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).     

Effect of a new nonoxide additive,Y3Si2C2, on the thermal conductivity and mechanical properties of Si3N4 ceramics

Enhancement of the thermal conductivity of silicon nitride is usually achieved by sacrificing its mechanical properties (bending strength). In this study, β-Si₃N₄ ceramics were prepared using self-synthesized Y₃Si₂C₂ and MgO as sintering additives. It was found that the thermal conductivity of the Si₃N₄ ceramics was remarkably improved without sacrificing their mechanical properties. The microstructure and properties of the Si₃N₄ ceramics were analyzed and compared with those of the Y₂O₃-MgO additives. The addition of Y₃Si₂C₂ eliminated the inherent SiO₂ and introduced nitrogen to increase the N/O ratio of the grain-boundary phase, inducing Si₃N₄ grain growth, increasing Si₃N₄ grain contiguity, and reducing lattice oxygen content in Si₃N₄. Therefore, by replacing Y₂O₃ with Y₃Si₂C₂, the thermal conductivity of the Si₃N₄ ceramics was significantly increased by 31.5% from 85 to 111.8Wm⁻¹K⁻¹, but the bending strength only slightly decreased from 704 ± 63MPa to 669 ± 33MPa.     

Thermal and mechanical properties of Si3N4 ceramics obtained via two-step sintering

Si₃N₄ ceramics were prepared using novel two-step sintering method by mixing α-Si₃N₄ as raw material with nanoscale Y₂O₃–MgO via Y(NO₃)₃ and Mg(NO₃)₂ solutions. Si₃N₄ composite powders with in situ uniformly distributed Y₂O₃–MgO were obtained through solid–liquid (SL) mixing route. Two-step sintering method consisted of pre-deoxidization at low temperature via volatilization of in situ-formed MgSiO₃ and densification at high temperature. Variations in O, Y, and Mg contents in Si₃N₄–Y₂O₃–MgO during first sintering step are discussed. O and Mg contents decreased with increasing temperature because SiO₂ on Si₃N₄ surface reacted with MgO to form low-melting-point MgSiO₃ compound, which is prone to volatilize at high temperature. By contrast, Y content hardly changed due to high-temperature stability of Y–Si–O–N quaternary compound. In the second sintering step, skeleton body was densified, and the formation of Y₂Si₃O₃N₄ secondary phase occurred simultaneously. Two-step sintered Si₃N₄ ceramics had lower total oxygen content (1.85 wt%) than one-step sintered Si₃N₄ ceramics (2.51 wt%). Therefore, flexural strength (812 MPa), thermal conductivity (92.1 W/m·K), and fracture toughness (7.6 MPa?m^1/2) of Si₃N₄ ceramics prepared via two-step sintering increased by 28.7%, 16.9%, and 31.6%, respectively, compared with those of one-step sintered Si₃N₄ ceramics.     

Effect of MgF2 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Dense silicon nitride (Si₃N₄) ceramics were prepared using Y₂O₃ and MgF₂ as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF₂ contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si₃N₄ ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF₂. For optimized mechanical properties and thermal conductivity of Si₃N₄, optimum value for MgF₂ content existed. Sample with 3 mol.% Y₂O₃ and 2 mol.% MgF₂ obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m^1/2 and 76 W m⁻¹ K⁻¹, respectively). It was observed that excessive MgF₂ reduced the performance of the ceramic, which was caused by the presence of excessive volatiles.     

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.     

Fabrication and properties of pressure-sintered reaction-bonded Si3N4 ceramics with addition of Eu2O3–MgO–Y2O3

Dense pressure-sintered reaction-bonded Si₃N₄ (PSRBSN) ceramics were obtained by a hot-press sintering method. Precursor Si powders were prepared with Eu₂O₃–MgO–Y₂O₃ sintering additive. The addition of Eu₂O₃–MgO–Y₂O₃ was shown to promote full nitridation of the Si powder. The nitrided Si₃N₄ particles had an equiaxial morphology, without whisker formation, after the Si powders doped with Eu₂O₃–MgO–Y₂O₃ were nitrided at 1400 °C for 2 h. After hot pressing, the relative density, Vickers hardness, flexural strength, and fracture toughness of the PSRBSN ceramics, with 5 wt% Eu₂O₃ doping, were 98.3 ± 0.2%, 17.8 ± 0.8 GPa, 697.0 ± 67.0 MPa, and 7.3 ± 0.3 MPa m^1/2, respectively. The thermal conductivity was 73.6 ± 0.2 W m^?1 K^?1, significantly higher than the counterpart without Eu₂O₃ doping, or with ZrO₂ doping by conventional methods.     

Effect of in-situ formed Y2O3 by metal hydride reduction reaction on thermal conductivity of β-Si3N4 ceramics

The effect of YH₂ on densification, microstructure, and thermal conductivity of Si₃N₄ ceramics were investigated by adjusting the amount of YH₂ in the range of 0–4 wt% using a two-step sintering method. Native SiO₂ was eliminated, and Y₂O₃ was in situ formed by a metal hydride reduction reaction, resulting in various Y₂O₃/SiO₂ ratios. Full densification of YH₂-doped samples could be achieved after sintering at 1900 °C for 4 h. The Y₂O₃/SiO₂ ratio had a significant influence on the composition of crystalline secondary phases. Besides, the increased Y₂O₃/SiO₂ ratio is conducive not only to the grain growth but also to the reduction of activity of SiO₂ in the liquid phase, resulting in enlarged purified grains, reduced volume fraction of intergranular phases and increased Si₃N₄-Si₃N₄ contiguity. Ultimately, the thermal conductivity increased by 29 % from 95.3 to 123.0 W m⁻¹ K⁻¹ after sintering at 1900 ℃ for 12 h by the substitution of Y₂O₃ with YH₂.     

Fabrication of highly dense Si3N4 via record low-content additive system for low-temperature pressureless sintering

Dense Si₃N₄ ceramics were fabricated by pressureless sintering at a low temperature of 1650°C with a short holding period of 1 h under a nitrogen atmosphere. The role of ternary oxide additives (Y₂O₃–MgO–Al₂O₃, Y₂O₃–MgO–SiO₂, Y₂O₃–MgO–ZrO₂) on the phase, microstructure, and mechanical properties of Si₃N₄ was examined. Only 5 wt.% of Y₂O₃–MgO–Al₂O₃ additive was sufficient to achieve >98% of theoretical density with remarkably high biaxial strength (∼1200 MPa) and prominent hardness (∼15.5 GPa). Among the three additives used, Y₂O₃–MgO–Al₂O₃ displayed the finest grain diameter (0.54 μm), whereas Y₂O₃–MgO–ZrO₂ produced the largest average grain diameter (∼0.95 μm); the influence was seen on their mechanical properties. The low additive content Si₃N₄ system is expected to have superior high-temperature properties compared to the system with high additive content. This study shows a cost-effective fabrication of highly dense Si₃N₄ with excellent mechanical properties.     

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.     

Effect of post-sintering heat-treatment on thermal and mechanical properties of Si3N4 ceramics sintered with different additives

To improve the thermal conductivity of Si₃N₄ ceramics, elimination of grain-boundary glassy phase by post-sintering heat-treatment was examined. Si₃N₄ ceramics containing SiO₂–MgO–Y₂O₃-additives were sintered at 2123 K for 2 h under a nitrogen gas pressure of 1.0 MPa. After sintering, the SiO₂ and MgO could be eliminated from the ceramics by vaporization during post-sintering heat-treatment at 2223 K for 8 h under a nitrogen gas pressure of 1.0 MPa. Thermal conductivity of 3 mass% SiO₂, 3 mass% MgO and 1 mass% Y₂O₃-added Si₃N₄ ceramics increases from 44 to 89 Wm⁻¹ K⁻¹ by the decrease in glassy phase and lattice oxygen after the heat-treatment. Relatively higher fracture toughness (3.8 MPa m^1/2) and bending strength (675 MPa) with high hardness (19.2 GPa) after the heat-treatment were achieved in this specimen. Effects of heat-treatment on microstructure and chemical composition were also observed, and compared with those of Y₂O₃–SiO₂-added and Y₂O₃–Al₂O₃-added Si₃N₄ ceramics.     

Effect of YF3 on densification behavior and thermal conductivity of AlN ceramics with Y2O3–YF3 additives under reducing atmosphere

The effect of yttrium fluoride (YF₃) on the densification behavior, microstructure, phase composition and thermal conductivity of aluminium nitride (AlN) ceramics with yttrium oxide (Y₂O₃) and YF₃ additives were studied. Since YF₃ provided liquid phases and promoted densification at a lower temperature, the sintering temperature required to reach the full density of AlN samples decreased with the increase in YF₃ content. Appropriate addition of YF₃ could improve the thermal conductivity of AlN ceramics, but the values of thermal conductivity decreased as YF₃ increased further. It is attributed to the ability of YF₃ to react with oxygen impurity was worse than that of Y₂O₃. Moreover, the reducing atmosphere significantly affected the phase composition, and the oxygen content in grain boundary phases decreased at 1750 °C and 1800 °C. Therefore, the proper proportion of Y₂O₃–YF₃ additives could simultaneously improve densification and the thermal conductivity of AlN samples at a low sintering temperature.     

Mechanical properties and thermal conductivity of Si3N4 ceramics with YF3 and MgO as sintering additives

Si₃N₄ ceramic was densified by hot pressing sintering at 1750 °C for 1 h under the uniaxial pressure of 20 MPa in N₂ atmosphere with YF₃ and MgO as sintering additives. The thermal conductivities of SN-YF specimen were both higher than that of Si₃N₄ ceramic sintered with Y₂O₃ and MgO before and after annealing treatment. The grain size and aspect ratio in SN-YF specimen were both bigger than those in the SN-YO specimen, which was beneficial for the creation of high thermal conductive path. On the other hand, the improvement of thermal conductivity by the addition of YF₃ might be attributed to the reduction of the grain boundary phase due to the evaporation of SiF₄, and the resultant reduction of the lattice oxygen due to the reduction of SiO₂ in the grain boundary phase. The {0001} direction of grains had the probability of growing along the hot pressing direction in the SN-YF specimen, which was beneficial for the improvement of thermal conductivity while the {0001} direction grew along the X₀-Y₀ plane in the SN-YO specimen. The mechanical properties of SN-YF specimen were comparable to those of SN-YO specimen.     

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.     

Improved thermal conductivity of β-Si3N4 ceramics by lowering SiO2/Y2O3 ratio using YH2 as sintering additive

A two-step sintering process was conducted to produce β-Si₃N₄ ceramics with high thermal conductivity. During the first step, native SiO₂ was eliminated, and Y₂O₃ was in situ generated by a metal hydride reduction process, resulting in a high Y₂O₃/SiO₂ ratio. The substitution YH₂ for Y₂O₃ endow Si₃N₄ ceramics with an increase of 29% in thermal conductivity from 95.3 to 123 W m⁻¹ K⁻¹ after sintered at 1900°C for 12 hours despite an inferior sinterability. This was primarily attributed to the purified enlarged grains, devitrified grain boundary phase, and reduced lattice oxygen content in the YH₂-MgO-doped material.     

Microstructure and thermal conductivity of gas-pressure-sintered Si3N4 ceramic: the effects of Y2O3 additive content

Si₃N₄ ceramics were sintered at 1900 °C under a nitrogen pressure of 1 MPa using Y₂O₃-MgO additives. The effects of Y₂O₃ content (0.5-4 mol%) on microstructure and thermal conductivity were systematically investigated. The increasing Y₂O₃ content led to increases in amount and viscosity of liquid phase during sintering, which induced a “bimodal to normal” transition in distribution of grain size, decreased Si₃N₄/Si₃N₄ contiguity and enhanced devitrification degree of intergranular phase in sintered bulks. Moreover, the decreasing Y₂O₃ content was found to improve the elimination efficiency of SiO₂ impurity during sintering, resulting in lower lattice oxygen content in densified specimens. The microstructure had a strong effect on thermal conductivity. The samples sintered for 3 h gained an increase of thermal conductivity from 65 to 73 W·m^-1 K^-1 with increasing Y₂O₃ content, while the samples sintered for 12 h obtained a substantial increase of thermal conductivity from 87 to 132 W·m^-1 K^-1 with decreasing Y₂O₃ content.     

Effects of yttria and magnesia on densification and thermal conductivity of sintered reaction-bonded silicon nitrides

A variety of combinations of Y₂O₃ and MgO were used as additives in preparing Si₃N₄ ceramics by the sintering of reaction-bonded silicon nitride (SRBSN) method. By varying the amount of Y₂O₃ in the range of 0-5 mol% and that of MgO in the range of 0-8 mol%, the effects of Y₂O₃ and MgO additives on nitridation and sintering behaviors as well as thermal conductivity were studied. It was found that appropriate amount and combination of Y₂O₃ and MgO additives were essential for attaining full densification and achieving high thermal conductivity. The sample doped with 2.5 mol% of Y₂O₃ and 5 mol% of MgO attained a thermal conductivity of 128 Wm⁻¹K⁻¹ when sintered at 1900°C for 6 hours, and the sample doped with 2 mol% of Y₂O₃ and 4 mol% of MgO achieved a thermal conductivity of 156 Wm⁻¹K⁻¹ when sintered for 24 hours.     

Influence of Sintering Aid on the Translucency of Spark Plasma‐Sintered Silicon Nitride Ceramics

Single additives (Y₂O₃, MgO, and Al₂O₃) were used as sintering aid of Si₃N₄. The density, crystal phase, microstructure, transmittance, hardness, and fracture toughness of sintered Si₃N₄ were investigated. Highly densified sintered bodies were obtained in single Y₂O₃ and MgO systems, but not in single Al₂O₃ system. The XRD results indicated that sintered bodies were composed of β‐Si₃N₄. The SEM images showed that all the sintered bodies had a fine‐grained microstructure with an average diameter of 0.29–0.37 μm. The thickness of grain boundary was changed with additive content. The transmittance (T) and the wavelength (λ) followed the relationship of T∝ exp(?λ^?2.3) due to the light scattering. The transmittance was mainly influenced by the refractive index of additives and the thickness of grain boundary phase. The hardness and fracture toughness of sintered ceramics were 12.6–15.5 GPa and 6.2–7.2 MPam^1/2, respectively.     

Texture,microstructures, and mechanical properties of AlN‐based ceramics with Si3N4–Y2O3 additives

Textured AlN‐based ceramics with improved mechanical properties were prepared by hot pressing using Si₃N₄ and Y₂O₃ as additives. The introduction of Si₃N₄–Y₂O₃ into AlN matrix led to the formation of secondary Y₃AlSi₂O₇N₂ and fiber‐like 2H^δ AlN‐polytypoid phases, the partial texture of all crystalline phases, and the fracture mode change from intergranular to transgranular. Consequently, Vickers hardness, fracture toughness and flexural strength of AlN‐based ceramics by the replacement of Y₂O₃ by Si₃N₄–Y₂O₃ increased significantly from 10.4±0.3 GPa, 2.4±0.3 MPa m^½ and 333.3±10.3 MPa to 14.2±0.4 GPa, 3.4±0.1 MPa m^½ and 389.5±45.5 MPa, respectively.     

Silicothermal synthesis and densification of x-sialon in the presence of metal oxide additives

The effect is reported of seven inorganic oxide additives on both the formation mechanism and the densification of X-sialon prepared by a silicothermal process. The oxides were added to the starting mixture of halloysite clay, alumina and elemental silicon at a level of 1 wt% of the calculated final product, and fired in nitrogen at 1200–1500°C. The formation of X-sialon was monitored by thermal analysis, powder XRD and ²⁷Al and ²⁹Si solid state MAS NMR. The effects of the additives are temperature dependent, and influence the various stages of the reaction by differing degrees. The oxides which best promote the formation of crystalline X-sialon (Y₂O₃, CaO and MgO) are also those which facilitate the conversion of initially-formed Si₃N₄ to SiO₂N₂ and SiO₃N units, the latter being particularly enhanced by Y₂O₃, Fe₂O₃ enhances the initial nitridation of Si but suppresses X-sialon formation by stabilising the preceding mullite phase. Densification is most enhanced by Y₂O₃, CaO and CeO₂; MgO exerts its maximum effect on sintering at lower temperatures. The beneficial influence of MgO and Y₂O₃ on both X-sialon formation and sintering is due to the formation of liquid phases.     

Enhanced thermal conductivity in Si3N4 ceramic with the addition of Y2Si4N6C

Si₃N₄ ceramic was densified at 1900°C for 12 hours under 1 MPa nitrogen pressure, using MgO and self‐synthesized Y₂Si₄N₆C as sintering aids. The microstructures and thermal conductivity of as‐sintered bulk were systematically investigated, in comparison to the counterpart doped with Y₂O₃‐MgO additives. Y₂Si₄N₆C addition induced a higher nitrogen/oxygen atomic ratio in the secondary phase by introducing nitrogen and promoting the elimination of SiO₂, resulting in enlarged grains, reduced lattice oxygen content, increased Si₃N₄‐Si₃N₄ contiguity and more crystallized intergranular phase in the densified Si₃N₄ specimen. Consequently, the substitution of Y₂O₃ by Y₂Si₄N₆C led to a great increase in ~30.4% in thermal conductivity from 92 to 120 W m^?1 K^?1 for Si₃N₄ ceramic.