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.     

Densification and properties of ZrO2 nanoparticles added magnesia–doloma refractories

In this work the effect of nano- and microZrO₂ addition on the densification and hydration resistance of MgO–CaO refractories was investigated. 0, 2, 4, 6 and 8 wt% ZrO₂ was added to MgO–CaO refractories that contain 35 wt% CaO. The crystalline phases and microstructure characteristics of specimens sintered at 1650 °C for 5 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. Results show that with addition of ZrO₂ the bulk density and hydration resistance of the samples increased while apparent porosity decreased. Also the hydration resistance of the samples was appreciably improved by the addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories, promotion of densification as well as modification of the microstructure. Also it revealed that the nanoZrO₂ addition was more effective than microZrO₂ due to its higher activity.     

Preparation of ZTA ceramic by aqueous gelcasting with a low-toxic monomer DMAA

The present work reports the development of aqueous gelcasting of ZTA ceramic with a low-toxicity monomer DMAA. The rheological properties and the gelation behaviors of the slurries for gelcasting were investigated. It was proved that the time available for casting the slurry (idle time) can be controlled by the amounts of initiator. The ZTA green bodies exhibited a mechanical strength as high as 21 MPa. After sintered at 1600 °C for 2 h, the highest bending strength and fracture toughness of the sintered ZTA samples were as high as 643.3 ± 75 MPa and 6.3 ± 0.3 MPa m^1/2, respectively. SEM photographs revealed that the green bodies and sintered part had a uniform microstructure. The volume fraction of tetragonal phase zirconia was as high as 90%. Dense ZTA green bodies and ceramic parts with complex shaped were produced through the new gelcasting system.     

Densification and Properties of Fe2O3 Nanoparticles added CaO Refractories

Up to 8 wt. % of Nano-iron oxide was added to CaO refractory matrix. The crystalline phases and microstructure characteristics of specimens sintered at 1650 °C for 5 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. The mechanical behavior was studied by a cold crushing strength (CCS) and flexural strength at 1200 °C test. As a result, it was found that the presence of Nano-iron oxide in the CaO refractory matrix induced 2CaO.Fe₂O₃ (C₂F), CaO.Fe₂O₃ (CF) and 3CaO.Al₂O₃ (C₃A) phase’s formation, which improved the sintering process. Nano-iron oxide also influenced the bonding structure through a direct bonding enhancement. On the Other hand, the presence of Nano-iron oxide resulting in improvement properties of CaO refractory matrix refractories such as bulk density, hydration resistance and cold crushing strength. The maximum flexural strength at 1200 °C is achieved by the samples containing 4 wt. % nano-Fe₂O₃.     

Rapid and uniform in-situ solidification of alumina suspension via a non-contamination DCC-HVCI method using MgO sintering additive as coagulating agent

A novel rapid, uniform and non-contamination in-situ solidification method for alumina suspension by DCC-HVCI method using MgO sintering additive as coagulating agent was reported. MgO was used to release Mg²⁺ in suspensions via reaction with acetic acid generated from glycerol diacetate (GDA) at elevated temperature as well as to improve density and suppress grain growth of alumina ceramics during sintering. Influence of adding 0.7 wt% MgO with 2.0 vol% GDA in alumina suspension on coagulation process and properties of green bodies and sintered samples were investigated. It was indicated that the controlled coagulation of the suspension could be achieved after treating at 70 °C for 10 min. Homogeneous composition distribution of Mg element in EDS result indicated the uniform solidification of suspensions. Compressive strength of wet-coagulated bodies is 2.09±0.25 MPa. Dense alumina ceramics with relative density of 99.2% and flexural strength of 354±16 MPa sintered at 1650 °C for 4 h present homogeneous microstructure. The result indicated that the novel DCC-HVCI method via a sintering additive reaction with no contamination, short coagulation time and uniform in-situ solidification is a promising colloidal forming method for preparing high-performance ceramic components with complex shape.     

Effect of MgAl2O4 nanoparticles addition on the densification and properties of MgO-CaO refractories

MgAl₂O₄nanoparticles were added to MgO–CaO refractory ceramic composites in the range of 0–8 wt%. Refractory specimens were obtained by sintering at 1650 °C for 3 h in an electric furnace. Refractory specimens were characterized by measurements of bulk density, apparent porosity, hydration resistance, cold crushing strength, crystalline phase formation, and microstructural analysis. Results show that with additions of MgAl₂O₄ nanoparticles the bulk density of the samples increased. But the apparent porosity and cold crushing strength decreased and increased, respectively with addition MgAl₂O₄ nanoparticles up to 6 wt% and for further MgAl₂O₄ nanoparticles, due to the thermal expansion mismatch, the results is reversed. Also, the hydration resistance of the samples was appreciably improved by the addition of MgAl₂O₄ nanoparticles due to its effect on decreasing the amount of free CaO in the refractory composite and promotion of densification by creating a dense microstructure.     

3D multiscale controlled micropatterning of lead-free piezoelectric electroceramics via Epoxy Gel Casting and lift-off

We report on the fabrication of multiscale three-dimensional (3D) micropatterning of 0.5Ba(Zr_0.2Ti_0.8)O₃ − 0.5(Ba_0.7Ca_0.3)TiO₃ (BZT − BCT) electroceramic material using soft lithographic PDMS moulds. The ceramic microcomponents were successfully consolidated by Epoxy Gel Casting from stable aqueous suspension of BZT-BCT material. A successful aqueous processing of electroceramic material was employed with surface protection of powder particles against hydrolysis as considerably an important parameter to achieve high solid loading suspension with pseudoplastic behaviour suitable for casting in micro scale moulds. The process to obtain micro sized pattern of material was divided in two steps: (i) production of high quality SU-8 master moulds and the respective negative replicas in PDMS (soft mould) by photolithography and soft mould replication, respectively; (ii) casting the soft moulds by stable high solid loading BZT-BCT suspensions in presence of gelation agents followed by de-moulding, drying and sintering. The resulting green micropattern structures show interesting features, including complex structures with a periodic variations through their length and, multiscale array of hexagonal shaped pillars of different aspect ratios (AR) (max AR ≈ 8 with approx ∼320 μm of height and side length ∼40 μm) with smooth side wall features along height. The green pillar arrays were further sintered to show the suitability of aqueous processing of material and proposed casting method.     

Binder jetting yttria stabilised zirconia ceramic with inorganic colloid as a binder

ABSTRACT

The new inorganic colloid is a particle-free decomposable binder that can solve the nozzle clogging problem and precipitate zirconia particle upon heating. The inorganic colloidal binder can be used as a binder precursor to replace commonly used PVP binder in printing ZrO₂ ceramics parts using binder-jet technology. Green bodies were printed using inorganic colloid binders with different saturation level and PVP binder separately, then cured and sintered. The effects of binder saturation level and sintered temperature on the properties of the green and sintered bodies were investigated. After being deposited into the powder bed interstices and cured, the inorganic colloidal binder containing zirconium basic carbonate decomposes and forms ZrO₂ ceramic particles that are filled the interstices and sintered to provide bonding strength to the printed parts. Samples with inorganic colloidal binder eventually perform better surface quality and denser sintered body than polymer binder when using same saturation ratio.     

New laser machining technology of Al2O3 ceramic with complex shape

A new method was developed for the fabrication of complex-shaped Al₂O₃ ceramic parts by combining laser machining and gelcasting technique. The unwanted ceramic powders parts were selectively removed by laser machining specified by a computer program, and the gelcast Al₂O₃ green bodies were machined to a designed shape by a CO₂ laser at a wavelength of 10.6 μm. The influences of solid loading, laser output power, scanning speed and nitrogen purge on the machining of green Al₂O₃ ceramic bodies were studied. The experimental parameters were optimized, the green Al₂O₃ bodies with solid loading of 40 vol% or below were easier to be machined, while the green bodies with solid loading of 45 vol% or above were hard to be further machined due to the surface sintering. Better machining quality and deeper machining depth could be obtained when the laser power is 30 W. The green Al₂O₃ bodies with complex shape were obtained by the laser machining.     

Porous co-continuous mullite structures obtained from sintered aluminum hydroxide and synthetic amorphous silica

Porous materials produced from sintered Al(OH)₃ show a potentially useful α-Al₂O₃-based coral-like co-continuous microstructure of high porosity (above 70%) and chemical resistance. However, due to the lack of efficient connections among the particles of the solid phase, their poor mechanical properties limit their use in biomechanical and thermo-mechanical applications, as scaffolds for bone tissue and hot air filters, respectively. In this study, authors improved these connections reinforcing the structure with a sintering aid (synthetic amorphous silica, SAS). Al(OH)₃ particles (previously sintered at 1500 °C, 5 h) were imbibed with SAS particles, compacted and sintered at 1300 °C, which generated a coral-like mullite-based porous structure. The porosity levels of the material (47%) were similar to those of the initial green state (50%) and achieved high levels of mechanical properties (flexural strength of 50.29 MPa, elastic modulus of 26.00 GPa), with small linear thermal shrinkage (lower than 6% at 1500 °C).     

Influence of the structure of MgO·nAl2O3 spinel lattices on transparent ceramics processing and properties

It is demonstrated that a complete elimination of pores on sintering is governed not only by the size of the ceramic powder particles and by the homogeneity of their mutual coordination but similarly strongly by the state of the crystal lattice: with different cation disorder at fixed stoichiometry (n = 1) the sintering temperatures may differ by as much as 200 °C at constant powder particle size and equal homogeneity of the green bodies. Additionally, the impact of stoichiometry was investigated over the range between n = 1 and n = 3 with retarded reactive sintering at moderately increased Al₂O₃ concentrations but promoted densification of alumina-rich compositions. Taking advantage of the observed effects, sintered spinel ceramics were derived by reactive sintering of undoped MgO/Al₂O₃ mixtures resulting in an in-line transmittance which equals spinel single crystals of similar composition from 200 nm wave length up to the IR range.     

Additive manufacturing of zirconia parts by indirect selective laser sintering

Thermally induced phase separation (TIPS) was used to produce spherical polypropylene–zirconia composite powder for selective laser sintering (SLS). The influence of the composition of the composite starting powder and the SLS parameters on the density and strength of the composite SLS parts was investigated, allowing realizing SLS parts with a relative density of 36%. Pressure infiltration (PI) and warm isostatic pressing (WIPing) were applied to increase the green density of the ZrO₂–PP SLSed parts. Infiltrating the SLS parts with an aqueous 30 vol.% ZrO₂ suspension allowed to increase the sintered density from 32 to 54%. WIPing (135 °C and 64 MPa) of the SLS and SLS/infiltrated complex shape green polymer–ceramic composite parts prior to debinding and sintering allowed raising the sintered density of the 3 mol Y₂O₃ stabilized ZrO₂ parts to 92 and 85%, respectively.     

Improved Compaction in Multilayer Capacitor Fabrication

The most common defects found in multi-layer ceramic capacitors are derived from residual porosity formed when solvents and binders are released from a ceramic green body. Without a well-controlled compaction technique, defects between sheets in the stacked body are often present, leading to lamination problems. This paper suggests alternative approaches, using compaction applied before and/or after the process of binder burnout with the intention of reducing the number of stacking-generated defects and the volume of voids formed after binder burn-out. Barium titanate tapes and stacked multilayer electroded discs have been investigated and characterised by microstructural examination and density measurement. The resulting properties are described, and the behaviour of the multi-layers discussed in terms of the microstructure and processing procedures. It is shown that the porosity content of sintered samples compacted at 300 MPa is approximately half that of samples compacted at 30 MPa when both are sintered at 1300°C for 2 h. A further improvement of densification can be achieved by a postcompaction treatment after binder burn out.     

Fabrication of three-dimensional ceramic photonic crystals and their electromagnetic properties

Fabrication process for three-dimensional ceramic photonic crystals with a diamond structure was investigated. A diamond structure composed of epoxy lattice including SiO₂–TiO₂ ceramic particles at 10 vol.% was fabricated as a precursor by stereolithography. After burning off the epoxy resin in air, the diamond structure of SiO₂–TiO₂ was successfully sintered at 1400 °C for 2.5 h. The linear shrinkage ratio was 50%. Cracks were not found in the sintered diamond structure. Photonic bandgap was observed at around 19 GHz.     

Fabrication of transparent SiO2 glass by pressureless sintering and spark plasma sintering

Transparent SiO₂ bodies were prepared by pressureless sintering (PLS) and spark plasma sintering (SPS). The effects of sintering and annealing temperature on the transmittance of the SiO₂ bodies were investigated. The SiO₂ bodies sintered by SPS and PLS at 1073–1573 K were amorphous. With increasing the sintering temperature to 1673 K, the SiO₂ bodies sintered by PLS were crystallized while those sintered by SPS were still amorphous. The relative density of the SiO₂ bodies sintered by SPS was 98.5% at 1373 K and 100% at 1573 K, whereas that sintered by PLS was 92.6% at 1373 K and 98.9% at 1573 K. The transmittance was 91.0% and 81.5% at a wavelength (λ) of 2 μm for the SiO₂ sintered bodies by SPS and PLS, respectively. In the ultraviolet range, the transmittance of the SiO₂ bodies sintered by SPS at 1573 K was about 40% at λ = 200 nm and increased to 75% after annealing at 1073 K for 1 h, which was about three times of the transmittance of the SiO₂ bodies sintered by PLS (24.8%).     

Effect of the addition of silica sol on layered extrusion forming of Al2O3-based cores

Layered extrusion forming of ceramic cores with a nanoceramic suspension as a binder was conducted to explore a novel method to produce complex-shaped ceramic cores. Green bodies were prepared using Al₂O₃ particles as precursor materials and silica sol combined with aqueous polyvinyl alcohol solution as a binder. Increasing the silica sol content increased the viscosity of the slurry, enhanced the green bending strength, and decreased the green linear shrinkage. The green microstructure showed the nanosized silica particles were deposited on the surface of the Al₂O₃ particles and among the pores formed by Al₂O₃ particles irregular packing. In addition, increasing the silica sol content increased the bending strength, however, decreased linear shrinkage and open porosity of the sintered bodies. During sintering, the nanosized silica particles converted to the melting phase and reacted with Al₂O₃ and the microstructure of sintered bodies indicated the existence of sintering neck with silica sol addition.     

Fine-structured oxide ceramics from a novel replication method

Fine-structured oxide ceramics sintered bodies with sub-micrometer scales were fabricated by a replication method from mixed slurry of polyvinyl alcohol (PVA) and nanosized ceramic particles in water. By using this replication method and burn-out at various sintering temperatures, fine-structured line and space patterns with sizes as small as 300 nm were fabricated on surfaces of TiO₂ and ZnO. The mechanism of pattern deformation was investigated and a general expression to predict the minimum pattern size was proposed based on the relationship between pattern size and grain size. The results demonstrated the feasibility of using this simple method to produce sintered oxide ceramic materials with various compositions and structures.     

Fabrication of micro-scale textured grooves on green ZrO2 ceramics by pulsed laser ablation

The green ZrO₂ ceramics were fabricated by cold isostatic pressing. Pulsed laser ablation with a wavelength of 1064 nm was performed to fabricate micro-scale textured grooves on the surface of green ZrO₂ ceramics. The influence of laser parameters on surface quality was studied. The heat-affected zone around the machined grooves and micromorphology of laser-irradiated surface were investigated. Results showed that micro-scale textured grooves with a width of 30–50 µm and a depth of 15–50 µm on the green ZrO₂ ceramic surfaces were successfully fabricated by pulsed laser ablation. The laser parameters had a profound influence on the surface quality of micro-scale textured grooves. Better surface quality could be obtained with frequency below 40 Hz, power below 6 W, and scanning velocity above 200 mm/s. A sintering layer was found on the laser-irradiated surfaces when frequency was above 60 Hz, power was above 10 W, and scanning velocity was below 150 mm/s. Analysis of this sintering layer revealed clear melting and resolidification of ZrO₂ particles.     

Preparation of concentrated barium titanate suspensions incorporating nano-sized powders

In order to reduce agglomeration and overcome the low packing density issues of working with nano-sized powders, a colloidal processing route has been chosen in this study. Commercial BaTiO₃ (BT) powders with a particle size in the range of 50 nm have been dispersed in the aqueous media. Rheological properties have been analyzed on suspensions with different solids loading, dispersant concentration, and pH conditions. Optimum dispersing conditions were obtained for suspensions prepared at basic pH (pH 10) with 0.646 wt% ammonium poly (acrylic acid) (NH₄PAA) as a dispersant. Suspensions have been centrifugally cast to obtain the green body, and the sintering conditions have been investigated by examining the phase evolution, microstructures and electrical properties of the sintered samples through XRD, SEM and dielectric measurements, respectively. The results show that for a 45 vol% suspension sintered at 1325 °C, the density of bulk ceramic can reach 5.85 g/cm³, nearly 97.0% of the theoretical density.     

Curing green fibres infusible by electron beam irradiation for the preparation of SiBNC ceramic fibres

Curing green fibres infusible is an essential procedure for the preparation of SiBNC ceramic fibres. Previously, green fibres had been fabricated by one-pot synthesis of polyborosilazane (PBSZ) and melt-spinning. In this paper, we attempted to use the method of electron beam irradiation to crosslink green fibres. The variation of molecular structures from green fibres to cured fibres and the properties of sintered SiBNC fibres were investigated. Via electron beam irradiation, the free radicals are formed at the C atoms and Si atoms on the -N-SiH(CH₃)- main chain units and terminal -Si(CH₃)₃ groups. The radicals react with each other to produce cross-linking, coupling and grafting among PBSZ chains, which all contribute to improvement of the cross-linking density of green fibres. The cured fibres performed a high ceramic yield of 80.4 wt%. After pyrolysis at 1500 °C, SiBNC ceramic fibres were acquired, which exhibited a good flexibility with 12 µm in diameter and 1.22 GPa in tensile strength. The obtained fibres could remain amorphous up to 1700 °C and showed no mass loss at this temperature.