Sintering kinetics and properties of NiFe2O4-based ceramics inert anodes doped with TiN nanoparticles

Sintering kinetics of NiFe₂O₄-based ceramics inert anodes for aluminum electrolysis doped 7 wt% TiN nanoparticles were conducted to investigate densification and grain growth behaviors. The linear shrinkage increased gradually with the increasing sintering temperature between 1000 and 1450°C, whereas the linear shrinkage rate exhibited a broad peak. The maximum linear shrinkage rate was obtained at 1189.4°C, and the highest densification rate was achieved at the relative density of 75.20%. Based on the pressureless sintering kinetics window, the sintering process was divided into the initial stage, the intermediate stage, and the final stage. The grain growth exponent reduced with increased sintering temperature, whereas the grain growth activation energy decreased by increasing sintering temperature and shortening dwelling time. The grain growth was mainly controlled by atomic diffusion. NiFe₂O₄-based ceramics possessed high-temperature semiconductor essential characteristics. The electrical conductivity of NiFe₂O₄-based ceramics first increased and then decreased with increasing sintering temperature, reached their maximum value (960°C) of 33.45 S/cm under 1300°C, mainly attributed to the relatively dense and uniform microstructure. The thermal shock resistance of NiFe₂O₄-based ceramic was improved by a stronger grain boundary bonding strength and lower coefficient of linear thermal expansion.     

Preparing EBCO superconducting targets with uniform phase structure by pressure slip casting

The grain size and phase structure uniformity of europium barium copper oxide (EBCO) superconducting targets would affect the superconducting properties of sputtered film. A uniformly dispersed EBCO slurry was used to prepare high-density green EBCO compacts using slip casting. The effects of milling time and slurry pH on EBCO slurries were studied. Based on the shrinkage data of the EBCO superconducting targets, the densification process was estimated and the apparent activation energy was calculated. The effects of the sintering process on the microstructure and phase composition of EBCO superconducting targets were investigated using scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). When the sintering temperature was 950–1050 °C, the EBCO superconducting target comprised EuBa₂Cu₃O₇ and CuO, where a new phase Eu₂BaCuO₅ formed at a sintering temperature of 1100 °C. The Tc and ΔT of the EBCO superconducting target sintered at 950 °C for 30 h were 91.18 K and 2.35 K.     

The orientation mechanism of (Ca,Sr)Bi4Ti4O15 ceramics prepared by slip casting in high magnetic field and subsequent sintering

A high magnetic field of 10 T was introduced into a processing of slip casting for fabricating (Ca,Sr)Bi₄Ti₄O₁₅ (abbreviated as CSBT) ceramics. Feeble magnetic CSBT particles in green compacts were partially aligned through rotating a gypsum mold containing the CSBT slurry in the magnetic field. The green compacts were sintered at 1200 °C for different time without magnetic field. With increasing of the sintering time, the preferable orientation degree of CSBT ceramics rapidly went up at the initial stage, and then slowly increased at the medium and final stages. The mechanism of the orientation degree increasing during the sintering can be attributed to a processing in which large oriented particles coarsen small randomly oriented particle.     

Effect of TiN content on properties of NiFe2O4-based ceramic inert anode for aluminum electrolysis

NiFe₂O₄-based ceramic inert anodes for aluminum electrolysis doped with various TiN nanoparticles were prepared by a two-step cold-pressing sintering process to investigate how TiN affected the sintering behavior and properties of the composites. The differential scanning calorimetry-thermogravimetry (DSC-TG), X-ray diffraction (XRD), and microstructure analysis results indicated that the Ti and N were evenly distributed in the NiFe₂O₄ matrix to form a solid solution. The maximum linear shrinkage and linear shrinkage rate were enhanced with the increase of TiN nanoparticles contents, and the sintering activation energy of initial stage was lowered from 382.63 to 279.58 kJ mol⁻¹ with the TiN nanoparticles additive range from 0 to 9 wt%. When the content of TiN nanoparticles was 7 wt%, the relative density, bending strength, and elastic modulus reached their maximum values of 97.24%, 73.88 MPa, and 3.77 GPa, respectively, whereas the minimum static corrosion rate of NiFe₂O₄-based ceramic of 0.00114 g cm⁻² h⁻¹ was obtained, mainly attributed to the relatively dense and stable microstructure. The electrical conductivity of NiFe₂O₄-based ceramics presented a clear ascending trend with increasing TiN nanoparticles content and elevated temperature, attributed to the increased concentration and migration rate of carrier.     

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.     

Ultrafast high-temperature sintering of dense and textured alumina

Crystallographic texture engineering in ceramics is essential to achieve direction-specific properties. Current texture engineering methods are time-consuming, energy extensive, or can lead to unnecessary diffusion of added dopants. Herein, we explore ultrafast high-temperature sintering (UHS) to prepare dense and textured alumina using templated grain growth (TGG). From a slurry containing alumina microplatelets coated with Fe₃O₄ nanoparticles dispersed in a matrix of alumina nanoparticles, green bodies with oriented microplatelets were prepared using magnetic assisted slip casting (MASC). The effects of the sintering temperature, time and heating rate on the density and microstructure of the obtained ceramics were then studied. We found that TGG occurs for a temperature range between 1640 and 1780 °C and 10 s sintering time. Sintering at 1700 °C for 10 s led to dense and textured alumina with anisotropic grains thanks to the Fe₃O₄ coating, which did not have the time to diffuse. The highest texture and relative density were obtained with a heating rate of ~5500 °C/min, leading to texture-dependent anisotropic mechanical properties. This study opens new avenues for fabricating textured ceramics in ultra-short times.     

Porous SiC/SiCN composite ceramics fabricated by foaming and reaction sintering

Porous SiC/SiCN composite ceramics with heterogeneous pore structure and rod-like SiCN grains were fabricated by foaming and reaction sintering. The mixture slurry containing SiC and silicon as raw materials, cornstarch as binder, Y₂O₃ as sintering additive and an electrosteric dispersant was stirred with foams derived from pre-foaming using foaming agent. The casted green body was sintered at 1650 °C under nitrogen atmosphere. The results demonstrated that the porous SiC/SiCN ceramics exhibited hierarchical vias ranging from 1 μm to 1 mm and the rod-like crystalline SiCN grains generated in the SiC matrix.     

Porous YSZ Ceramics Reinforced by Different Kinds of Fibers

Porous YSZ ceramics reinforced by different fibers were prepared by gel‐casting with 15% solid content and pressureless sintering. The four kinds of fibers (mullite, aluminosilicate, Al₂O₃, and YSZ fibers) were added into the YSZ ceramics with the same 10% vol content. After sintered at 1500°C for 2 h, aluminosilicate and mullite fibers could not be found in the samples of porous YSZ ceramics, which showed they reacted with YSZ ceramics at high temperature, while YSZ and Al₂O₃ fibers still kept perfect after sintering. Furthermore, the influences of fiber content, sintering temperature, porosity of matrix materials on compressive strength and porosity of the porous YSZ ceramics were studied. The results showed that Al₂O₃ fiber showed more obvious reinforcing effect than YSZ fiber on porous YSZ ceramics. The fiber‐reinforcing effects depend on fiber content, sintering temperature, and porosity of matrix materials. The fiber addition can improve the shrinkage behavior of porous ceramics during sintering and strengthen the skeleton of porous ceramics.     

Pressureless ultrafast sintering of near-net-shaped superhard isotropic B4C/rGO composites with Ti-Al additives

Superhard composites of B₄C reinforced with randomly-oriented reduced graphene oxide (rGO) nanoplatelets are manufactured by a near-net-shape fabrication route based on three successive steps. Firstly, aqueous colloidal processing is used for the environmentally-friendly preparation of a semi-concentrated multi-component slurry (B₄C as main component, Ti-Al as sintering additive, and rGO as toughening reinforcement), whose suitability for wet shaping is demonstrated by rheological measurements. Secondly, slip casting is used to produce robust green parts with shapes on demand and microstructures free of macro- and micro-defects. And thirdly, pressureless spark-plasma sintering (PSPS) is used for the ultrafast and energy-efficient densification of the green parts with shape retention. Measurements of shrinkage and hardness, as well as the microstructural observations, are used to identify suitable PSPS temperatures leading to obtaining isotropic B₄C/rGO composites that are superhard and almost twice as tough as the monolithic B₄C ceramics.     

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.     

Colloidal Filtration of Silicon Nitride Aqueous Slips,Part II: Slip Casting and Pressure Casting Performance

In a previous part, the rheological behaviour of silicon nitride aqueous slips was optimized by dispersing with TMAH up to pH 11·5 using different mixing procedures and including different concentrations of sintering aids (Al₂O₃ and Y₂O₃). In this part, the obtention of pressureless sintered silicon nitride bodies by colloidal filtration techniques is studied. The kinetics of the different compositions is studied for both slip casting and pressure casting. The pressure casting kinetics is up to 20 times faster than that of slip casting, which allows the scale-up of the process for a low cost production, The obtained green density is slightly >58%th for slip casting and 57–55%th for pressure casting, depending on the applied pressure. This small difference does not influence sintered density. At 1750°C/2h, a final density around 96%th is obtained. The sintering conditions are studies considering the time, temperature, atmosphere and sintering bed. The best results are obtained when the sintering bed has the same composition to that of the sample to be sintered. The room temperature properties of the sintered materials show a K_IC value higher than 6 MPam^1/2, comparable to those found in the literature for pressure sintered materials.     

Study on the sintering behavior and characterization of the IGZO ceramics by slip casting

High-purity In₂O₃, Ga₂O₃ and ZnO (In:Ga:Zn = 1:1:1) mixtures were employed to fabricate the IGZO (In-Ga-Zn-O) ceramics by slip casting. The rheological properties and zeta potential of IGZO slurry were analyzed in details. The optimum amount of dispersant used in the IGZO slurry is studied in details. This study investigated the relationship between the phase composition of sintered IGZO ceramics and sintering temperatures. The IGZO ceramics with high density and low resistivity are successfully synthesized in an air atmosphere, and their grain size is distributed uniformly.     

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.     

Colloidal Processing of Zirconium Diboride Ultra‐High Temperature Ceramics

Colloidal processing of the Ultra‐High Temperature Ceramic (UHTC) zirconium diboride (ZrB₂) to develop near?net‐shaping techniques has been investigated. The use of the colloidal processing technique produces higher particle packing that ultimately enables achieving greater densification at lower temperatures and pressures, even pressureless sintering. ZrB₂ suspension formulations have been optimized in terms of rheological behavior. Suspensions were shaped into green bodies (63% relative density) using slip casting. The densification was carried out at 1900°C, 2000°C, and 2100°C, using both hot pressing at 40 MPa and pressureless sintering. The colloidally processed materials were compared with materials prepared by a conventional dry processing route (cold pressed at 50 MPa) and subjected to the same densification procedures. Sintered densities for samples produced by the colloidal route are higher than produced by the dry route (up to 99.5% relative density by hot pressing), even when pressureless sintering is performed (more than 90% relative density). The promising results are considered as a starting point for the fabrication of complex‐shaped components that can be densified at lower sintering temperatures without pressure.     

Non-shrinkage porous ceramics by In-situ hollow sphere method based on the Kirkendall effect of Al particle

The significant shrinkage of porous ceramics after sintering has produced a number of issues with their use and development. As a result, we proposed an in-situ hollow sphere method for producing non-shrinkage alumina porous ceramics. The obtained green samples were made up of Al₂O₃ and Al powders, with pores emerging inside the materials due to the Kirkendall effect of Al particles after sintering. The expansion of hollowing particles exactly offsets the shrinkage generated by sintering throughout the process. When 50 vol. % Al powder (10 µm) is added, the linear shrinkage rate of the sample after sintering at 1500 °C can reach −3.47 %, and its apparent porosity and flexural strength are 30.69 % and 44.03 MPa, respectively. According to approximate calculations, the pores formed by the oxidation of Al powder are smaller than the initial size of Al powder. This method suggests a novel approach for producing controlled shrinkage porous ceramics.     

Alumina nanocrystalline ceramic by centrifugal casting

Centrifugal casting is an established molding method to prepare ceramics with high strength and high reliability and it has been well demonstrated in Al₂O₃. However, it has not yet been applied to Al₂O₃ nanocrystalline ceramic with < 100 nm grain size, primarily due to the unavailability of high-quality α-Al₂O₃ nanoparticles. In addition, ultrafine nanoparticles may be difficult to cast from the solution unless high-speed ultracentrifuge (e. g., >60,000 rpm) is used. Here we addressed these two challenges by home-made dispersed α-Al₂O₃ nanoparticles with 10 nm average particle size and HCl-assisted casting under a “normal” centrifuge condition and report the first attempt to produce Al₂O₃ nanocrystalline ceramic by centrifuge casting and pressureless sintering. The sintering kinetics and microstructure were analyzed, which assists the design of optimal two-step sintering schedule. We showed that dense Al₂O₃ nanocrystalline ceramic with 65 nm average grain size and ultra-uniform microstructure (the standard deviation of the grain size distribution to the average grain size is 0.358) can be obtained by two-step sintering at 1175 °C without holding followed by holding at 1025 °C for 20 h. The ultra-uniform microstructure may result from the denser and more uniform packing of particles in the green bodies produced by centrifugal casting. The two-step sintered Al₂O₃ nanocrystalline ceramic has a microhardness of 19.9 GPa. The microhardness indicates potential softening (inverse Hall-Petch relationship) of Al₂O₃ nanocrystalline ceramic at such a grain size.     

Recycling of coal ash for production of dense β-Sialon/ZrN/ZrON-based ceramics without sintering aids via pressureless sintering

β-Sialon(z = 2, Si₄Al₂O₂N₆)/ZrN composite powders have been synthesized from coal ash, zircon, and active carbon at 1550°C for 6 hours, and β-Sialon(z = 2)/ZrN/ZrON-based composite ceramics (SZZCCS) have been prepared from as-synthesized β-Sialon/ZrN composite powders via pressureless sintering process. The effects of sintering temperatures (1450, 1500, 1550°C) on the phase compositions, microstructures, linear shrinkage ratio, bulk density, and oxidation characteristics of the SZZCCS were investigated in detail, and the oxidation process was also analyzed. It was found that the dense SZZCCS could be prepared at 1550°C for 1 hour, and they mainly consisted of β-Sialon(z = 2), ZrN, and ZrON. The ZrN and ZrON particles were uniformly distributed on the β-Sialon matrix. The sintering properties of the SZZCCS were greatly improved with increasing the sintering temperature. The SZZCCS sintered at 1550°C for 1 hour possessed excellent oxidation resistance at 900°C for 6 hours due to their dense microstructures.     

The effects of ball milling time on the rheological,optical, and microstructural properties of YAG transparent ceramics

Stable slurries dispersed mixture of commercial Al₂O₃ and Y₂O₃ powders, ammonium poly meta acrylate (Dolapix CE64) as the dispersant, and tetraethyl orthosilicate (TEOS) as the sintering aid were prepared by ball milling process. The effects of the milling time on the fabrication of transparent polycrystalline yttrium aluminum garnet (YAG) ceramics were investigated by slip casting and vacuum sintering. The results showed that the best milling time for the deagglomeration of the powder mixture was 16 hours and the slurry prepared during this time showed a near-Newtonian behavior due to the better deagglomeration and lower viscosity. X-ray patterns also showed that all samples were pure YAG phase. The results also revealed that the sample prepared by 16 hours ball milling time suspensions exhibited higher relative green and final density, as well as the maximum transmittance at 1064 nm (≈ 77%). These samples had a more uniform microstructure too.     

Effect of MnO2 addition on properties of NiFe2O4-based cermets

The impact of MnO₂ as an additive on the properties of NiFe₂O₄-based cermets prepared by the two-steps sintering method has been investigated. The new material was characterized in terms of the crystal structure, microstructure, linear shrinkage, relative density and porosity. Moreover, the bending strength of NiFe₂O₄-based cermets was measured. Differential scanning calorimetry (DSC) and X-ray diffraction analysis (XRD) shows that the addition of MnO₂ has no obvious influence on the crystal structure of the cermets. Scanning electron microscope (SEM) studies reveals that the grain sizes of cermets decreases slightly with doped MnO₂. The results show that the linear shrinkage, relative density and bending strength increase at first and then decrease slightly. A high-density (99.56%) and high-strength (84.28 MPa) NiFe₂O₄-based cermets has been obtained by adding 0.50 wt% MnO₂ into the matrix.     

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.