resistance increases by 2 orders of magnitude). The improved properties are mainly ascribed to the contribution of hard Ti₅Si₃ particles, and the strength degradation is mainly due to the lower Young's modulus and strength of Ti₅Si₃.     

Impurity control in pressureless reactive synthesis of pure Ti3SiC2 bulk from elemental powders

The impurity control in pressureless reactive synthesis of pure Ti₃SiC₂ from elemental powders is reported. Ti₃SiC₂ bulk samples were prepared by sintering compacts of ball-mixed elemental powders at 1500 °C for 2 h in lidded alumina crucibles under Ar atmosphere. Undesirable TiC impurity was successfully eliminated from the synthesized product. Product with desired phase constituent can be fabricated by preparing samples according to phase diagram data. Keeping away from the phase fields that involve TiC is a vital way to obtain pure Ti₃SiC₂ without containing the undesirable TiC. The key for successful impurity control in the sintering process is the conservation of mass in the reactants.     

Rapid synthesis of Ti3AlC2/TiB2 composites by the spark plasma sintering (SPS) technique

Dense Ti₃AlC₂/TiB₂ composites were successfully fabricated from B₄C/TiC/Ti/Al powders by spark plasma sintering (SPS). The microstructure, flexural strength and fracture toughness of the composites were investigated. The experimental results indicate that the Vickers hardness increased with the increase in TiB₂ content. The maximum flexural strength (700 ± 10 MPa) and fracture toughness (7.0 ± 0.2 MPa m^1/2) were achieved through addition of 10 vol.% TiB₂, however, a slight decrease in the other mechanical properties was observed with TiB₂ addition higher than 10 vol.%, which is believed to be due to TiB₂ agglomeration.     

Combined TiO2/SiO2 mesoporous photocatalysts with location and phase controllable TiO2 nanoparticles

Combined TiO₂/SiO₂ mesoporous materials were prepared by deposition of TiO₂ nanoparticles synthesised via the acid-catalysed sol–gel method. In the first synthesis step a titania solution is prepared, by dissolving titaniumtetraisopropoxide in nitric acid. The influences of the initial titaniumtetraisopropoxide concentration and the temperature of dissolving on the final structural properties were investigated. In the second step of the synthesis, the titania nanoparticles were deposited on a silica support. Here, the influence of the temperature during deposition was studied. The depositions were carried out on two different mesoporous silica supports, SBA-15 and MCF, leading to substantial differences in the catalytic and structural properties. The samples were analysed with N₂-sorption, X-ray diffraction (XRD), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) to obtain structural information, determining the amount of titania, the crystal phase and the location of the titania particles on the mesoporous material (inside or outside the mesoporous channels). The structural differences of the support strongly determine the location of the nanoparticles and the subsequent photocatalytic activity towards the degradation of rhodamine 6G in aqueous solution under UV irradiation.     

氯化 Ti3AlC2 陶瓷制备碳化物衍生碳的结构表征

以三元碳化物陶瓷Ti_3AlC_2为原料,在500°C~1000°C温度范围内氯化制备具有纳米孔结构的碳化物衍生碳(Ti_3AlC_2-CDC)。高温氯化制备得到的Ti_3AlC_2-CDC由无定形碳和石墨组成。氯化温度越高,石墨化程度越明显,石墨有序度越高。Ti_3AlC_2-CDC的结构与前驱体Ti_3AlC_2的层状结构保持一致。但随着温度升高,Ti_3AlC_2-CDC会逐渐裂解为单片层或多片层。采用N2吸附技术研究了700°C、800°C和1000°C下制备的Ti_3AlC_2-CDC的孔隙结构特征,通过分析试样的吸附等温线特征和孔径分布探讨了温度对CDC孔结构的影响。     

Carbon reduction reaction in the Y2O3–SiO2 glass system at high temperature

In order to assess the role of carbon with respect to the grain boundary chemistry of Si₃N₄-based ceramics model experiments were performed. Y₂O₃–SiO₂ glass systems with various amount of carbon (from 1 to 30 wt.%) were prepared by high-temperature treatment in a graphite furnace. High carbon activity of the furnace atmosphere was observed. EDX analysis proved the formation of SiC by the carbothermal reduction of SiO₂ either in the melt or in the solid state. The melting temperature of the Y₂O₃–SiO₂ system is strongly dependent on the amount of reduced SiO₂. XRD analysis of the products documented the presence of Y₂Si₂O₇, Y₂SiO₅ and Y₂O₃ crystalline phases in that order with an increasing amount of free C in the starting mixture. The reduction of Y₂O₃ was not confirmed.     

Synthesis of LaAlO3 powder using triethanolamine

LaAlO₃ powders were successfully synthesized by pyrolysis of complex compounds of lanthanum and aluminum with triethanolamine (TEA). The precursors and the derived powders were characterized by simultaneous thermogravimetry analysis (TG) and differential scanning calorimetry analysis (DSC), X-ray diffractometry (XRD), specific surface area measurements, and transmission electron microscopy (TEM). Pure LaAlO₃ phase was obtained at 775 °C for 2 h or 750 °C for 4 h, without formation of any intermediate phase. Pores were found from TEM images of LaAlO₃ powders prepared at 800 °C for 2 h.     

Fine-sized LiNi0.8Co0.15Mn0.05O2 cathode particles prepared by spray pyrolysis from the polymeric precursor solutions

Fine-sized LiNi_0.8Co_0.15Mn_0.05O₂ cathode particles with high discharge capacities and good cycle properties were prepared by spray pyrolysis from the polymeric precursor solutions. The cathode particles obtained from the spray solution without polymeric precursors had irregular morphology and hardly aggregated morphology. On the other hand, the cathode particles obtained from the spray solution with citric acid and ethylene glycol had fine size and regular morphologies. The cathode particles obtained from the spray solution containing adequate amounts of citric acid and ethylene glycol had several hundreds nanometer and narrow size distribution. The maximum discharge capacity of the cathode particles was 218 mAh/g when the excess of lithium component added to the spray solution was 6 mol% of the stoichiometric amount to obtain the LiNi_0.8Co_0.15Mn_0.05O₂ particles. The discharge capacities of the fine-sized LiNi_0.8Co_0.15Mn_0.05O₂ particles dropped from 218 to 213 mAh/g by the 50th cycle at a current density of 0.1 C.     

Preparation and characterization of Ti3SiC2 powder

Pressureless sintering in vacuum was applied to synthesize Ti₃SiC₂ from elemental powders of Ti, Si and C. Based on the phase compositions and purities of the products obtained by X-ray diffraction, the elemental powders composition and sintering condition were optimized. It was found that the sample sintered at 1450 °C for 240 min from a mixture of 3Ti/1.75Si/2C (molar ratio) contained Ti₃SiC₂ with the volume fraction as high as 93%. It was proposed that loss of Si through gaseous vaporization and contamination of C might be the main obstacles against obtaining high-purity material by this way.     

TiO2 as a sintering additive for KNbO3 ceramics

Improved densification during the conventional sintering of KNbO₃ ceramics was achieved by using small additions of TiO₂. This improved densification can be explained on the basis of high-temperature chemical reactions in the system. X-ray diffractometry and electron microscopy were used in combination with diffusion-couple experiments in order to elucidate the chemical reactions between KNbO₃ and TiO₂. TiO₂ reacts with KNbO₃ forming KNbTiO₅, and a low concentration of Ti incorporates in the KNbO₃ structure resulting in the formation of oxygen vacancies and, consequently, in an improvement in the densification. At ∼1037 °C eutectic melting between the KNbO₃ and the KNbTiO₅ further improves the densification of the KNbO₃ ceramics.     

Magneli phase Ti4O7 electrode for oxygen reduction reaction and its implication for zinc-air rechargeable batteries

In this paper, Magneli phase Ti₄O₇ was successfully synthesized using a TiO₂ reduction method, and characterized using X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrode coated with this Ti₄O₇ material showed activities for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). For the ORR, several parameters, including overall electron transfer number, kinetic constants, electron transfer coefficient, and percentage H₂O₂ production, were obtained using the rotating ring-disk electrode (RRDE) technique and the Koutecky-Levich theory. The overall electron transfer number was found to be between 2.3 and 2.9 in 1, 4, and 6 M KOH electrolytes, suggesting that the ORR process on the Ti₄O₇ electrode was a mixed process of 2- and 4-electron transfer pathways. Electrochemical durability tests, carried out in highly concentrated KOH electrolyte, confirmed that this Ti₄O₇ is a stable electrode material, suggesting that it should be a feasible candidate for the air-cathodes of zinc-air batteries. To understand the stability of this material, Raman and XPS spectra were also collected for the Ti₄O₇ samples before and after the stability tests. The results and analysis revealed that a thin layer of TiO₂ formed on the Ti₄O₇ surface, which may have prevented further oxidation into the bulk of the Ti₄O₇ electrode.     

Characterization of SnO2 nanowires synthesized from SnO by carbothermal reduction process

In this paper, single-crystalline SnO₂ nanowires have been successfully prepared by a carbothermal reduction process employing SnO as the starting material and CuO as the catalyst. Their morphologies, purity and sizes of the products were characterized by transmission electron microscopy (TEM), selected area electron diffraction, X-ray diffraction, field emission scanning electron microscopy (FESEM) and Raman spectroscopy, respectively. The FESEM images reveal wire-like and rod-shaped nanowires of about 100–800 μm in length and 30–200 nm in the transverse dimensions. The three observed Raman peaks at 474, 634 and 774 cm⁻¹ indicate the typical rutile phase of the SnO₂ which is in agreement with the X-ray diffraction results. The influence of some reaction parameters, including the temperature and the reaction duration, on the forming, morphology and particle size of SnO₂ crystallize is discussed.     

The characteristics of X1 type Y2SiO5:Tb phosphor particles prepared by high temperature spray pyrolysis

The X1 type Y₂SiO₅:Tb phosphor particles with high brightness were prepared by spray pyrolysis from spray solution with NH₄F flux material. The phosphor particles prepared by spray pyrolysis at high preparation temperature had spherical shape, fine size and dense morphology. The mean sizes of the phosphor particles prepared at 900 and 1650 °C were 1.3 and 0.9 μm. The emission spectrum of the phosphor particles prepared by spray pyrolysis at 1650^oC had the characteristics of X1 type Y₂SiO₅:Tb phosphor. The photoluminescence intensity of the phosphor particles directly prepared by spray pyrolysis from spray solution with 20 wt.% NH₄F flux of the product at temperature of 1650 °C was 127 and 184% of the X1 and X2 type Y₂SiO₅:Tb phosphor particles post-treated at 1100 and 1300 °C, respectively. The Y₂SiO₅:Tb phosphor particles prepared by spray pyrolysis at 1650 °C had X1 type crystal structure because of short residence time of particles inside hot wall reactor of 0.4 s.