Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Spark plasma sintering of silicon nitride/barium aluminum silicate composite

Si₃N₄ based composites were successfully sintered by spark plasma sintering using low cost BaCO₃, SiO₂ and Al₂O₃ as additives. Powder mixtures were sintered at 1600–1800 °C for 5 and 10 min. Displacement-temperature-time (DTT) diagrams were used to evaluate the sintering behavior. Shrinkage curve revealed that densification was performed between 1100 and 1700 °C. The specimen sintered at 1700 °C showed the maximum relative density (99.8±0.1%), flexural strength (352±16 MPa), Vickers harness (11±0.1 GPa) and toughness (5.6±0.05 MPa m^1/2).     

Cost effective preparation of Si3N4 ceramics with improved thermal conductivity and mechanical properties

High-purity silicon powder is used as the starting material for cost-effective preparation of silicon nitride ceramics with both high thermal conductivity and excellent mechanical properties using RE₂O₃ (RE=Y, La or Er) and MgO as sintering additives. Nitridation is a key procedure that would affect the properties of green bodies and the sintered samples. The β: (α+β) ratio can be increased as the samples nitrided at 1450ºC and a large amount of long rod-like β-Si₃N₄ grains were developed in the samples. It was found that the addition of Er₂O₃-MgO could help to improve the mechanical properties of the sintered Si₃N₄ ceramics, the thermal conductivity, flexural strength and fracture toughness of the sample were 90 W/(m∙K), 953±28.3 MPa and 10.64±0.61 MPa·m^1/2, respectively. The RE³⁺ species with larger ionic radius tended to increase the oxygen of nitrided samples and decrease N/O ratio (triangle grain boundary) of sintered samples.     

Effect of rare earth additives on properties of silicon nitride ceramics

Abstract

Silicon nitride ceramics with rare earth (Re) compound (5 wt-%) and MgO (3 wt-%) additives were fabricated by spark plasma sintering and following heat treatment. The Re compounds included two groups: ReF₃ ((Re?=?La,Nd,Gd) and Re₂O₃ (Re?=?La,Nd,Gd). Specimens show the same tendency in the sintering shrinkage rate, relative density, grain size and bending strength with the increasing Re cation (Re³⁺) radius both in ReF₃ and Re₂O₃ added samples. However, as to aspect ratios and thermal conductivity, the change rules are completely opposite between the two groups of specimens.     

Effect of carbon nanoparticle reinforcement on mechanical and thermal properties of silicon carbide ceramics

This research presents an analysis of the influence of graphene reinforcement on the thermal and mechanical properties of silicon carbide ceramics, at 2.5% (wt%) graphene content. The SiC composites, containing various carbon nanofillers (graphene oxide and graphene nanoparticles), were sintered by the classical two stage spark plasma sintering method. Two current modes were used, the continuous mode and the pulsed current mode. The results from photothermal radiometry and investigations of the mechanical properties showed that graphene additives significantly improve the thermal properties and toughness of material, sintered from a SiC powder. An 45% growth in the toughness was observed, which increased from 1.21 to 1.75?MPa/m^1/2. The thermal diffusivity value also increased from 0.60 to 0.71?cm²/s and giving an improvement in thermal properties of 18%. The friction coefficient reached 7% giving an increase in value from 0.62 to 0.66. Microscopic investigations supported the photothermal radiometry (PTR) results. Whilst, thermal imaging revealed homogeneity of the local thermal properties of the products fabricated from the starting SiC powder.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Effect of rare earth oxides addition on the mechanical properties and coloration of silicon nitride ceramics

The aim of this study was to evaluate the mechanical properties and coloration of silicon nitride ceramics in the presence of RE₂O₃ (RE = Nd, Eu or Dy). Dense Si₃N₄ ceramics were prepared by gas pressure sintering at 1800 °C for 2 h. XRD analysis confirmed the complete transformation of α-Si₃N4 to β-Si₃N₄. The fracture toughness and flexure strengths were 11.93 ± 0.56 MPa·m^1/2, 667 ± 40.98 MPa with the addition of Eu₂O₃ (SE). Base on the SEM image, the pull-out, bridging and deflection of large grains were observed and contributed to the increase in mechanical properties. The chromaticity of sintered bodies was measured using a spectrophotometer. The color difference of the ceramics is due to the formation of different color developing compounds according to the EDS. Results showed that high-toughness and colorful Si₃N₄ ceramics can be prepared using YAG:Ce³⁺ as sintering additive and RE₂O₃ as the colorant.     

Improved electrical and thermal properties of silicon oxycarbide/spodumene composites

Novel bulk SiOC/spodumene composites have been developed by spark plasma sintering (SPS) at relatively low temperature (1200–1400 °C). Spodumene is a cheap and natural available lithium aluminosilicate mineral which acts as meltable/active filler. At 1300–1400 °C, the Al migrates toward the glassy matrix producing a Si-Al-O network and the crystallization of α-cristobalite. The C_free phase also experiences a deep transformation. The epitaxial growth of few-layered graphene over SiC particles occurs at 1400 °C. An increase in the phonon transport is observed (36%, 1.28 – 2.14 Wm⁻¹K⁻¹) associated to the reduction of the interface resistance between the partially crystallized SiO₂ matrix and the SiC nano-wires/graphene-like carbon conductive phase. The electrical conductivity increases (1.14 ×10⁻² – 8.1 Sm⁻¹) due to the densification reached and an increasing ordering degree of the tortuous C_free phase with a high quality of interconnection and crystallization. Raman parameters are determinant to understand the thermal and electrical response.     

Fabrication and mechanical properties of boron nitride nanotube reinforced silicon nitride ceramics

In this study, silicon nitride (Si₃N₄) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated by hot-pressing method. The influence of sintering temperature and BNNTs content on the microstructures and mechanical properties of Si₃N₄ ceramics were investigated. It was found that both flexural strength and fracture toughness of Si₃N₄ were improved when sintering temperature increases. Moreover, α-Si₃N₄ phase could transform into β-Si₃N₄ phase completely when sintering temperature rises to 1800 °C and above. BNNTs can enhance the fracture toughness of Si₃N₄ dramatically, which increases from 7.2 MPa m^1/2 (no BNNTs) to 10.4 MPa m^1/2 (0.8 wt% BNNTs). However, excessive addition of BNNTs would reduce the fracture toughness of Si₃N₄. Meanwhile, the flexural strength and relative density of Si₃N₄ decreased slightly when BNNTs were added. The related toughening mechanism was also discussed.     

High-pressure spark plasma sintering of silicon nitride with LiF additive

High-pressure spark plasma sintering of Si₃N₄ with Y₂O₃, Al₂O₃ and LiF additives was employed to fabricate high quality dense ceramics comprising approximately 92% α-Si₃N₄ phase and 8% β-Si₃N₄ phase. The relatively high pressure applied (up to 650 MPa) had a substantial effect on densification by enhancing particle rearrangement, making it possible to obtain dense Si₃N₄ at a significantly lower sintering temperature (1350 °C). Consequently, virtually no α to β phase transformation transpired during the liquid phase sintering process. The LiF additive had an indispensable influence on the densification process by lowering the viscous glass formation temperature, which also contributed to enhanced particle rearrangement. The nearly fully dense samples (theoretical density ≥99%) obtained displayed a good combination of mechanical properties, namely elastic modulus (304–316 GPa), hardness (1720–1780 HV2) and fracture toughness (6.0 MPa m^1/2).     

Synergistic effect of binary fluoride sintering additives on the properties of silicon nitride ceramics

A variety of combinations of YF₃ and MgF₂ were used as sintering aids in the fabrication of Si₃N₄ ceramics via gas pressure sintering (GPS). The synergistic effects of YF₃ and MgF₂ on the liquid viscosity, mechanical properties, thermal conductivities, and grain growth kinetics of the Si₃N₄ ceramics were investigated. The results showed that appropriately adjusting the YF₃/MgF₂ ratio could decrease liquid viscosity, reducing the diffusion energy barrier of the solute atom and promoting mass transfer. Meanwhile, the chemical bonding strength in the grain boundary complexions formed by the metal cations also influenced grain boundary migration. Samples doped with 4 mol% YF₃ and 2 mol% MgF₂ achieved the lowest grain growth exponent (n = 2.9) and growth activation energy (Q = 616.7 ± 16.5 kJ mol^?1) as well as the highest thermal conductivity (83 W m^?1 K^?1) and fracture toughness (8.82 ± 0.13 MPa m^1/2).     

Effects of Y2O3/MgO ratio on mechanical properties and thermal conductivity of silicon nitride ceramics

In this work, the effects of Y₂O₃/MgO ratio on the densification behavior, phase transformation, microstructure evolution, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were investigated. Densified samples with bimodal microstructure could be obtained by adjusting the ratio of Y₂O₃/MgO. It was found that a low Y₂O₃/MgO ratio facilitated the densification of Si₃N₄ ceramics while a high Y₂O₃/MgO ratio benefited the phase transformation of Si₃N₄ ceramics. Best mechanical properties (flexural strength of 875 MPa, and fracture toughness of 8.25 MPa·m^1/2, respectively) and optimal thermal conductivity of 98.04W/(m·K) were achieved in the sample fabricated with Y₂O₃/MgO ratio of 3:4 by sintering at 1900°C for 4 h.     

Thermal conductivity and stability of Al-doped ZnO nanostructured ceramics

Pure and Al-doped ZnO powders have been sintered by Spark Plasma Sintering. Al doping allows the ceramics to reach a relative density greater than 90% at a sintering temperature of 500?°C. The morphology of powder nanoparticles impacts the final grain size of the sintered bulk compounds. A ceramic sintered from isotropic nanoparticles of 30?nm in diameter can reach an average grain size of 110?nm, whereas a ceramic sintered from platelets and isotropic nanoparticles exhibits an average grain size in the submicrometric range. The influence of ceramic grain size on the thermal conductivity has been investigated. It shows that substantial decrease of the grain size from several microns down to 100?nm reduces the thermal conductivity from 29.5 to 7.8?W/m?K at 100?°C. The stability of nanostructured ceramic has also been checked. After SPS, an annealing at 500?°C in air also leads to grain growth.     

Electrical and thermal response of silicon oxycarbide materials obtained by spark plasma sintering

In order to obtain dense silicon oxycarbide (SiOC) materials that maintain the properties of glass, non-conventional spark plasma sintering was used to sinter SiOC powders from 1300 to 1700 °C and with 40 MPa of pressure. The concurrence of electrical current, high pressure and low vacuum while the material is being heating produces a dense SiOC-derived material composed of a SiO₂ glassy matrix reinforced with SiC nanowires grown in situ, graphene-like carbon and turbostratic graphite. SiOC materials with high electrical and thermal response are obtained as a result of this new processing technique. Electrical resistivity undergoes an extraordinary decrease of five orders of magnitude from 1300 (1.0 × 10⁵ Ω m) to 1700 °C (0.78 Ω m), ranging from insulate to semiconductor material; and thermal conductivity increases by 30%, for these sintering temperatures.     

Effects of two-step sintering on thermal and mechanical properties of aluminum nitride ceramics by impedance spectroscopy analysis

The effects of two-step sintering on the microstructure, mechanical and thermal properties of aluminum nitride ceramics with Yb₂O₃ and YbF₃ additives were investigated. AlN samples prepared using different sintering methods achieved almost full density with the addition of Yb₂O₃–YbF₃. Compared with the one-step sintering, the grain sizes of AlN ceramics prepared by the two-step sintering were limited, and the higher flexural strength and the larger thermal conductivity were obtained. Moreover, the electrochemical impedance spectroscopy of AlN ceramic was associated with thermal conductivity by analyzing the defects and impurities in AlN ceramics. The fitting grain resistance and the activation energy for the grain revealed the lower concentrations of aluminum vacancy in the two-step sintered AlN ceramics, which resulted in the higher thermal conductivity. Thus, mechanical and thermal properties for AlN ceramics were improved with Yb₂O₃ and YbF₃ additives sintered using two-step regimes.     

The effect of Al2O3-MgO additives on the microstructure of spark plasma sintered silicon nitride

It was shown that spark plasma sintered silicon nitride with a high content of Al₂O₃ and MgO consists of α and β silicon nitride, the main phase being α silicon nitride. The increase in the sintering temperature did not lead to significant changes in the phase composition as occurs in silicon nitride added with Al₂O₃-Y₂O₃. It was found that increasing in SPS temperature above 1650 °C leads to an insignificant increase in the density. A complex shaped equiaxed grain microstructure was shown in both cases. However, doping with aluminum and yttrium oxides allows obtaining an elongated grain microstructure. The Hall-Petch effect was observed for the microhardness of the investigated SPSed silicon nitride. The microhardness of the described ceramics was rather high and more than 1900 HV compared to the pressureless sintered at 1800 °C silicon nitride with the microhardness equal to 1511 HV.     

Electrical resistivity of silicon nitride produced by various methods

It has been shown that the grain growth and amount of the glass phase influence the electrical resistivity of pressureless sintered and spark plasma sintered silicon nitride. Sintering additives strongly affect the impurity conductivity of pressureless sintered silicon nitride and slightly influence the intrinsic conductivity due to the longer sintering process as compared with the spark plasma sintering. It was demonstrated that Al₂O₃-Y₂O₃ lead to decrease in the electrical resistivity of SPSed silicon nitride due to increase in the band gap width as opposed to Al₂O₃-MgO. Effect of the sintering additive on the impurity conductivity is practically absent but there is a strong dependence of the sintering temperature for reported spark plasma sintered silicon nitride. However, intrinsic conductivity of SPSed silicon nitride is affected by both sintering temperature and sintering additive. It was also shown that electrical resistivity of produced ceramics is linearly depends on the content of β-Si₃N₄ and microhardness. Electrical resistivity of manufactured silicon nitride varied from 3.16·10⁹ to 1.73·10¹¹ Ω?m. It has been observed strong influence of the sintering additive and sintering temperature on the electrical properties of SPSed and pressureless sintered silicon nitride.     

Fabrication of high thermal conductivity silicon nitride ceramics by pressureless sintering with MgO and Y2O3 as sintering additives

The fabrication of silicon nitride (Si₃N₄) ceramics with a high thermal conductivity was investigated by pressureless sintering at 1800 °C for 4 h in a nitrogen atmosphere with MgO and Y₂O₃ as sintering additives. The phase compositions, relative densities, microstructures, and thermal conductivities of the obtained Si₃N₄ ceramics were investigated systemically. It was found that at the optimal MgO/Y₂O₃ ratio of 3/6, the relative density and thermal conductivity of the obtained Si₃N₄ ceramic doped with 9 wt% sintering aids reached 98.2% and 71.51 W/(m·K), respectively. EDS element mapping showed the distributions of yttrium, magnesium and oxygen elements. The Si₃N₄ ceramics containing rod-like grains and grain boundaries were fabricated by focused ion beam technique. TEM observations revealed that magnesium existed as an amorphous phase and that yttrium produced a new secondary phase.     

Electrical conductivity and thermal properties of spark plasma sintered Al2O3-SiC-CNT hybrid nanocomposites

In this study, we report the electrical conductivity and thermal properties of Al₂O₃-SiC-CNT hybrid nanocomposites processed via ball milling (BM) and spark plasma sintering (SPS). The initial powders and consolidated samples were characterized using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM), respectively. A multifunction calibrator and a high-resolution digital multimeter were used to measure the electrical conductivity. The thermal properties were measured using a thermal constants analyser. The SiC and CNT-reinforced alumina hybrid nanocomposites exhibited a significant increase in their room-temperature electrical conductivity, which made them suitable for electrical discharge machining. The Al₂O₃-5SiC-2CNTs had a high electrical conductivity value of 8.85 S/m compared to a low value of 6.87×10⁻¹⁰ S/m for the monolithic alumina. The addition of SiC and CNTs to alumina decreased its room-temperature thermal properties. The increase in temperature resulted in a decrease in the thermal conductivity and thermal diffusivity but an increase in the specific heat of the monolithic alumina and the hybrid nanocomposites. These properties were correlated with the microstructure, and possible transport mechanisms were discussed.     

Microstructure control and mechanical properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with a fibrous interlocking microstructure were synthesized by carbothermal nitridation of silicon dioxide. The influences of different starting powders on microstructure and mechanical properties of the samples were studied. The results showed that the microstructure and mechanical properties of porous silicon nitride ceramics depended mostly on the size of starting powders. The formation of single-phase β-Si₃N₄ and the microstructure of the samples were demonstrated by XRD and SEM, respectively. The resultant porous Si₃N₄ ceramics with a porosity of 71% showed a relative higher flexural strength of 24 MPa.