Synthesis of high purity titanium silicon carbide from elemental powders using arc melting method

The objective of this study is to investigate the formation of Ti₃SiC₂ from Ti/Si/C powders using the arc melting method. The results show that the sample sintered at 80 s produced a near single-phase of Ti₃SiC₂ (99.2 wt.%) with a relative density of 88.9%. These results were confirmed by phase determination using XRD analysis and were supported with micrographs from FESEM/EDX analyses. The relative density and porosity of all samples were dependent on the formation of macropores in bulk samples and micropores in TiC_x grains. The proposed reaction mechanisms for the synthesis of Ti₃SiC₂ by arc melting is that Ti₃SiC₂ might be formed from TiC_x + Si, Ti₅Si₃C_x + C, and Ti₅Si₃C_x + TiC_x at early arcing time (≤ 10 s), while TiC_x + TiSi₂ take place at 15 s to 80 s. After 80 s, decomposition of Ti₃SiC₂ into TiC_x, TiSi₂ and C was observed.     

Radiation tolerance of Mn+1AXn phases,Ti3AlC2 and Ti3SiC2

During investigations of novel material types with uses in future nuclear technologies (ITER/DEMO and GenIV fission reactors), ternary carbides with compositions Ti₃AlC₂ and Ti₃SiC₂ have been irradiated with high Xe fluences, 6.25 × 10¹⁵ ions cm^?2 (～25–30 dpa), using the IVEM-TANDEM facility at Argonne National Laboratory. Both compositions show high tolerance to damage, and give indications that they are likely to remain crystalline to much higher fluences. There is a visible difference in tolerance between Ti₃AlC₂ and Ti₃SiC₂ that can be related to the changes in bonding within each material. These initial findings provide evidence for a novel class of materials (+200 compounds) with high radiation resistance, while, significantly, both of these materials are composed of low-Z elements and hence exhibit no long-term activation.     

Influence of SPS parameters on the density and mechanical properties of sintered Ti3SiC2 powders

The densification of Ti₃SiC₂ MAX phase was performed by the Spark Plasma Sintering (SPS) technique. The SPS parameters, such as sintering temperature, pressure and soaking time, were optimized to obtain fully densified samples which were characterized to obtain the best mechanical properties. The sintering temperature was varied from 1070 to 1300 °C, the soaking time from 1 to 10 min and the applied pressure from 60 to 180 MPa. The best full densified samples were sintered at 1300 °C applying 60 MPa for 7 min. Ti_xC_y and TiSi₂ secondary phases were found in samples densified at 1200, 1250 and 1300 °C, due to decomposition of Ti₃SiC₂. These secondary phases, detected by XRD patterns, were confirmed by microhardness testing, FESEM observations and EDAX analyses.     

Optimization of the Ti3SiC2 MAX phase synthesis

The synthesis of Ti₃SiC₂ MAX phase by self-propagating high-temperature synthesis (SHS) and pressureless argon shielding synthesis has been investigated following different pathways pertaining to the reactant systems Ti/Si/C, Ti/SiC/C and Ti/TiC/Si. Silicon in excess ranging from 10 to 50 mol% was employed to obtain powders mainly constituted by Ti₃SiC₂.Optimizing the excess of silicon and the pressing technique, the resultant powders with Ti₃SiC₂ content near to 100% were obtained. Result was consequent to the use of pressureless argon shielding synthesis obtained with 30 mol% of silicon excess in the examined different systems. The Ti₃SiC₂ was also obtained by SHS, but with lower proportion (88% and 86% from 3Ti + 1.2SiC + 0.8C and 3Ti + 1.3Si + 2C respectively). These results driving from XRD patterns were confirmed by FESEM observations and the EDAX analyses.     

Intermetallic protective coatings on titanium

In the presented work, the powder siliconizing and liquid phase alloying were used for a surface hardening of titanium and to protect titanium against high-temperature oxidation. The powder siliconizing was carried out in a pure Si powder at 900–1100 °C/3–48 h and the liquid phase alloying was realized in an Al–20 wt.% Si melt at 800 °C/5–40 min. It was shown that the coating methods produced hard multi-phase surface layers composed of various kinds of silicides (Ti₅Si₃, Ti₃Si and TiSi) and ternary Ti(Al_XSi_1−X)₂ (τ₂) phase. The binary silicide layers grew in accordance with the parabolic law while the ternary layer grew very rapidly. It was shown that the powder siliconizing at 900–1100 °C/3 h produced sufficiently thick and compact protective layers. The liquid phase alloying at 800 °C/10 min was efficient for preparation of protective layers. The oxidation experiments were conducted at 850 °C in air. Both the powder siliconized and liquid phase alloyed coatings were shown to provide a good protection against high-temperature oxidation.     

Mechanical behavior of diamond matrix composites with ceramic Ti3(Si,Ge)C2 bonding phase

Polycrystalline diamond, PCD, compacts are usually produced by high pressure–high temperature (HP–HT) sintering. This technique always introduces strong internal stresses into the compacts, which may result in self-fragmentation or graphitization of diamond. This may be prevented by a bonding phase and Ti₃(Si,Ge)C₂ was so investigated. This layered ceramic was produced by Self Propagating High Temperature Synthesis and the product milled. The Ti₃(Si,Ge)C₂ milled powder was mechanically mixed, in the range 10 to 30 wt.%, with 3–6 μm diamond powder (MDA, De Beers) and compacted into disks 15 mm in diameter and 5 mm high. These were sintered at a pressure of 8.0 GPa and temperature of 2235 K in a Bridgman-type high pressure apparatus. The amount of the bonding phase affected the mechanical properties: Vickers hardness from 20.0 to 60.0 GPa and Young's modulus from 200 to 500 GPa, with their highest values recorded for 10 wt.% Ti₃(Si,Ge)C₂. For this composite fracture toughness was 7.0 MPa m^1/2, tensile strength 402 MPa and friction coefficient 0.08. Scanning and transmission electron microscopy, X-ray and electron diffraction phase analysis were used to examine the composites.     

Thermal stability of Ti3SiC2 thin films

The thermal stability of Ti₃SiC₂(0 0 0 1) thin films is studied by in situ X-ray diffraction analysis during vacuum furnace annealing in combination with X-ray photoelectron spectroscopy, transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray analysis. The films are found to be stable during annealing at temperatures up to ∼1000 °C for 25 h. Annealing at 1100–1200 °C results in the rapid decomposition of Ti₃SiC₂ by Si out-diffusion along the basal planes via domain boundaries to the free surface with subsequent evaporation. As a consequence, the material shrinks by the relaxation of the Ti₃C₂ slabs and, it is proposed, by an in-diffusion of O into the empty Si-mirror planes. The phase transformation process is followed by the detwinning of the as-relaxed Ti₃C₂ slabs into (1 1 1)-oriented TiC_0.67 layers, which begin recrystallizing at 1300 °C. Ab initio calculations are provided supporting the presented decomposition mechanisms.     

High hydrogen permeability in the Nb-rich Nb–Ti–Ni alloy

The microstructure and the hydrogen permeability of the Nb-rich Nb–Ti–Ni alloy, i.e., the Nb₅₆Ti₂₃Ni₂₁ alloy were investigated and compared with those of the Nb₄₀Ti₃₀Ni₃₀ alloy. The Nb₅₆Ti₂₃Ni₂₁ alloy consisted of a combination of the primary phase bcc- (Nb, Ti) solid solution with the eutectic phase {bcc- (Nb, Ti) + B2-TiNi}. The volume fraction of the former and the latter phases were 62 and 38 vol.%, respectively. The Nb₅₆Ti₂₃Ni₂₁ alloy showed the higher Φ value of 3.47 × 10⁻⁸ (mol H₂ m⁻¹ s⁻¹ Pa^−0.5) at 673 K, which is 1.8 times higher than that of the Nb₄₀Ti₃₀Ni₃₀ alloy, which has been reported to be highest in the Nb–Ti–Ni system. The present work demonstrated that the Nb-rich Nb–Ti–Ni alloys consisting of only the primary phase bcc- (Nb, Ti) and the eutectic phase {bcc- (Nb, Ti) + B2-TiNi} are promising for the hydrogen permeation membrane.     

Metastable phase formation in Ti–Al–Nb undercooled melts

Metastable phase formation processes in ternary Ti–Al–Nb alloys were studied by containerless electromagnetic levitation for melt undercooling up to 300 K below the liquidus temperature. Dendrite growth velocities of 15–25 m s⁻¹ for highly undercooled Ti–Al–Nb melts were consistent with primary β-phase formation, which is promoted by Nb addition. From double-recalescence events in Ti₄₀Al₅₀Nb₁₀ and Ti₄₅Al₅₀Nb₅ melts beyond a critical undercooling a subsequent β to α phase transformation in the semi-solid state was inferred. A second recalescence near 1300 °C, which was attributed to an α to γ solid state transformation, was observed in the pyrometer trace for the Ti₄₅Al₅₀Nb₅ and Ti₄₀Al₅₀Nb₁₀ alloys. The γ phase formation was suppressed in favour of a homogeneous α₂ phase in undercooled Ti₄₅Al₄₅Nb₁₀ samples quenched onto a chill substrate, whereas in Ti₄₀Al₅₀Nb₁₀ high undercooling enabled a direct γ phase solidification.     

Effect of silicidation pretreatment with gaseous SiO on sinterability of TiC powders

Silicidation pretreatment with gaseous SiO at 1350 °C for 30 min is employed for chemically modifying commercially available TiC powder. Phase composition and microstructural features of the pretreated powder are discussed. Densification behavior of the pretreated TiC powder during hot pressing is studied in comparison with that of non-pretreated one. Significantly improved densification behavior and sinterability of TiC powder after silicidation pretreatment are explained by the effect of Ti₃SiC₂ acting as a solid lubricant. Nearly fully dense TiC-based ceramics having flexural strength of 370 MPa, fracture toughness of 5.6 MPa m^½, and microhardness of 24 GPa is obtained by hot pressing under conditions as mild as 1600 °C and 20 MPa.     

Phase diagram of the Sn–Zn–Ti system

Equilibrated Sn–Zn–Ti alloys and (Sn + Zn)/α-Ti diffusion couples have been studied by scanning electron microscopy, metallography, and differential scanning calorimetry. For the first time an isothermal section, at 600 °C, of the ternary Sn–Zn–Ti system has been constructed. A previously unknown ternary phase with approximate formula Ti₃Sn₂Zn₆ (probable homogeneity interval in the range Ti₅Sn₄Zn₁₁ to Ti₅Sn₃Zn₁₂) has been found.The solubility ranges of the titanium based solid solutions and the intermetallic phases have been looked for. As far as we could detect and in agreement with theoretical considerations, zinc dissolves more in Ti–Sn phases than tin into Ti–Zn compounds. Titanium additions of 3 and 4 at.% Ti do not influence significantly the Sn–Zn eutectic temperature. The experimentally determined melting enthalpies of the nearly eutectic alloys have values around 100 J g⁻¹.     

Effect of oxygen vacancy and Mn-doping on electrical properties of Bi4Ti3O12 thin film grown by pulsed laser deposition

An amorphous Bi₄Ti₃O₁₂ phase was formed when films were grown at <400 °C while Bi₂Ti₂O₇ and Bi₂Ti₄O₁₁ transient phases were developed when films were grown at 400–500 and 600 °C, respectively. A homogeneous Bi₄Ti₃O₁₂ crystalline phase was formed in the film grown at 700 °C. The high leakage current density (5 × 10^?7 A cm^?2 at 0.2 MV cm^?1) of the film grown at 300 °C under 100 mTorr oxygen partial pressure (OPP) decreased to 2 × 10^?8 A cm^?2 for the film grown at 200 mTorr OPP, due to the decreased number of intrinsic oxygen vacancies. However, when OPP exceeded 200 mTorr, the electrical properties were deteriorated due to the formation of oxygen interstitial ions. Mn-doping at a suitable level improved the electrical properties of the films by producing extrinsic oxygen vacancies that reduced the number of intrinsic oxygen vacancies. Schottky emission was suggested as the leakage current mechanism of the Bi₄Ti₃O₁₂ film.     

Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

First-principles study of Ti3AC2 (A = Si,Al) (0 0 1) surfaces

We have systematically studied the mechanical properties and surface properties of (1 × 1) Ti₃AC₂ (A = Si, Al) (0 0 1) using the density functional theory (DFT). The calculated cleavage energy for each possible cleavage site shows that Ti–Si and Ti–Al are the weakest layers, while the Ti–C layer is the strongest one. It reveals that the main difference between Ti₃SiC₂ and Ti₃AlC₂ is that the Ti–Si bond is stronger than the Ti–Al bond. This shows the different mechanical and surface properties between them. The surface rumpling and surface energy of Ti₃SiC₂ and Ti₃AlC₂ are calculated. The study shows that the cleavage energy affects both the surface rumpling and the surface energy. The higher the cleavage energy, the larger the surface energy and surface rumpling. Furthermore, the most stable surface structures are predicted for different experimental conditions. The predicted surface structures agree with the available experimental results.     

Formation and stability of tungsten boride nanocomposites in WO3–B2O3–Mg ternary system: Mechanochemical effects

Mechanochemical behavior of WO₃–B₂O₃–Mg ternary system to produce tungsten boride-based nanocomposites was investigated in terms of milling duration. To provide essential conditions for the occurrence of a mechanically induced self-sustaining reaction (MSR), a mixture of tungsten trioxide, boron oxide and elemental magnesium with the stoichiometric composition was activated using a high-energy planetary ball mill. Based on the obtained data, the adiabatic temperature was around 3659 K which confirmed that the reaction mode was MSR. Due to the occurrence of a combustion reaction at the beginning of milling, the phase compositions were W, WB, W₂B, and MgO. After 60 min of milling, WB disappeared completely and a ternary nanocomposite with the phase constituents of W₂B, W and MgO was obtained. With increasing the milling time to 1800 min, no phase transformation was observed and thus a nanocomposite powder with similar phase compositions was produced. However, the percentage of the detected phases fluctuated in terms of milling time. During the leaching process, MgO (unwanted phase) was completely removed and consequently a nanocomposite powder with the phase compositions of W₂B and W was formed. From the microscopic observations, the size of the composite particles was varied from 38 to 500 nm.     

Oxidation-induced strength behavior of Ti3SiC2

Oxidation behavior and subsequent mechanical properties of Ti₃SiC₂ were studied. The oxide scale has significant effect on strength and hardness, which is mainly attributed due to the mismatch of coefficient of thermal expansion (CTE) of substrate and oxide phases. The improved flexural strength (～650 MPa) could be noticed at 1000 °C; however, at elevated temperature the ductility of Ti₃SiC₂ was predominant and reduced the strength.     

Effect of ZrB2 and SiC addition on TiB2-based ceramic composites prepared by spark plasma sintering

TiB₂-based ceramic composites with different amounts of ZrB₂ and SiC were prepared by spark plasma sintering at 1700 °C with an initial pressure of 40 MPa and a holding time of 10 min. The (Ti_xZr_y)B₂ solid solution was found in the sintered TiB₂/ZrB₂/SiC composites by XRD. The microstructural and mechanical properties of the prepared samples were investigated. The composite with the addition of 30 vol.% ZrB₂ shows better comprehensive performances, and the bending strength and the fracture toughness of the composite are 780.5 MPa and 7.34 MPa m^1/2, respectively. The generation of the (Ti_xZr_y)B₂ solid solution makes the microstructures of the composites finer and more homogeneous, which has played a very important role in grain refinement and interface fusion.     

Expanding microwave plasma process for thin molybdenum films nitriding: Nitrogen diffusion and structure investigations

Transition metal nitrides exhibit very interesting properties for mechanical and catalytic applications. Thin nitride layers are expected to prevent metal films from oxidizing under working conditions. Molybdenum thin films of about 200 nm thick and deposited on Si (100) wafers are processed in pure N₂, (Ar–35%N₂) and (Ar–25%N₂–30%H₂) expanding microwave plasma at 673 K. Secondary neutral mass spectrometry (SNMS) and Raman spectroscopy are both used to make correlations between the composition and the structure of the as-formed compounds. The low nitrogen diffusion up to a depth of about 40 nm is correlated to the formation of well crystallized MoO₂ of monoclinic structure acting as a barrier of diffusion for nitrogen, in molybdenum films exposed to pure N₂ and (Ar–35%N₂) plasma. The large nitrogen diffusion into the film exposed to ternary (Ar–25%N₂–30%H₂) plasma is correlated to the reduction of MoO₂ oxides by hydrogen species such as atomic hydrogen, NH_x<3… contained in ternary plasma. The formation of Mo–N phases with defects could take place in molybdenum films processed at 673 K compared to the results obtained at 873 K.     

Epitaxial Ti2AlN(0 0 0 1) thin film deposition by dual-target reactive magnetron sputtering

Ultrahigh-vacuum dual-target reactive magnetron sputtering, in a mixed Ar/N₂ discharge was used to deposit epitaxial single-crystal MAX phase Ti₂AlN(0 0 0 1) thin films, without seed layers, onto Al₂O₃(0 0 0 1) substrates kept at 1050 °C. By varying the N₂ partial pressure a narrow process window was identified for the growth of single-crystal Ti₂AlN. The film microstructure was characterized by a combination of X-ray diffraction, spherical aberration (C_s) corrected transmission electron microscopy (TEM), high-resolution image simulation and high-resolution scanning TEM. Nitrogen-depleted deposition conditions resulted in the concurrent formation of N-free Ti–Al intermetallics at the film/substrate interface and a steady-state growth of Ti₂AlN together with N-free intermetallic phases. At higher N₂ partial pressures the growth assumes a columnar epitaxial nature. 1 Å resolution of the lattice enabling location of all elements in the Ti₂AlN unit cell is demonstrated.     

Superior hydrogen storage kinetics of MgH2 nanoparticles doped with TiF3

MgH₂ nanoparticles were obtained by hydriding ultrafine magnesium particles which were prepared by hydrogen plasma–metal reaction. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that the obtained sample is almost pure MgH₂ phase, without residual magnesium and with an average particle size of ∼300 nm. Milled with 5 wt.% TiF₃ as a doping precursor in a hydrogen atmosphere, the sample desorbed 4.5 wt.% hydrogen in 6 min under an initial hydrogen pressure of ∼0.001 bar at 573 K and absorbed 4.2 wt.% hydrogen in 1 min under ∼20 bar hydrogen at room temperature. Compared with MgH₂ micrometer particles doped with 5 wt.% TiF₃ under the same conditions as the MgH₂ nanoparticles, it is suggested that decrease of particle size is beneficial for enhancing absorption capacity at low temperatures, but has no effect on desorption. In addition, the catalyst was mainly responsible for improving the sorption kinetics and its catalytic mechanism is discussed.