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.     

Sintering densification behavior of ZnO–SnO2 binary ceramic targets

In this work, high-performance ZnO–SnO₂ binary ceramic targets for magnetron sputtering of transparent conductive oxide (TCO) films were prepared by pressureless oxygen atmosphere sintering. The sintering behavior and densification mechanism of the ZnO–SnO₂ binary targets were analyzed by systematically studying the oxide powder state, formation process of the solid reaction phase, and evolution of the target microstructure. The data revealed that the ZnO–SnO₂ powder treatment improved the sintering activity and the powder dispersion; furthermore, it promoted a mutual reaction between the different components during sintering and the homogeneity of the target composition. The densification of the pure SnO₂ ceramic target was difficult to achieve, and the addition of ZnO led to a continuous interaction between the ZnO and SnO₂ components. The Zn₂SnO₄ phase started to form, and a temporary shrinkage of the target occurred above 800°C. After formation of the stable Zn₂SnO₄ and SnO₂ phases, the target shrunk rapidly with increasing temperature, densification occurred during growth, and the two phases started to interact. The sintering temperature provided the driving force for the target densification, with the densification activation energy of the ZnO–SnO₂ binary ceramic target estimated to be 580 kJ/mol based on the master sintering curve. A binary ceramic target with a high density (99.78% relative density), a fine grain size, and a homogeneous phase structure was achieved at a temperature of 1600°C. These findings are promising for the further improvement and performance enhancement of SnO₂-based materials.     

Densification and grain growth of Gd2Zr2O7 nanoceramics during pressureless sintering

Gd₂Zr₂O₇ nanoceramics were fabricated using pressureless sintering method, in which the nanopowders were synthesized via solvothermal approach. The effects of starting powders on grain growth and densification during sintering of ceramics were revealed. Two distinct pressureless sintering methods were investigated, including conventional and two-step sintering. The sample grain size increases abruptly as sintering temperature increases during conventional sintering. In contrast, in two-step sintering, abnormal or discontinuous grain growth was suppressed in the second step, leading to Gd₂Zr₂O₇ nanoceramics formation (average grain size 83 nm, relative density ∼93%). Such distinct behaviors may originate from the interplay between kinetic factors such as grain boundary migration and diffusion. Moreover, suppression of grain growth and promotion of densification in the two-step sintering are mainly due to dominant role of grain boundary diffusion during the second-step sintering process.     

Nonisothermal sintering of Cr2AlC powder

This work studies the sintering behavior of ultrafine powder of Cr₂AlC compound during nonisothermal heating using a dilatometer. Sample exhibits relatively slower densification under pressureless condition where relative density increased from about 61% (green compact) to about 73% during heating up to 1350°C. Axial shrinkage data were analyzed through the volume diffusion and grain boundary diffusion‐based sintering models. Estimated activation energy of sintering (352 kJ/mol) for volume diffusion mechanism, was found to be closely matching with the reported activation energy of chromium diffusivity, indicating volume diffusion as a controlling mechanism and chromium as a major diffuser in Cr₂AlC.     

Sintering and densification of nanocrystalline ceramic oxide powders: a review

Abstract

Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sliding, plastic deformation, and pore shrinkage. Low temperature mass transport by surface diffusion during the spark plasma sintering of nanoparticles can lead to rapid densification kinetics with negligible grain growth.     

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-stoichiometry of (TiZrHfVNbTa)Cx and its significance to the microstructure and mechanical properties

(TiZrHfVNbTa)C_x with variable stoichiometry are fabricated by pressureless sintering utilizing self-synthesized carbide powders via carbothermal reduction reaction. The densification behavior and microstructure evolution coupled with corresponding adjustable mechanical properties are investigated. The single-phase rock-salt crystal structure is retained despite the carbon stoichiometry approaching 0.6, indicating (TiZrHfVNbTa)C_x can maintain structural stability even containing high carbon vacancy. The carbon vacancy is beneficial for promoting densification procedure. The relative density of (TiZrHfVNbTa)C_0.6 sintered at 2150 °C can reach 97.9 %, while the similar value for (TiZrHfVNbTa)C_1.0 is obtained even at 2400 °C. While, remarkable grain growth accompanied by decline in relative density also occurs for lower carbon stoichiometry. With the variation of carbon content, the concentration of carbon-metal bonds changes gradually, leading to the adjustable mechanical properties. This work provides a potential approach to synthesize non-stoichiometric high-entropy carbides with high carbon vacancy via low-temperature pressureless sintering.     

Fabrication of transparent MgAl2O4 ceramics by gelcasting and cold isostatic pressing

Highly transparent MgAl₂O₄ ceramics have been fabricated by aqueous gelcasting combined with cold isostatic pressing (CIP), pressureless sintering and hot isostatic pressing (HIP) from high purity spinel nanopowders. The gelling system used AM and MABM as monomer and gelling agent. The influences of dispersant and PH on the rheological behavior of the MgAl₂O₄ slurries were investigated. The spinel slurry with low solids loading (25 vol%) and low viscosity (0.15 Pa s) was obtained by using 6 wt% Duramax-3005 (D-3005) as dispersant. After CIP, the green body had a relative density of 48% with a narrow pore size distribution. The influence of sintering temperature on densification and microstructure was studied, choosing 1500 °C as the sintering temperature. After HIP (1650 °C/177 MPa/5 h), transparent MgAl₂O₄ ceramic with the thickness of 3 mm was obtained, whose in-line transmittance was 86.4% at 1064 nm and 79.8% at 400 nm, respectively. The ceramic exhibited a dense microstructure with the average grain size of 23 μm. The Vickers hardness and flexure strength of the sample reached 13.6 GPa and 214 MPa, respectively.     

Construction and validation of master sintering curve for TiO2 for pressureless sintering

Abstract

One of the ultimate objectives for sintering research is to predict densification results under different thermal profiles for a given processing method. This paper studies the construction and validation of the master sintering curve (MSC) for rutile TiO₂ for pressureless sintering. The MSC was constructed using dilatometry data at two heating rates and was then validated using isothermal holds at three different temperatures. The scanning electron microscopy (SEM) observation shows that the partially sintered samples have the same density under different heating procedures, which demonstrates that the assumptions of the model are reliable. The concept of the MSC could be used to predict the sintering shrinkage and final density and calculate the activation energy. A value of 105 kJ mol^-1 for TiO₂ was obtained. The MSC could be applied to predict the sintering profile to prepare ceramics with required density and a minimum of grain growth.     

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.     

Pressureless sintered Al4SiC4 ceramics with Y2O3 addition

Highly densified Al₄SiC₄ ceramics with a relative density of 96.1% were prepared by pressureless sintering using 2 wt% Y₂O₃ as additives. The densification mechanism, phase composition, microstructures and mechanical properties of Al₄SiC₄ ceramics were investigated. Y₂O₃ in-situ reacted with the oxygen impurities in Al₄SiC₄ powder to form a yttrium aluminate liquid phase during sintering, which promoted the densification and anisotropic grain growth. The final Al₄SiC₄ ceramics were composed of equiaxed grains and columnar grains, and presented a bimodal grain distribution. The mechanical properties of the pressureless sintered Al₄SiC₄ ceramics were better than those reported for hot pressed Al₄SiC₄, including a flexural strength of 369 ± 24 MPa, fracture toughness of 4.8 ± 0.1 MPa m^1/2 and Vickers hardness of 11.3 ± 0.2 GPa. Pressureless sintering of Al₄SiC₄ ceramics is of great significance for the development and practical application of Al₄SiC₄ ceramic parts, especially with big size and complex shape.     

Fabrication of Er:Y2O3 transparent ceramics for 2.7 μm mid-infrared solid-state lasers

Laser grade 7 at.% Er:Y₂O₃ transparent ceramics with submicron grain size were fabricated by using one-step vacuum sintering followed by hot isostatic pressing (HIPing) technique. Through studying the sintering trajectory of Er:Y₂O₃ ceramics, the sintering temperature zone where sufficient relative density (>96%), no pore-boundary separation, and sub-micron grain size (<1 μm) ceramic samples could be identified. The samples pre-sintered in this zone were readily densified by HIPing. To maximum the densification and achieve high transparency, it is critical to suppress the final-stage grain growth. After HIPing at 1520 °C, the Er:Y₂O₃ ceramics were fully densified without further grain growth, and exhibited in-line transmission of about 81.6% at 2000 nm. Continuous wave (CW) room temperature laser operation of the Er:Y₂O₃ transparent ceramic at 2.7 μm was demonstrated.     

Densification behaviour and two stage master sintering curve in lithium sodium niobate ceramics

The Master Sintering Curve (MSC) has received much attention in recent years due to its ability to predict sintering behaviour of a given powder and green body process regardless of its thermal history. In this paper MSC, based on the combined stage sintering model is constructed for one of the most important lead-free piezoelectric viz. lithium sodium niobate, Na_1-xLi_xNbO₃ (x=0.12, LNN-12), ceramic using shrinkage data. The present study has been carried out to understand and control the densification behaviour during pressureless sintering. Two distinct stages of densification have been observed en route to the upper limit to sintering temperature. The activation energies of densification for the two temperature ranges viz. 800–1150 °C and 1150–1300°C were found to be 365 kJ/mol and 2530 kJ/mol, respectively, through the construction of MSC. The MSC should be useful in predicting the densification behaviour and the final density and for designing a reproducible fabrication schedule for the LNN-12 ceramics.     

Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering

SPS (spark plasma sintering) process was used to sinter nanocrystalline ZrO₂ (3Y). It was found to be different with the usual rapid sintering method, the density of the samples kept increasing with the rising of the sintering temperature. A higher density could be reached at a lower temperature and shorter dwelling time than that by hot-pressing under the similar pressures. In contrast to the samples with a differential densification from edge to center prepared by a rapid hot-pressing, no obvious densification gradient could be found in the samples sintered by SPS. The grain sizes of the Y-TZP obtained by SPS were smaller than those by the pressureless sintering method, while the grain growth speed was much higher under SPS conditions. All these unique sintering behaviors were explained by the special sintering process of SPS.     

Mechanical modelling of microwave sintering and experimental validation on an alumina powder

Microwave sintering (MW) allows fast heating (≤30 min) and densification of ceramic materials, like alumina Al₂O₃. In order to predict the final material properties (density, size and grain size) the mechanical SOVS (Skorohold Olevsky Viscous Sintering) model is adapted and validated for conventional sintering of alumina. The model is implemented on ABAQUS with UMAT subroutine. Secondly, the SOVS model is modified for the microwave sintering by adapting the shear viscosity Arrhenius type law. Pre-exponential and exponential coefficients are modified for MW sintering. The calculated relative densities are compared to experimental results from conventional and microwave sintering and the relative difference remains under 3%. The coefficients identified for the MW sintering reveal a decrease in the shear viscosity by around 10 and an increase by up to 50 times in the grain boundaries diffusion coefficient.     

3D printing of dense structural ceramic microcomponents with low cost: Tailoring the sintering kinetics and the microstructure evolution

In this study, we have successfully developed a unique 3D printing approach based on mask-image-projection stereolithography (MIP-SL) to fabricate structural ceramics microcomponents with low cost and high efficiency. Ultra-dense submicron crystalline ceramics without fierce grain growth could be obtained via tailoring the sintering kinetics. The ZrO₂ ceramic microcomponents reached the highest relative density (RD) of 99.7% with the average grain size of 0.52 μm upon sintering at 1550°C while the Al₂O₃ ceramic microcomponents reached its highest RD of 98.31% with the average grain size of 2.6 μm upon sintering at 1600°C. Oxide ceramics microcomponents of fully flexible design can be produced easily without visible defects via the method developed in this study, which demonstrates significant potential in the applications of microelectromechanical systems, micro-optical electronics systems and micro-opto-electro-mechanical systems. The method developed in this study has addressed the problem successfully by healing the interlayer interface defects in densification process via the sintering kinetic window and microstructure evolution. The current work provides a promising opportunity to fabricate structural ceramic microcomponents with complex shape, high precision, and high surface smoothness.     

Fabrication of high-density NiFe2O4 ceramics by slip casting and pressureless sintering

High-density NiFe₂O₄ ceramics with homogeneous microstructure were produced by slip casting and pressureless sintering. The slurry stability, sintering behavior, and microstructure of NiFe₂O₄ ceramics were investigated. A stable slurry can be obtained by adding 12.5 wt% NiFe₂O₄ nanoparticle and 5 wt% nano-binder at a slurry pH around 11.0. The linear shrinkage and linear shrinkage rate for both NiFe₂O₄ ceramic green bodies shaped by cold press molding and slip casting showed nearly the same trends. The temperature associated with the maximum linear shrinkage rate of slip casted green body was 1263.5°C, which was lower than that of cold press molded sample (1272.0°C). The sintering activation energy of slip casted green body was also lower than that of cold press molded sample (279.18 vs 288.47 kJ mol⁻¹), owing to high density and homogeneity of slip-casted green compact. A high-density NiFe₂O₄ ceramics with uniform grain size distribution can be produced by slip casting and pressureless sintering at 1350°C for 6 hours, attributed to the ability of slip casting to minimize agglomerates and micropores. It demonstrated that slip casting was more suitable to prepare high-density NiFe₂O₄ ceramics with good homogeneity.     

Optical,thermal properties,and hydrolysis resistance of transparent YF3-doped CaO ceramics fabricated by vacuum sintering

Transparent calcium oxide (CaO) ceramic was successfully fabricated by pressureless vacuum sintering technique with 0.5 at% YF₃ as the sintering additive. In consideration of the hydrolysis of CaO, the precursor powders were calcined at 600 °C. Within the sintering temperature range of 1150–1450 °C, the effect of YF₃ additive on the phase structure, relative density, transmittance, and microstructure of CaO ceramics was studied. Benefiting from the assisted liquid-phase sintering mechanism and vacancy diffusion mechanism of YF₃, full-dense (3.35 g/cm³) YF₃-doped CaO transparent ceramic was fabricated at 1350 °C with high thermal conductivity of 15.42 W/(m·K). The in-line transmittance attained 47.10% at the wavelength of 1200 nm. Moreover, the hydrolysis resistance was extremely enhanced due to the pore-free structure. Compared to the CaO ceramic with low relative density of 78.66%, the weight gain of full-dense CaO ceramic greatly decreased from 22% to 1.2% within 35 days.     

Building submicron crystalline piezoceramics: One-step pressureless sintering partially amorphized nanopowder

The key to miniaturization of piezoelectric devices is to build high-performance fine-grained piezoceramics. Although the preparation of fine-grained ceramics can be achieved by hot press sintering or spark plasma sintering, complex processes, and high costs hinder the popularization of such technologies. In this work, a one-step conventional pressureless sintering technique based on partially amorphized nano-precursors has been proposed for low-cost preparation of submicron-structured ceramics. Using this technology, 0.36BiScO₃–0.64PbTiO₃ ceramics with an average particle size of 170 nm and a relative density of 95% can be prepared at temperature as low as 900°C, while the samples still have excellent piezoelectric properties (d₃₃ = 220 pC/N, g₃₃ = 40 × 10⁻³ Vm/N). In the process of sintering partially amorphized nano-precursors, the initial densification rate is faster than the grain growth rate, which can be attributed to two effects promoting low temperature densification, one is the liquid phase sintering mechanism associated with the amorphous phase, and the other is the filling effect of small particles deposited at the grain boundaries. This work is simple and easy to implement, and it is expected to be extended to the preparation of other types of fine-grained ceramics.     

Fabrication of TiO2 components by Fields Activated Sintering Technology (FAST)

The ceramic materials exhibit many intriguing properties relating to higher melting temperature and good thermomechanical properties. In present work, a newly developed Fields Activated Sintering Technology (FAST), named as Micro-forming Fields Activated Sintering Technology (Micro-FAST), has been applied to successfully sinter the TiO₂ samples. The influence of sintering temperature and heating schedule has been investigated. Based on the analysis of the relative density, grain size and microstructure evolution of TiO₂ samples, the densification process could be divided into three stages: I) Preheating period: densification by particle rearrangement mainly driven by the external pressure; II) Rapid heating period: rapid densification by fast atomic diffusion under the coupling effects of stress and temperature; III) Final heating period: the densification further proceeds by slower atomic diffusion. High density (up to 96.95%), fine grain sizes and direct grain-to-grain contact could be obtained by Micro-FAST to sinter TiO₂ bodies without use of any sintering aid. These results should encourage systematic exploration of Micro-FAST in engineering applications.