Porous TiB2 Research Articles

Ultra-low shrinkage and high-strength porous TiB/TiB2 ceramics via low temperature in-situ reaction sintering combined with the freeze-drying method

Ultra-low shrinkage porous TiB 2 -based ceramics reinforced by the TiB whiskers are firstly fabricated through the in-situ reaction between TiB 2 and Ti at a low temperature (1450 °C). The growth of TiB whiskers with a high aspect ratio at pore channels is achieved through a vapor-solid growth mechanism, while low aspect ratio TiB whiskers at pore walls are dominated by a solid-state reaction diffusion growth, forming bimodal distribution whiskers in porous TiB 2 -based ceramics. The overlapping TiB whiskers with low-speed growth at particle contact points can significantly inhibit the shrinkage and improve the strength of porous TiB 2 -based ceramics. When the solid content is fixed at 20 vol% and target TiB content changes from 0 to 80 vol%, the porous ceramics show slight sintering shrinkage (from 1.1 to 4.7%) and high porosity (from 79.3 to 73.7%) while keeping high compressive strength of 1.8–18.2 MPa, which is higher than most reported porous ceramics at the same porosity.

Thermal Stability of TiN Coated Cubic Boron Nitride Powder.

Wear-resistant, super hard ceramic composites based on cubic boron nitride (cBN) are of great interest to industry. However, cBN is metastable under sintering conditions at normal pressure and converts into the soft hexagonal BN (hBN). Therefore, efforts are being made to avoid this process. Besides short sintering times, the use of coated cBN-particles is a way to minimize this process. Therefore, the thermal stability of TiN coated cBN powders in high purity argon and nitrogen atmospheres up to temperatures of 1600 °C was investigated by thermogravimetry, X-ray phase analysis, scanning electron microscopy and Raman spectroscopy. The TiN coating was prepared by the atomic layer deposition (ALD)-method. The investigations showed that the TiN layer reacts in Ar at T ≥ 1200 °C with the cBN and forms a porous TiB2 layer. No reaction takes place in nitrogen up to temperatures of 1600 °C. Nevertheless, the 20 and 50 nm thin coatings also undergo a recrystallization process during heat treatment up to temperatures of 1600 °C.

Pore and microstructure change induced by SiC whiskers and particles in porous TiB2–TiC–Ti3SiC2 composites

TiB2–TiC–Ti3SiC2 porous composites were prepared through a plasma heating reaction using powder mixtures of Ti, B4C SiC whiskers (SiCw) and SiC particles (SiCp). The effects of the SiCw and SiCp content on pore structures, phase constituents, microstructure, and crystal morphology of TiC were studied. The results show that TiC, TiB, Ti3B4 phases are formed within the 5Ti+B4C system. With the addition of SiCw and SiCp, the TiB and Ti3B4 phases are reduced, sometimes even disappeared. Interestingly, the content of TiB2 and TiC increased, resulting in Ti3SiC2 and TiSi2 being formed. The porosity of composites increases notably with the addition of SiCw. However, with the increase of SiCp, the porosity of the composites first decreases, followed by an increase. After adding the specified amount of SiCw/SiCp, the compressive strength of composites are improved significantly. Additionally, the pore size of the composites are decreased significantly with the addition of SiCw/SiCp. During the plasma heating process, some Si atoms will diffuse into the TiC lattice, which in turn made the cubic TiC grains into hexagonal lamellar TiC or Ti3SiC2 grains.

Strengthening of porous TiB2⿿SiC ceramics by pre-oxidation and crack-healing

Low porosity TiB2⿿SiC ceramics were prepared by Cold Isostatic Pressing⿿Pressureless Sintering (CIP⿿PS). The crack-healing behavior of these ceramics was investigated by pre-oxidizing the samples at 800⿿1200°C in air for various durations ranging from 5 to 240min. The effects of pre-oxidation temperature and time on the microstructure and mechanical properties of porous TiB2⿿SiC ceramics were studied. The strength of porous ceramics obviously improved after pre-oxidation, with a maximum increase in strength of 134.4% for samples pre-oxidized at 1200°C for 120min. The strengthening mechanism of porous TiB2⿿SiC ceramics after pre-oxidation was also investigated and discussed. This study provides a novel concept for improving the strength and structural integrity of porous ceramics in practical applications.

Effects of B4C particle size on pore structures of porous TiB2–TiC by reaction synthesis

Porous TiB2–TiC were prepared by SHS using Ti and B4C powder mixtures. The pores, phase constituents, microstructures and composition distribution of TiB2–TiC were analyzed. The influences of B4C particle size and pressure in green compacts on pore sizes and structures of final products were also discussed. Via combustion front quenching, the reaction process between Ti and B4C particles was achieved. The results showed that the pore size decreased and combination between TiB2 and TiC became closer with the decrease of B4C particle size and increase of pressure. During the reaction, B4C particles decomposed layer by layer, and reacted with the melted Ti to form poor boron phases of TiB and Ti3B4 with needle-like shapes, which existed in quenched samples. With further reacting, TiB and Ti3B4 phases gradually changed to TiB2 with rectangle and hexagonal-plate shapes. The regular TiB2 particles were bound together by TiC, which constructed the skeleton of pores.

Fabrication and thermophysical properties of porous TiB2 ceramics fabricated by reactive spark plasma sintering

Titanium oxide (TiO2) and boron carbide (B4C) were added to TiB2 raw powders to prepare porous TiB2 ceramics by reactive spark plasma sintering, and the gas escape (such as CO and B2O3) resulted in higher porosity. X-ray Diffraction results indicated that the reduction reaction was completed after the reactive spark plasma sintering process. The porosity could be controlled by changing the ratio of synthesized TiB2 to raw TiB2 powders. The porosity of porous TiB2 ceramics with 20wt.% and 40wt.% synthsized TiB2 ceramics are 18.5% and 22.2%, respectively. The thermal diffusivity of the porous TiB2 ceramics decreased with the porosity due to the low diffusivity behavior of gas and vacuum in pores, and the thermal conductivity for porous TiB2 ceramics decreased as the temperature increased throughout the measured temperature range. The results here pointed to a potential method for fabricating porous TiB2 ceramics with controllable thermophysical properties.

Investigating the Potential of TiB<sub>2</sub>-Based Composites with Ti and Fe Additives as Wettable Cathode

In this work, porous TiB2 ceramics were consolidated by pressureless sintering method using metallic Ti and Fe as additives in order to perform sintering at temperatures lower than 1700°C. It was shown that processing parameters including milling time of the starting mixture had a considerable effect on final properties of sintered specimens and their behavior in molten aluminum. Microstructural studies were carried out using optical microscope, SEM and EPMA. It was found that specimens with uniform and crack-free microstructure could be produced using the pre-mixed powders milled for as low as 30 min prior to compaction and sintering. Sessile drop test was performed on the specimens milled for 30 and 240 minutes. Their interaction with molten aluminum was also studied. It was found that 30 min milling time resulted in better electrical conductivity, wettability and stability in liquid aluminum.

Investigating the Potential of TiB<sub>2</sub>–Based Composites with Ti and Fe Additives as Wettable Cathode

In this work, porous TiB2 ceramics were consolidated by pressureless sintering method using metallic Ti and Fe as additives in order to perform sintering at temperatures lower than 1700°C. It was shown that processing parameters including milling time of the starting mixture had a considerable effect on final properties of sintered specimens and their behavior in molten aluminum. Microstructural studies were carried out using optical microscope, SEM and EPMA. It was found that specimens with uniform and crack-free microstructure could be produced using the pre-mixed powders milled for as low as 30 min prior to compaction and sintering. Sessile drop test was performed on the specimens milled for 30 and 240 minutes. Their interaction with molten aluminum was also studied. It was found that 30 min milling time resulted in better electrical conductivity, wettability and stability in liquid aluminum.

High-strength porous titanium diboride ceramics: Microstructure and mechanical properties

Porous TiB2 ceramics have been fabricated by compacting TiB2 powders with cold isostatical pressing, followed by pressureless sintering. The microstructural feature, relative density, flexural strength, and fracture toughness were investigated. The experimental results revealed that the porosity of the fabricated porous TiB2 was controllable between ∼ 50 and ∼ 84 per cent depending on the sintering temperature and initial compacted pressures. The flexural strength and fracture toughness were improved with the increased relative density. The improved mechanical properties of the sintered porous TiB2 were attributed to the enhanced neck bonding and growth.

Preparation of Titanium Diboride Reticulated Porous Ceramics

Using a reticulated polyurethane sponge with interconnected pores as primal framework and immersing into TiB2 slurry consisting of Ni and Mo as sintering additives, a porous TiB2 ceramics with high porosity and interconnected pores was prepared by immersing and high temperature sintering process. The rheology of TiB2 slurry which used silica sol as a binder was studied. The optimum condition of the slurry suitable for impregnating the polyurethane sponge was obtained. The flexural strength of the porous reticulated TiB2 ceramics can reach 1 MPa.

Fabrication and Characterization of Porous TiB<sub>2</sub> Ceramic with Three-Dimensional Interconnected Skeleton

Porous TiB2 ceramics with a three-dimensional interconnected skeleton were fabricated by high temperature pressureless sintering from fine TiB2 powders. The microstructure of the porous TiB2 ceramic was characterized by the enhanced neck growth between the initially touching particles. This neck growth was ascribed to the selective heating of TiB2 particles with different dimension. The porous structure prepared by the high-temperature sintering exhibited higher bending strength and fracture toughness in the present experiment. The improved mechanical properties of the sintered composites were attributable to the enhanced neck growth by surface diffusion.

The Application of Self-Propagating High-Temperature Synthesis of Engineered Porous Composite Biomedical Materials

The term “self-propagating high-temperature synthesis” (SHS), or “combustion synthesis”, refers to an exothermic chemical reaction process that utilizes the heat generated by the exothermic reaction to ignite and sustain a propagating combustion wave through the reactants to produce the desired product(s). The products of combustion synthesis normally are extremely porous: Typically 50 percent of theoretical density. Advantages of combustion synthesis over traditional processing routes, e.g., sintering, in the production of advanced materials such as ceramics, intermetallic compounds and composites include process economics, simplicity of operation, and low energy requirements. However, the high exothermicity and rapid combustion propagation rates necessitate a high degree of control of these reactions. One research area being conducted in the Center for Space Resources (ISR) at the Colorado School of Mines (CSM) is the application of SHS to synthesize advanced, engineered porous TiB–Ti, NiTi, NiTi–TiC, TiC–Ti, and multiphase/heterogeneous calcium phosphate (HCaP) for bone tissue engineering and drug delivery systems. Processing conditions, such as reaction stoichiometry, green density, and gasifying agent, greatly affect the structure and properties of the SHS porous products. In general, all porous composite materials produced in this work for tissue engineering, including NiTi, NiTi + TiC, TiC + Ti, TiB + Ti, and multiphase/heterogeneous calcium phosphate (HCaP), appear to be biocompatible with no overt toxicity observed either in vitro or in vivo. An example of an SHS reaction is Ni + Ti = NiTi. The extent of the porosity and the size of the pores can be controlled using certain ‘gasifying agent' such as B2O3, CaCO3. The article discusses the synthesis of porous TiB–Ti, NiTi, Ni3Ti–TiC, TiC–Ti, and multiphase/heterogeneous calcium phosphate (HCaP) for bone tissue engineering and drug delivery systems.

Fabrication and mechanical properties of porous TiB2 ceramic

Porous material of TiB2 with improved mechanical properties was fabricated by vacuum and pressureless sintering. The microstructure of the porous ceramic was characterized by the enhanced neck growth between the initially touching particles. This neck growth was ascribed to the selective heating of TiB2 particles with different dimension. The porous structure prepared by the high-temperature sintering exhibited higher bending strength and fracture toughness in the present experiment. The improved mechanical properties of the sintered composites were attributable to the enhanced neck growth by surface diffusion.

Porous TiB2 electrodes for the alkali metal thermoelectric convertor

Porous TiB2 electrodes for the alkali metal thermoelectric convertor have been developed and tested. The electric performance of these new electrodes turned out to be superior to that of other electrodes known so far, like TiN or Mo. Because of its low reactivity, TiB2 (like TiN) can be expected to show a long-time stable operation. The performance of TiB2 electrodes of different sizes is described, using current-voltage relationships and impedance measurements.

Conditions of production of high-porosity graphite-containing sintered titanium diboride materials

The feasibility is demonstrated of producing, by the powder metallurgy method, a porous TiB2 material containing graphite microspheres acting as a pore-forming agent. A sintered material composition was determined giving the best combination of porosity and strength, and optimum conditions of production of such a material were established. A study was made of the sintering densification kinetics of a material of optimum composition. It was found that its sintering densification is linked with the behavior of its titanium boride powder matrix, since the volume of the graphite microspheres remains virtually unchanged during heating. It is shown that Ivensen's equation can be employed for describing curves of densification experienced by the composite material investigated during heating.