Application of polypropylene carbonate for the tape casting of silicon nitride ceramics

QPAC40 (polypropylene carbonate), with a little decomposition residue, is commonly used as a binder in aluminum nitride (AlN) tape casting. In this paper, we tried to explore its application in silicon nitride (Si₃N₄) tape casting. By studying the influence of dispersant, binder, plasticizer/binder ratio, and solid loading on slurry and green tape properties, the optimum formulation of the tape casting of Si₃N₄ slurry was determined, and the green tape with a uniform structure and relative density up to 63.16% was prepared. Si₃N₄ ceramics were obtained by debinding at 600°C for 1 h in vacuum and gas-pressure sintering at 1830°C for 2 h in N₂. The thermal conductivity and flexural strength of Si₃N₄ ceramics were 56.28 ± 1.21 W/(m·K) and 1130.67 ± 23.58 MPa, respectively. These results indicated that QPAC40 can be used to prepare Si₃N₄ sheets through tape casting.     

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

In this paper, high thermal conductivity Si₃N₄ ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si₃N₄ slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si₃N₄ ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm⁻³ were prepared by the investigation and optimization of the Si₃N₄ slurry formulation. After gas pressure sintering, Si₃N₄ ceramics with a density of 3.23 g cm⁻³, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si₃N₄ ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si₃N₄ ceramics was 100.5 W m⁻¹ K⁻¹, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m^1/2.     

Production of Si3N4/Glass Composites for LTCC Substrate by Aqueous Tape Casting Process

In this work, Si₃N₄/glass sheets were prepared by aqueous tape casting, and the preparation process and properties of green and sintered tapes were discussed thoroughly. The effect of Si₃N₄ content on the rheological behavior of the slurries was investigated, and the results showed that the viscosity of the slurry increased with the solid loading and all the slurries exhibited shear‐thinning behavior within the entire range of shear rate. Green tapes were fabricated successfully by aqueous tape casting which have excellent properties such as uniform pore distribution, low porosity, and high bulk density. The sintered samples exhibited low shrinkage, and low dielectric constant and loss.     

Sintered silicon nitride/nano-silicon carbide materials based on preceramic polymers and ceramic powder

A flexible method is presented, which enables the fabrication of porous as well as dense Si₃N₄/nano-SiC components by using Si₃N₄ powder and a preceramic polymer (polycarbosilazane) as alternative ceramic forming binder. The SiCN polymer benefits consolidation as well as shaping of the green body and partially fills the interstices between the Si₃N₄ particles. Cross-linking of the precursor at 300 °C increases the mechanical stability of the green bodies and facilitates near net shape machining. At first, pyrolysis leads to porous ceramic bodies. Finally, subsequent gas pressure sintering results in dense Si₃N₄/nano-SiC ceramics. Due to the high ceramic yield of the polycarbosilazane binder, the shrinkage during sintering is significantly reduced from 20 to 15 lin.%. Investigations of the sintered ceramics reveal, that the microstructure of the Si₃N₄ ceramic contains approx. 6 vol.% nano-scaled SiC segregations, which are located both at the grain boundaries and as inclusions in the Si₃N₄ grains.     

Tape casting and characterizations of MgO ceramics

We report a high density MgO ceramic substrate produced by the tape casting technology. The tape casting formulation and process produced a uniform tape free of cracking. Y₂O₃ and SiO₂ were used as the sintering aid for the pressureless sintering of the green tape. X-ray diffraction phase identification indicates that MgO is the main phase, while both Y₂O₃ and SiO₂ sintering aids react with MgO to form MgY₄Si₃O₁₃ as the second phase. Liquid phase sintering occurs in the temperature range from 1030°C to ~1500°C, which is confirmed by the simultaneous Thermal Gravitation Analysis/Differential Scanning Calorimeter (TGA/DSC) and the percent linear shrinkage and densification. A 96.5% theoretical density was achieved by presureless sintering at 1650°C for 2 hours, which was further increased to a fully dense structure using hot-isostatic-pressing(HIP) at 1650°C and 207 MPa in argon. Scanning electron microscopy (SEM) and energy dispersive(EDS) spectroscopic analysis on the HIP’ed sample show that MgY₄Si₃O₁₃is located at the MgO grain boundary and the sample has a fully dense structure. The refractive indices and extinction coefficient were measured on the HIP’ed sample along with thermal properties and dielectric properties. Thermal diffusivity and heat capacity were measured to calculate the thermal conductivity.     

Tape casting of non-aqueous silicon nitride slips

Tape casting is a powerful method for the manufacturing of flat, large area ceramic components. Silicon nitride is a reference material for high temperature structural applications. Between them, thick film/coating technologies and ceramic–ceramic joining are receiving an increased attention. In this work, the rheological behavior on non-aqueous silicon nitride slips for tape casting was investigated considering different solvent and binding systems, by controlling the total binder and plasticizer content and the binder to plasticizer ratio. A phosphate ester was used as dispersant. The characteristics of the green tapes obtained in different conditions were studied in terms of density, thickness and microstructure and related with the rheological properties of the slips. Once the slip properties were adjusted, manufacturing parameters, such as the casting speed and the gap between the blades and the carrier substrate, were also analyzed in order to improve the green tape properties and the process reliability. In order to obtain sinterable compacts, Al₂O₃ and Y₂O₃ were used as sintering aids. The effect of the sintering aids in both the rheological behavior and the green characteristics was also studied.     

A strategy for fabricating textured silicon nitride with enhanced thermal conductivity

C-axis textured Si₃N₄ with a high thermal conductivity of 176 W m⁻¹ K⁻¹ along the grain alignment direction was fabricated by slip casting raw α-Si₃N₄ powder seeded with near-equiaxed β-Si₃N₄ particles and Y₂O₃–MgSiN₂ as sintering additives in a rotating strong magnetic field of 12 T, followed by gas pressure sintering at 1900 °C for 12 h at a nitrogen pressure of 1 MPa. The green material reached a relative density of 57%, with slip casting and the sintered material exhibited a relative density of 99% and a Lotgering orientation factor of 0.98. The morphology of the β-Si₃N₄ seeds had little effect on the texture development and thermal anisotropy of textured Si₃N₄. The technique developed provides highly conductive Si₃N₄ with conductivity to the thickness direction, which is a major advantage in practical use. The technique is also simple, inexpensive and effective for producing textured Si₃N₄ with high thermal conductivity of over 170 W m⁻¹ K⁻¹.     

Tuning the dielectric properties of porous BN/Si3N4 composites by controlling porosity

To solve the shortcomings of traditional layered structure for broadband wave-transmitting applications, a novel porosity gradient structure is proposed. The precise turning of the relative permittivity of each layer and their simultaneous shrinkage during sintering are the keys to design and fabrication. Porous BN/Si₃N₄ composites with different porosities were prepared by incorporation of pore former, tape casting and gas pressure sintering. Results show that the obtained composites with different porosities exhibit low and similar shrinkage after sintering at 1900 °C, which is mainly due to the bridging effect between rapidly growing β-Si₃N₄ rod. When the porosity increases from 33.2% to 59.4%, the relative permittivity decreases from 4.00 to 2.38. Moreover, the composites with different porosities have excellent mechanical properties, with bending strength and fracture toughness of 180 ± 10–36 ± 4 MPa and 3.2 ± 0.30–0.85 ± 0.10 MPa·m^?1/2, respectively, which is attributed to the bridging and pull-out strengthening effects of the elongated β-Si₃N₄ grains.     

Effect of additives on slip casting rheology,microstructure and mechanical properties of Si3N4/SiC composites

The SiC/Si₃N₄ composites were fabricated with sintering process. To produce SiC/Si₃N₄ composite components, slurry mixtures containing Si/SiC powders were used by the slip casting method. In order to investigate the effect of dispersants and additives on the rheological properties and the body casted, slurries with concentration of 70% solid weight were prepared. It included a mixture of silicon and silicon carbide with weight ratios of 30 wt% and 70 wt%, respectively, and various weight percentages of Ball clay as lubricant and Tiron (sodium salt of benzene disulfonic acid) as dispersant at pH value of 7. After preparing the green bodies by slip casting method by using plaster mold, the samples were sintered at 1450 °C inside an atmospheric-controlled furnace under a pressure of 0.12 MPa of nitrogen gas for 2 h. By examining the rheological properties of the slurry and the sintering properties, it was concluded that the best slurry was obtained in terms of viscosity, density, porosity and strength using 5 wt% Ball clay and 0.5 wt% Tiron. Phase transformations, microstructure and morphology of the sintered specimens were accomplished by Field Emission Scanning Electron Microscopy (FESEM) examination and X-ray diffraction experimental analysis. XRD and FESEM results demonstrated that the composite fabricated by slurry containing 5 wt% Ball clay and 0.5 wt% Tiron had the least porosity without SiO₂ phase.     

Low temperature sintering of Si3N4 ceramics by spark plasma sintering technique

Abstract

Abstract

Low temperature sintering of α‐Si₃N₄ matrix ceramics was developed in the present study using 4?wt‐%MgO together with Al₂O₃ or AlPO₄ as the sintering additives and spark plasma sintering technique. The results suggested that α‐Si₃N₄ ceramics could be densified at low sintering temperature by adjusting both the sintering temperature and sintering additive content. For low temperature sintered α‐Si₃N₄ ceramics, using MgO and Al₂O₃ as the sintering additives, the densification is not complete at a temperature lower than 1600°C, and the mechanical strength is <200?MPa. When MgO and AlPO₄ were used as the sintering additives, the increase in AlPO₄ content not only declines the sintering temperature but also promotes the mechanical property of the sintered Si₃N₄ ceramics. It was the AlPO₄ phosphate binder that played a significant role in low temperature sintering of Si₃N₄ ceramics.     

Effects of AlN inorganic binder on the properties of porous Si3N4 ceramics prepared by selective laser sintering

Porous Si₃N₄ ceramics are widely used in the aerospace field due to its lightweight, high-strength, and high wave transmission. Traditional manufacturing methods are difficult to fabricate complex structural and functional ceramic parts. In this paper, selective laser sintering (SLS) technology was applied to prepare porous Si₃N₄ ceramics using AlN as an inorganic binder. And the effects of AlN content on the properties of the obtained ceramic samples were explored. As the AlN content increased, nano-Al₂O₃ and nano-SiO₂ formed the eutectic liquid phase, enhancing the sintering densification and phase transformation of Si₃N₄ poly-hollow microspheres (PHMs). The island-like partial densification structures in Si₃N₄ green bodies increased. During the high-temperature sintering, the eutectic liquid phase partially transformed into the mullite phase or reacted with AlN and Si₃N₄ to form the Sialon phase. With the increase of AlN content, the fracture mode of Si₃N₄ ceramics changed from fracturing along PHMs to fracturing across PHMs. The bonding depth between PHMs increased and the connection between the grains was tighter, so the Si₃N₄ ceramics became denser. With the increase of AlN addition, the total porosity of the porous Si₃N₄ ceramics tended to decrease and the flexural strength gradually increased. When AlN content was 20 wt%, the total porosity and the flexural strength were 33.6% and 23.9 MPa, respectively. The addition of AlN inorganic binder was carried out to develop a novel way to prepare high-performance porous Si₃N₄ ceramics by SLS.     

Diamond coating of coloured Si3N4 ceramics

The effect of Si₃N₄ secondary phases on chemical vapour deposition (CVD) diamond film growth was analyzed. Silicon nitride substrates were obtained by pressureless sintering, placing the green samples inside a powder bed of Si₃N₄/BN. Local variations in the sintering atmosphere led to samples with different grey colouration as well as chemical and physical characteristics, determined by X-ray diffraction and thermal conductivity tests, which affected the diamond film growth. A complete characterization of the films, including thickness, average crystal size, surface roughness, texture and adhesion, was done. The Si₃N₄ substrate with glassier phase gave thicker diamond films, with smaller crystal sizes and better film adhesion to the substrate than the diamond films grown on ceramic substrates with less vitreous phase.     

Preparation of Si3N4 ceramics with high strength and high reliability via a processing strategy

In this study, the preparation of Si₃N₄ ceramics with high mechanical reliability is investigated. The influences of several processing steps on the bending strength and the Weibull modulus are reported including: (i) coating of the Si₃N₄ powder with its sintering aids, (ii) oxidation of the coated powder, (iii) cold isostatic pressing, (iv) gelcasting of the green bodies and (v) gas pressure sintering. It was found that all the aforementioned steps contribute to improvements of strength and reliability of Si₃N₄ ceramics. Via an optimised processing strategy, Si₃N₄ ceramics with a bending strength and a Weibull modulus as high as 944.7±29.5 MPa and 33.9, respectively, could be prepared. Additionally, it was also found that surface modifications, i.e. coating and oxidation of Si₃N₄ powder, increased the rheological properties of the powder suspension in aqueous media, which is favourable for in situ colloidal forming such as gelcasting.     

Pressureless sinterability of slip cast silicon nitride bodies prepared from coprecipitation-coated powders

The sinterability of compositions from different powder preparation methods (coprecipitation-coating of Si₃N₄ powder or mechanical mixing of Si₃N₄ with Y₂O₃ and Al₂O₃) and compaction routes (dry pressing or slip casting) was compared. Both the coating method and the slip casting process improved silicon nitride sinterability over the mechanical mixing method and dry pressing route. However, the minimisation of powder agglomeration in the green bodies achieved by slip casting is more determinant to the sintering behaviour than the homogeneous distribution of the sintering additives around the Si₃N₄ offered by the coated powder. The coating powder method in combination with the slip casting process is the most favourable processing route, leading to a homogeneous and fully dense microstructure by pressureless sintering at a relatively low temperature of 1750°C. This technique produced materials with hardness of 15·2 GPa, fracture toughness of 7 MPa  m^1/2 and flexural bending strength of 910 MPa.     

Fused deposition modeling of Si3N4 ceramics: A cost-effective 3D-printing route for dense and high performance non-oxide ceramic materials

Dense Si₃N₄ ceramics were prepared by fused deposition molding method accompanied by gas pressure sintering. In this study, the surface steps, inter layer bonding and microstructure evolution were characterized and dense Si₃N₄ ceramics without obvious defects were obtained. It was verified that layer thickness and nozzle diameter have little impact on the density and flexural strength of both green and sintered parts. As to the filling strategy, contour offset path was more effective to obtain sintered part with higher flexural strength than parallel lines and grid path, which was due to the possible voids appeared at the intersection of print paths with different directions. The highest flexure strength 824.74 ± 85 MPa was obtained with layer thickness 0.15 mm, nozzle size 0.6 mm and contour offset path. The reliability of the obtained Si₃N₄ ceramics was also investigated and complex shaped Si₃N₄ ceramic parts with good shape keeping was prepared successfully.     

Preparation,mechanical and thermal properties of Si3N4 ceramics by gelcasting using low–toxic DMAA gelling system and gas pressure sintering

In this work, Si₃N₄ ceramics were fabricated through an aqueous gelcasting method using a low–toxic monomer called N, N–dimethylacrylamide (DMAA) followed by gas pressure sintering at 1850 °C for 2 h under 6 MPa N₂ atmosphere. The effect of solid loading on performance of slurries, green and sintered bodies was investigated systematically. The results show that the slurries with a solid loading as high as 50 vol% (viscosity 0.17 Pa.s at 100 s^–1) were achieved. With the increase of solid loading (30–50 vol%), the green bodies exhibited a monotonically decreased, however high enough in general, flexural strength of 16.50–11.52 MPa, which was comparable to that of widely–used neurovirulent acrylamide (AM) gelling system. In regard to the sintered bodies, increasing solid loading significantly promoted sintering and improved mechanical properties and thermal conductivity as a result of the increased density, bimodal distribution structure, as well as suitable interfacial bonding strength. The best performance parameters of Si₃N₄ ceramics, bulk density of 3.25 g/cm³, apparent porosity of 0.67%, flexural strength of 898.92 MPa, fracture toughness of 6.42 MPa m^1/2, Vickers hardness of 2.81 GPa, and thermal conductivity of 34.69 W m^–1 K^–1, were obtained at 50 vol% solid loading. This work renders low–toxic DMAA gelling system promising prospect in preparation of high–performance Si₃N₄ ceramics by gelcasting.     

A Novel Hot Pressing Flowing Sintering for Preparation of Texturing Ceramics

In this paper, we proposed a novel hot‐pressing flowing sintering (HPFS) for texturing ceramics. The perfectly two‐dimensional textured Si₃N₄ ceramics were fabricated by HPFS. The Lotgering orientation factor f_L for Si₃N₄ texture was 0.9975. During earlier sintering stage, the specimen flowed along the plane which perpendicular to the hot‐pressing direction under pressure, through the controlling of the graphite die movement. The rod‐like β‐Si₃N₄ nuclei was easily to texture during the flowing process, due to the small size of the β‐Si₃N₄ nuclei and the high porosity of the flowing specimen. As a result, the perfectly two‐dimensional textured Si₃N₄ was obtained. After aligned, the β‐Si₃N₄ grains grew along the materials flowing direction with little constraint, which accelerated the growth of the grains extensively. So, in addition to texture, the Si₃N₄ ceramics by HPFS also showed high aspect ratio. The present study indicated that the HPFS would be more simple and low‐cost method for texture of Si₃N₄, compared with conventional hot‐forging which contained the sintering and forging.     

High-strength and wave-transmitting Si3N4–Si2N2O-BN composites prepared using selective laser sintering

In this study, high-strength and wave-transmission silicon nitride (Si₃N₄) composites were successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding and before final sintering, and the optimal moulding process parameters for the SLS Si₃N₄ ceramics were determined. The effects of the sintering aids and secondary CIP on the bulk density, porosity, flexural strength, fracture toughness, and wave-transmitting properties of the Si₃N₄ composites were studied. The results showed that the increased CIP pressure was beneficial to the densification of SLS Si₃N₄ ceramics and improved their mechanical properties. However, the wave-transmitting performance decreased as the CIP pressure increased. The Si₃N₄ ceramics prepared by the moulding of sample S₁₁ were more in line with the performance requirements of the radomes. To obtain good comprehensive performance, an additional 3% of interparticle Y₂O₃ was added to the pre-printed mixed powder of granulated Si₃N₄ particles and resin and the secondary CIP pressure was adjusted to 280 MPa. After sintering, the bending strength, fracture toughness, and dielectric constant of the Si₃N₄ ceramics were 651 MPa, 6.0 MPa m^1/2, and 3.48 respectively. This study provides an important method for preparing of Si₃N₄ composite radomes using SLS process.     

The effect of BaTiO3 addition on the dielectric constant of Si3N4 ceramics

Si₃N₄ ceramics with different BaTiO₃ contents have been fabricated by pressureless sintering in a N₂ atmosphere at 1680°C for 2 h. Al₂O₃ and Nd₂O₃ were used as sintering additives to promote the densification of Si₃N₄ ceramics. The effect of BaTiO₃ addition on the densification, mechanical properties, phase compositions, microstructure, and dielectric properties of Si₃N₄ ceramics was investigated. The relative density and flexural strength of Si₃N₄ ceramics increased with the addition of BaTiO₃ up to 15 wt% and then decreased, while the dielectric constant increased continuously as the BaTiO₃ contents increased. The dielectric constant of Si₃N₄ ceramics can be tailored in the range from 8.42 to 12.96 by the addition of 5 wt%‐20 wt% BaTiO₃. Meanwhile, these Si₃N₄ ceramics all had flexural strength higher than 500 MPa.     

Effect of the molecular weight and concentration of PEG solution on microstructure and mechanical properties of Si3N4 ceramics fabricated by gel-casting

Gel-casting is a promising preparation technology of Si₃N₄ structural ceramics. The process involves drying of the “green” gel-cast parts before densification. And the drying of green gel-cast bodies is an important step in the gel-casting manufacturing process. In this work, the Si₃N₄ gel-cast green bodies were dried in polyethylene glycol (PEG) solution with the purpose of obtaining Si₃N₄ ceramics with good mechanical properties. The effect of the molecular weight and concentration of PEG solution on drying rate, microstructure and mechanical properties of Si₃N₄ ceramics was studied. The results indicated that with the increase of molecular weight of PEG, the drying rate increased obviously and the structure became more uniform and dense when the concentration of solution was 20?wt%. The Si₃N₄ ceramics after sintering have the excellent flexural strength (662.6?MPa) under PEG600 drying condition. Furthermore, the concentration of PEG600 solution had a positive effect on drying and sintering of the green body. Therefore, the bending strength reached 871.1?MPa under 65?wt% PEG 600 solution drying condition. Overall, the drying process (drying in 65?wt% PEG600 solution) promotes the efficiency and quality of drying of Si₃N₄ gel-cast green bodies, which is beneficial for the subsequent drying and sintering process.