Influence of nitrogen vacancy defects incorporation on densification behaviour of spark plasma sintered non-stoichiometric TiN1−x

Highly nitrogen deficient non-stoichiometric TiN_1?x powders within nitrogen vacancy defects (0.3<1?x<0.5) were prepared through mechanical alloying and consolidated through spark plasma sintering. Increasing nitrogen vacancy defects promoted densification behaviour of TiN_1?x. Nitrogen vacancies accelerated material transport and diffusion during sintering. The altered strong covalent bonding nature of TiN_1?x was believed to enhance the sinterability. TiN_1?x (1?x?=?0.32) ceramic reached relative density of 98.8%, Vickers hardness of 17.0 GPa and grain size of 200–300 nm after sintering at 1000°C, 40 MPa and 10 min.     

Densification and mechanical properties of TiC by SPS-effects of holding time,sintering temperature and pressure condition

The influence of sintering temperature, holding time and pressure condition on densification and mechanical properties of bulk titanium carbide (TiC) fabricated by SPS sintering has been systematically investigated. Experimental data demonstrated that relative density and Vickers hardness (H_V) increase with sintering temperature and holding time, but fracture toughness (K_IC) was not significantly influenced by sintering parameters. The H_V and relative density of samples consolidated by SPS technique at 1600 °C for 5 min under 50 MPa pressure (applied entire sintering cycle) reached 30.31 ± 2.23 GPa and 99.90%, respectively. H_V values of ～24–30 GPa and K_IC of ～3.7–5 MPa m^1/2 were obtained in all bulk samples with relative densities of 95.61–99.90% when fabricated under various conditions presented above, without abnormal grain growth. More pronounced effects of pressure condition on grain growth (promoted by grain-boundary diffusion) than on densification were observed. The relationship of fracture toughness and fracture mode is also discussed.     

The effect of Zn addition on the structure and transport properties of BaCe0.9−xZrxY0.1O3−δ

BaCe_0.9−xZr_xY_0.1O_3−δ (0.1 ≤ x ≤ 0.9) are ceramic proton conductors widely investigated for different electrochemical devices, such as Solid Oxide Fuel Cell (SOFC) electrolytes, however, their applications are limited by the high sintering temperatures necessary to achieve densification. Polycrystalline powders of BaCe_0.9−xZr_xY_0.1O_3−δ (BCZ) have been prepared by freeze-drying precursor method at 1000 °C. These powders were mixed with a zinc nitrate solution to decrease the sintering temperature to 1200–1300 °C. The addition of Zn has several effects on the structure and microstructure of BCZ: the densification and grain growth are enhanced at lower temperature and a reduction of the crystallographic symmetry is observed for samples with low Zr-content (x ≤ 0.3). Furthermore, the bulk and specific grain boundary conductivities are only slightly affected by the addition of Zn.     

HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering

HfB₂-x vol%CNTs (x=0, 5, 10, and 15) composites are prepared by spark plasma sintering. The influence of CNTs content and sintering temperature on densification, microstructure and mechanical properties is studied. Compared with pure HfB₂ ceramic, the sinterability of HfB₂-CNTs composites is remarkably improved by the addition of CNTs. Appropriate addition of CNTs (10 vol%) and sintering temperature (1800 °C) can achieve the highest mechanical properties: the hardness, flexural strength and fracture toughness are measured to be 21.8±0.5 GPa, 894±60 MPa, and 7.8±0.2 MPa m^1/2, respectively. This is contributed to the optimal combination of the relative density, grain size and the dispersion of CNTs. The crack deflection, CNTs debonding and pull-out are observed and supposed to exhaust more fracture energy during the fracture process.     

Enhancing the sinterability and fracture toughness of Al2O3–TiN0.3 composites

We introduce a new and effective method for improving the fracture toughness of Al₂O₃-based composites through the addition of a nonstoichiometric material. Al₂O₃–TiN_0.3 composites were sintered by spark plasma sintering with different TiN_0.3 content at temperatures between 1300 and 1600 °C for 10 min and a micro-region diffusion phenomenon was observed at the Al₂O₃–TiN_0.3 interface. Ti atoms from TiN_0.3 diffused into Al₂O₃ to occupy Al sites, which led to the formation of Al vacancies that enabled the transport of aluminum by a vacancy mechanism. The optimal densification temperature of the Al₂O₃–30vol% TiN_0.3 composite was approximately 1400 °C. The maximum fracture toughness measured was 6.91 MPa m^1/2, from the composite with 30 vol% TiN_0.3 sintered at 1500 °C.     

Densification behavior and microstructure evolution of yttrium-doped ThO2 ceramics

Sintering of Th_1-xY_xO_2-x/2 ceramics (x = 0.01, 0.08, 0.15 and 0.22), planned to be used as solid electrolytes in oxygen sensors for sodium-cooled fast nuclear reactors, was investigated. High densification state (i.e. up to 98% TD) was reached after 4 h of heat treatment at 1600 °C and beyond. In addition, ESEM observations showed a major effect of yttrium on grain size due to solute drag effects. Sintering maps were plotted for all the samples and evidenced different stages driven by densification and grain growth. Grain growth was found to be strongly slowed down for x > 0.01, resulting in high values of relative density correlated to submicrometric grain size. Also, activation energies related to densification and grain growth were evaluated around 450 and 500–650 kJ mol⁻¹, respectively. These results led to deliver guidelines for the formulation and sintering of Th_1-xY_xO_2-x/2 ceramics in prospect of their use as a solid electrolyte.     

Densification and grain growth kinetics of Ce0.9Gd0.1O1.95 in tape cast layers: The influence of porosity

The sintering behavior of Ce_0.9Gd_0.1O_1.95 (CGO) tape cast layers with different porosity was investigated by an extensive characterization of densification, microstructural evolution, and applying the constitutive laws of sintering. The densification of CGO tapes associates with grain coarsening process at the initial sintering stage at T < 1150 °C, which is mainly influenced by small pores and intrinsic characteristics of the starting powders. At the intermediate sintering stage, densification is remarkably influenced by large porosity. Moreover, the sintering constitutive laws indicate that increasing the initial porosity from 0.38 to 0.60, the densification at the late stage is thermally activated with typical activation energy values increasing from 367 to 578 kJ mol⁻¹. Similar effect of the porosity is observed for the thermally activated phenomena leading to grain growth in the CGO tapes. The analysis of sintering mechanisms reveals that the grain growth behavior at different porosity can be described using an identical master curve.     

Preparation and properties of an Al2O3/Ti(C,N) micro-nano-composite ceramic tool material by microwave sintering

One kind of Al₂O₃/Ti(C,N) micro-nano-composite ceramic tool material with acceptable properties was prepared by microwave sintering. Effects of sintering temperature and holding time on densification, mechanical properties and microstructure were studied. The optimal relative density, fracture toughness and Vickers hardness were 98.4±0.30, 6.72±0.28 MPa m^1/2 and 18.42±0.59 GPa, respectively, which were obtained at 1550 °C for 10 min. Compared to the conventional sintering, the sintering temperature and holding time of microwave sintering were reduced by 14% and 89%, respectively. The microwave sintering made the sizes of some particles keep in nano-scale, which leaded to the formation of intragranular structures. The residual stress in the intragranular structures increased the ratio of grain boundary toughness to grain toughness of matrix (K_cb/K_cg), and thus the micro-Al₂O₃ grains were more inclined to transgranular fracture.     

Improved radiation damage tolerance of titanium nitride ceramics by introduction of vacancy defects

TiN and TiN_0.7 were irradiated using a 100 keV Ar-ion beam at 600 °C to target doses of 3 × 10¹⁷ ions cm⁻². SRIM estimation, GIXRD and fluorescence analysis have been performed to evaluate the effect of pre-existing vacancy defect on the radiation tolerance. The lattice parameter of TiN increased after irradiation due to interstitial atoms and vacancies in as-irradiated TiN. In contrary, the lattice parameter decreased for as-irradiated TiN_0.7, which indicates that the nitrogen atom vacancies in TiN_0.7 acted as sinks for displacement atoms generated by irradiation to limit interstitial atoms existing. The intensity of peaks in fluorescence spectrum of as-irradiated TiN was higher than that of as-irradiated TiN_0.7. That attributed to the presence of color centers formed by Frenkel defects in as-irradiated TiN. All of the results indicate that introducing vacancy defect in materials would offer capability to realize self-heal of irradiation damage.     

Densification behaviour and microstructural development in undoped yttria prepared by flash-sintering

Conventional sintering of undoped Y₂O₃ requires temperatures above 1400 °C for a few hours. We show that it can be sintered nearly instantaneously to nearly full density at furnace temperature of 1133 °C under a DC applied field of 500 V/cm. At 1000 V/cm sintering occurs at 985 °C. The FLASH event, when sintering occurs abruptly, is preceded by gradually accelerated field-assisted sintering (FAST). This hybrid behaviour differs from earlier work on yttria-stabilized zirconia where all shrinkage occurred in the flash mode. The microstructure of flash-sintered specimens indicated that densification was accompanied by rapid grain growth. The single-phase nature of flash-sintered Y₂O₃ was confirmed by high-resolution transmission electron microscopy. The non-linear rise in conductivity accompanying the flash led to Joule heating. It is postulated that densification and grain growth were enhanced by accelerated solid-state diffusion, resulting from both Joule heating and the generation of defects under the applied field.     

High-strength AlN ceramics by low-temperature sintering with CaZrO3–Y2O3 co-additives

Aluminum nitride (AlN) ceramics with the concurrent addition of CaZrO₃ and Y₂O₃ were sintered at 1450-1700 °C. The degree of densification, microstructure, flexural strength, and thermal conductivity of the resulting ceramics were evaluated with respect to their composition and sintering temperature. Specimens prepared using both additives could be sintered to almost full density at relatively low temperature (3 h at 1550 °C under nitrogen at ambient pressure); grain growth was suppressed by grain-boundary pinning, and high flexural strength over 630 MPa could be obtained. With two-step sintering process, the morphology of second phase was changed from interconnected structure to isolated structure; this two-step process limited grain growth and increased thermal conductivity. The highest thermal conductivity (156 Wm⁻¹ K⁻¹) was achieved by two-step sintering, and the ceramic showed moderate flexural strength (560 MPa).     

Spark plasma sintered ZrC-Mo cermets: Influence of temperature and compaction pressure

The microstructure analysis and mechanical characterisation were performed on a ZrC-20 wt%Mo cermet that was spark plasma sintered at various temperatures ranging between 1600 and 2100 °C under either 50 or 100 MPa of compaction pressure. The composite reached ~98% relative density for all experiments with an average grain size between 1 and 3.5 µm after densification. The nature of SPS technology caused a faster densification rate when higher compaction pressures were applied. The difference in compaction pressures produced different behaviors in densification and grain structure: 1900 °C, 100 MPa produced excessive grain growth in ZrC; 1600 °C, 50 MPa revealed a very clear ZrC grain structure and Mo diffusion between carbide grains; and 2100 °C, 50 MPa exhibited the highest overall mechanical properties due to small clusters of Mo phases across the microstructure. In fact, this particular sintering regime gave the most optimal mechanical values: 2231 HV10 and 5.4 MPa*m^1/2, and 396 GPa Young's modulus. The compaction pressure of SPS played a pivotal role in the composites’ properties. A moderate 50 MPa pressure caused all three mechanical properties to increase with increasing sintering temperature. Conversely, a higher 100 MPa pressure caused fracture toughness and Young modulus to decrease with increasing sintering temperature.     

Synthesis and characterization of dense and fine nickel ferrite ceramics through two-step sintering

Two-step sintering (TSS) has been employed in the current study to suppress the accelerated grain growth of NiFe₂O₄ nanopowder compacts in the final sintering stage. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range from 1200 °C to 1300 °C is effective for the first-step sintering (T₁) due to its highest densification rate. The second-step sintering temperature (T₂) should be within the kinetic window, where grain boundary diffusion is maintained but grain boundary migration is suppressed. The grain sizes of high density (≥98% theoretical density) NiFe₂O₄ compacts produced by TSS are smaller than 700 nm, while that of those formed by CS are over 2.5 μm. The evidence indicates that the saturation magnetization of nearly full NiFe₂O₄ ceramics is independent of grain size and likewise high, with the corresponding values of approximately 54 emu/g. The Vickers hardness and fracture toughness both increase with the decrease of grain size and porosity.     

Microwave sintering and grain growth behavior of nano-grained BaTiO3 materials

In this paper, we investigated the effect of microwave sintering parameters on the development of the microstructure of nano-grained BaTiO₃ materials co-doped with Y and Mg species. It is observed that the materials can not only be sintered densely at a lower temperature (1150 °C) and a shorter soaking time (20 min), but also the grain growth can be suppressed by 2.45 GHz microwave heating process. However, the grain growth exhibits a unique tendency in some processing conditions such as microwave sintering for longer intervals (≧60 min) or at higher temperatures (1200 °C). The grain growth behavior after densification was investigated in terms of the phenomenological kinetics, and the activation energy for grain growth using microwave sintering (59.4 kJ/mol) is considerably less than that of the conventionally sintered ones (96.0 kJ/mol), which indicates that microwave sintering process can accelerate the densification rate of the BaTiO₃ materials comparing with the conventional sintering process.     

Two step sintering of a novel calcium magnesium silicate bioceramic: Sintering parameters and mechanical characterization

Two-step sintering (TSS) was applied to control the grain growth during sintering of a novel calcium magnesium silicate (Ca₃MgSi₂O₈ – Merwinite) bioceramic. Sol–gel derived nanopowders with the mean particle size of about 90 nm were sintered under different TSS regimes to investigate the effect of sintering parameters on densification behavior and grain growth suppression. Results showed that sintering of merwinite nanopowder under optimum TSS condition (T₁ = 1300 °C, T₂ = 1250 °C) yielded fully dense bodies with finest microstructure. Merwinite compacts held at T₂ = 1250 °C for 20 h had the average grain size of 633 nm while the relative density of about 98% was achieved. Mechanical testing was performed to investigate the effect of grain growth suppression on the hardness and fracture toughness. Comparison of mechanical data for samples sintered under two sintering regimes, including TSS and normal sintering (NS), showed TSS process resulted in significant enhancement of fracture toughness from 1.77 to 2.68 MPa m^1/2.     

Master sintering curves of a nanoscale 3Y-TZP powder compacts

The sintering behavior of commercially available granulated ZrO₂–3 mol% Y₂O₃ (3Y-TZP) powder compacts with an aggregate size of 75 nm was studied. The shrinkage response of the powder compacts during non-isothermal sintering was measured in a sensitive dilatometer at different heating rates. Densification and grain growth were also studied after isothermal firing in air according to different sintering cycles. The sintering and grain growth activation energy was estimated to be Q_S = 485 ± 12 kJ mol^?1 and Q_G = 546 ± 23 kJ mol^?1, respectively. Using the estimated Q-values, the master curves for sintering and grain growth were established and used for prediction of the densification and microstructural development under different thermal histories. A good agreement between the model predictions and experimental result was obtained.     

Spark plasma sintering of Ti1−xAlxN nano-powders synthesized by high-energy ball milling

The present study focused on the fabrication of bulk materials from Ti_1−xAl_xN nano-powders using a spark plasma sintering (SPS) apparatus. Super-saturated Ti_1−xAl_xN solid solutions containing differing fractions of AlN (10, 20, 30 and 50 mol%) were synthesized by high-energy ball milling (HEBM) of pure nitrides. The complete dissolution of AlN in TiN was achieved after 100 h of milling. The milled powders were characterized by X-ray diffraction, scanning electron microscopy, energy-filtered transmission electron microscopy spectra imaging and energy dispersive X-ray spectroscopy. The crystalline size of the mechanically alloyed powders after 100 h of milling was about 12–14 nm. Ti_1−xAl_xN powders of various compositions were sintered by SPS under pressure of 63 MPa at 1673 K. Maximal hardness and bending strength values (610 MPa and 18.6 GPa, respectively) were obtained for composites containing 20 mol%AlN. Powder with 20% mole%AlN was consolidated under pressure of 500 MPa in the 1273–1423K temperature range by high pressure SPS (HPSPS). A fully dense nano-structured specimen, processed at 1423 K, displayed a Young modulus of 420 GPa, hardness of 20.5 GPa, bending strength of 670 MPa and fracture toughness of 7.1 MPa m^0.5.     

Spark plasma sintering of TaC–HfC UHTC via disilicides sintering aids

Ta_0.8Hf_0.2C ceramic has the highest melting point among the known materials (4000 °C). Spark plasma sintering is a new route for consolidation of materials, specially ultra high temperature ceramics (UHTCs), which are difficult to be sintered at temperatures lower than 2000 °C.The purpose of this study is to consolidate Ta_0.8Hf_0.2C by spark plasma sintering at low temperature using MoSi₂ and TaSi₂ as sintering aid. In this regard, effect of different amounts of sintering aids and carbides ratio on densification behavior and mechanical properties of Ta_1?xHf_xC were investigated.Fully consolidation of Ta_0.8Hf_0.2C was achieved in presence of 12 vol.% sintering aid after sintering at 1650 °C for 5 min under 30 MPa. The first stage of sintering was due to plastic deformation of sintering aids particles and consequent rearrangement. The second stage was occurred via Ta_1?xHf_xC solid solution and liquid phase formation.     

Experimental study on the stability of graphitic C3N4 under high pressure and high temperature

The stability and decomposition of graphitic C₃N₄ (g-C₃N₄) were studied in the pressure and temperature range of 10–25 GPa and up to 2000 °C by multi-anvil experiments and phase characterization of the quenched products. g-C₃N₄ was found to remain stable at relatively mild temperatures, but decomposes to graphite and nitrogen at temperatures above 600–700 °C and up to 15 GPa, while it decomposes directly to diamond (plus nitrogen) above 800–900 °C and between 22 and 25 GPa. The estimated decomposition curve for g-C₃N₄ has a positive slope (~ 0.05 GPa/K) up to ~ 22 GPa, but becomes inverted (negative) above this pressure. The diamond formed through decomposition is characterized by euhedral crystals which are not sintered to each other, but loosely aggregated, suggesting the crystallization in a liquid (nitrogen) medium. The nitrogen release from the graphitic CN framework may also play an important role in lowering the activation energy required for diamond formation and enhancing the grain growth rate. No phase transition of g-C₃N₄ was found in the studied P–T range.     

Densification behaviour and two stage master sintering curve in lithium sodium niobate ceramics

The Master Sintering Curve (MSC) has received much attention in recent years due to its ability to predict sintering behaviour of a given powder and green body process regardless of its thermal history. In this paper MSC, based on the combined stage sintering model is constructed for one of the most important lead-free piezoelectric viz. lithium sodium niobate, Na_1-xLi_xNbO₃ (x=0.12, LNN-12), ceramic using shrinkage data. The present study has been carried out to understand and control the densification behaviour during pressureless sintering. Two distinct stages of densification have been observed en route to the upper limit to sintering temperature. The activation energies of densification for the two temperature ranges viz. 800–1150 °C and 1150–1300°C were found to be 365 kJ/mol and 2530 kJ/mol, respectively, through the construction of MSC. The MSC should be useful in predicting the densification behaviour and the final density and for designing a reproducible fabrication schedule for the LNN-12 ceramics.