This manuscript describes, defines, and discusses the process of cold sintering, which can consolidate a broad set of inorganic powders between room temperature and 300 °C using a standard uniaxial press and die. This temperature range is well below that needed for appreciable bulk diffusion, indicating immediately the distinction from the well-known and thermally driven analogue, allowing for an unconventional method for densifying these inorganic powders. Sections of this report highlight the general background and history of cold sintering, the current set of known compositions that exhibit compatibility with this process, the basic experimental techniques, the current understanding of physical mechanisms necessary for densification, and finally opportunities and challenges to expand the method more generically to other systems. The newness of this approach and the potential for revolutionary impact on traditional methods of powder-based processing warrants this discussion despite a nascent understanding of the operative mechanisms.

Texture-engineered ceramics enable access to a vast array of novel texture-property relations leading to property values ranging between those of single crystals and isotropic bulk ceramics. Recently developed templated grain growth and magnetic alignment texturing methods yield high quality crystallographic texture, and thus significant advances in achievable texture-engineered properties in magnetic, piezoelectric, electronic, optical, thermoelectric, and structural ceramics. In this paper, we outline the fundamental basis for these texture-engineered properties and review recent contributions to the field of texture-engineered ceramics with an update on the properties of textured lead-free and lead-based piezoelectrics. We propose that further property improvements can be realized through development of processes that improve crystallographic alignment of the grain structure, create biaxial texture, and explore a wider array of crystallographic orientations. There is a critical need to model the physics of texture-engineered ceramics, and more comprehensively characterize texture, thus enabling testing of texture orientation-property relations and materials performance. We believe that in situ measurements of texture evolution can lead to a more fundamental and comprehensive understanding of the mechanisms of texture development.

Many material manufacturing techniques require the use of nonaqueous colloidal suspensions, containing well dispersed oxide particles and various water insoluble functional components. We report an efficient method for the direct transfer of MnO2 and titania particles, synthesized in water, to an organic solvent through the interface of two immiscible liquids. Particle agglomeration during the drying stage was avoided, and stable suspensions of nonagglomerated particles in the organic phase were obtained. The benefits of this method were demonstrated by the fabrication of advanced composite MnO2-multiwalled carbon nanotube electrodes, containing a polymer binder, for electrochemical supercapacitors with high active mass loading and high active material to current collector mass ratio. The electrodes showed a capacitance of 5.13 F/cm2 and low impedance. High extraction efficiency from concentrated suspensions was achieved by the use of an advanced extractor, which allowed strong adsorption on the particles by the polydentate bonding. The extraction mechanism is discussed.

High-porosity metakaolin-based geopolymer foams (GFs) were fabricated by a gelcasting technique using hydrogen peroxide (foaming agent) in combination with Tween 80 (surfactant). Slurries processed in optimized conditions enabled to fabricate potassium based GFs with a total porosity in the range of ∼67 to ∼86 vol% (∼62 to ∼84 vol% open), thermal conductivity from ∼0.289 to ∼0.091 W/mK, and possessing a compressive strength from ∼0.3 to ∼9.4 MPa. Moreover, factors that influence the compressive strength, the porosity, the thermal conductivity, and the cell size distribution were investigated. The results showed that the cell size and size distribution can be controlled by adding different content of surfactant and foaming agent. The foamed geopolymer can also be used as adsorbents for the removal of copper and ammonium ions from wastewater. The foams, due to their low thermal conductivity, could also be used for thermal insulation. It was also possible to produce geopolymer formulations that could be printed using additive manufacturing technology (Direct Ink writing), which enabled to produce components with nonstochastic porosity.

Ceramic scaffolds are being developed to control both proliferation and differentiation of hematopoietic stem cells into desired cell products in bioreactors. These scaffolds mimic important aspects of the microenvironment or “niche” inside bone marrow. In particular, hematopoietic stem cell fate is thought to be effected by the architecture of trabecular bone and the presence of calcium. Here we report the effects of ceramics processing on the phase distribution and microstructure of biphasic ceramics used to culture hematopoietic stem cells. Processing of biphasic ceramics by powder mixing resulted in tetracalcium phosphate on the sintered surface. This correlated with observed surface deposits, weight gain, and the release of calcium ions in saline over 28 days. In contrast, impregnation of partially sintered hydroxyapatite with calcium nitrate resulted in calcium carbonate on the sintered surface. Impregnation correlated with the release of calcium ions into the saline, surface pitting, and weight loss.

A new rapid and energy saving method for the obtention of high performance nanoparticles and thin films of Nb2O5 by microwave-assisted hydrothermal synthesis is reported. The hydrothermal treatment of a sol–gel precursor solution in a microwave oven at 180 °C for 20 min was enough to obtain amorphous nanoparticles with average sizes of 40 nm. The calcination promotes the formation of different phases of Nb2O5 (TT and T) with pseudohexagonal and orthorhombic structure, respectively, that transform at higher temperatures in a mixture of orthorhombic and monoclinic phases. Crystalline phase composition was found to have a significant influence on the photocatalytic activity. The best photocatalytic performance was observed for the material mainly constituted by the TT-Nb2O5 phase. Thin films constituted by the TT phase were prepared by dip-coating. Photocatalytic experiments confirmed the high photocatalytic activity of this material, which showed a kinetic curve similar to that of a reference TiO2-P25 thin film.

Spark plasma sintering (SPS) is adopted to fabricate transparent AlON ceramics at 1350–1500 °C under 40 MPa, using a bimodal γ-AlON powder synthesized by the carbothermal reduction and nitridation method. After holding 10 min, high density samples are obtained, and their optical transmittance is investigated over the wavelength range of 1330–6000 nm. Despite the samples SPS-processed at 1350 °C indicate the presence of three-phases: γ-AlON, α-Al2O3, and h-AlN, they show high infrared transparency, i.e., the maximum transmittance for 1.2 mm thick specimens is up to 77.3% at ∼3900 nm. Also, the processed samples exhibit high hardness of 17.81 GPa. The high infrared transmittance should be mainly attributed to high density and rationally controlled grain size distribution, and the high hardness is apparently caused by a small grain size.

Macroporous ceramics having unique pore morphologies with ceramic bridges, unidirectional cellular pores, and bamboo-like cells were fabricated by freezing gels with ceramic powder and various gelatin contents. Varying gel strengths were found to be effective for control of the pore architecture from open to closed pore channels. The proposed process is a relatively simple and versatile way to produce tailored pore configurations via a gelation freezing route. In addition, the relationship between the microstructure and mechanical properties of the resulting ceramics was discussed using a multiscale modeling technique, in which a homogenization method was conducted with microscopic models created from three dimensional images, global stress distributions in macroscopic samples by finite element method and local stress distributions. The simulation results were relatively consistent with the experimental results. The multiscale modeling technique was thus confirmed to be a strong tool for the prediction of the mechanical responses of porous ceramics.

Highly porous and fully crystalline mullite fiber compacts were fabricated by uniaxial hot pressing of stacked short-fiber mats without binders or sintering aids. The special feature of this new fabrication method is the use of MAFTEC® organic bound mat (OBM) type fiber mats consisting of semicrystalline “MLS-2” short fibers. In contrast to conventional polycrystalline mullite fibers, semicrystalline MLS-2 fibers consist of amorphous silica and nanoscale transitional alumina. Due to this special microstructure, MLS-2 fibers are less prone to fiber breakage upon shear load and therefore can well withstand pressure-assisted consolidation. Hot pressing of stacked OBM to highly porous fiber compacts is facilitated by the thermally driven softening of amorphous silica above 900 °C; favorable deformation and consolidation rates are achieved above 1050 °C. Above 1250 °C the mullite is crystallized at the expense of amorphous silica and simultaneously both deformation and consolidation comes to an end. Obtained MAFTEC OBM-derived fiber compacts consist only of crystalline mullites, and therefore mechanical properties are favorably retained at high temperatures.

Dense ZrB2–SiC nanocomposites were fabricated at 1450 °C by the high-energy ball milling (HEBM) of ZrB2 powder with ZrSi2–B4C–C additives and reactive spark plasma sintering (R-SPS). The sizes of ZrB2 and SiC grains were 80–350 nm, and the phases were homogeneously distributed because of the molecular-level homogeneity of the constituents in ZrSi2 and the homogeneous mixing of the raw powders by HEBM. The deformation of ZrSi2 or the reactions between the additives during R-SPS did not strongly promote the densification. Fine and homogeneously distributed ZrB2 and SiC particles formed by HEBM and R-SPS were the major reasons for the low temperature densification. The fine particles which had high surface energy provided the driving force for densification at low temperatures. Also, the fine SiC grains suppressed the growth of ZrB2 grains during densification. The 4-point bending strength of the composites sintered at 1500 °C was 354 MPa.

In the quest for high real in-line transmittances for transparent polycrystalline alumina (PCA), we need defect free processing. One of the biggest advances in producing high density defect free ceramics over recent years has been the advent of spark plasma sintering (SPS) or pulsed electric current sintering. The production of PCA with high transmittances >60% has been demonstrated, but the mechanisms behind this fast, pressure aided sintering method are still much debated. Here, we investigate the sintering of doped α-alumina powders using traditional and pulsed electric current dilatometry. We demonstrate that at the final sintering stage, there is no major difference in the sintering mechanisms between conventional sintering and SPS sintering. High densification rates occurring in SPS are shown to be related to powder reorientation at the very early sintering stage and viscous-flow dominated densification in the intermediate sintering cycle. This paper clarifies what parameters in the processing–sintering domain have to be improved for even higher real in-line transmittances for PCA.

As more and more SiC–SiC ceramic matrix composites or CMC’s are being used in the hot sections of gas turbine engines, there is a greater need for surface temperature measurement in these harsh conditions. Thin film sensors are ideally suited for this task since they have very small thermal masses and are nonintrusive due to their thickness. However, if the bulk properties of SiC contributed to the sensor performance (thermoelectric response) rather than those of the thin films, superior resolution, and stability could be realized. Therefore, thermocouples utilizing the SiC–SiC CMC itself as one thermoelement and thin film platinum as the other thermoelement were developed. Large and stable thermoelectric powers (as large as 250 μV/°K) were realized with these Pt:SiC (CMC) thermocouples. The advantages in using this approach for surface temperature measurement are presented as well as the effects of fiber orientation on thermoelectric response and drift.

The development of ceramic nanomaterials with unique structure is necessary for discovery of novel property. We developed a novel niobium oxide nanoparticles with a spiky morphology. The spiky structure was composed of two kinds of component: niobium oxide hydrate sphere core and niobium pentoxide nanorods. These spiky niobium oxide nanoparticles are easily synthesized by hydrothermal treatment of niobium oxalate solution at 200 °C for 2 h, and their particle size could be tuned from 80 to 300 nm with 5–10 nm of nanorod on the surface by adjusting niobium concentration in the niobium oxalate solution. The band gap energy of the spiky nanoparticles was around 3.4 eV, and the spiky niobium oxide nanoparticles showed a light absorption in a wide wave length range from 380 to 700 nm. The niobium oxide nanoparticles are applicable as both solid acid catalyst and photocatalyst because of their spiky and two-layer structure.

The aqueous corrosion resistance of TiC–Ni3Al based cermets was examined with the specific aims of assessing the influence of both the raw materials and the test methods, using a 3.5 wt% NaCl containing solution. The effects of W contamination in the ceramic phase was investigated using a single-phase ceramic, with and without W, and with a stoichiometric intermetallic Ni3Al binder. The influence of the electrolyte O2 content was examined for TiC cermets with 30 vol% binder, for both the stoichiometric composition and sub-stoichiometric variants, containing either Zr and B (alloy IC50) or Zr, B and Cr (alloy IC221) additions. Electrochemical measurements were matched with chemical and microstructural analyses at various stages of oxidation, and the rate of material loss in combination with the corrosion mechanisms were identified. The effects of O2 concentration were most significant for the TiC based cermets with Ni3Al due to the diffusion controlled nature of the reaction.

Porous silicon nitride ceramics are attracting extensive attention due to its high strength and low dielectric loss. However, further strength enhancement at elevated temperatures is hindered by its intergranular phase, forming from sintering additives. This paper describes the fabrication of porous silicon nitride ceramic materials, by using a replacement method of carbothermal nitridation. The initial samples which were obtained from the sintering of mixed powder consisted of 95 wt% Si3N4 and 5 wt% Y2O3. After the removal of the oxide intergranular phase and the infiltration of mixtures of phenolic resins and silica sols, carbothermal nitridation process was carried out at 1550 °C for 2 h under nitrogen. X-ray diffraction and microstructural analysis revealed a complete replacement of oxide intergranular phases by the newly formed Si3N4 intergranular phase. The unmodified ceramic exhibited lower flexural strength at 1400 °C, which was only 50% of the room-temperature strength. Although the modified ceramic attained a slightly lower flexural strength at room temperature after the replacement of intergranular phase, its strength measured at 1400 °C could attain 90% of room-temperature strength.

We report the structure and synthesis approach for obtaining a ceramic nanocomposite pellet comprising ∼50 nm-sized TaC nanoparticles. A mixture of Ta metal powder and the carbon precursor 1,2,4,5-tetraphenylethynyl benzene, pelletized by vacuum pressing at 131 MPa, on further thermal treatment with Ar at 1400 °C yields such a ceramic composite. On air oxidation, the TaC nanoparticles are converted to Ta2O5 nanoparticles at 760 °C. Hardness measurements revealed that the composite exhibited a global hardness in the range of 1.23–1.57 GPa. However, nanoindentation studies showed that, locally, hardness of the TaC nanoparticles (∼15 GPa) approached that of the densified TaC ceramic. Superconducting studies of the pellet consistently exhibited two transitions with T c values of 10 K and 8.5 K, respectively, that corresponded to bulk TaC and to a component of unknown origin. The results discuss the morphological and constitutional characterizations of the TaC nanoparticle-containing composite.

Extremely hard, wear-resistant SiC-bonded diamond materials with diamond contents of approximately 45–60% by volume can be prepared by pressureless infiltration of shaped diamond compacts with silicon. Materials with diamond grain sizes in the range of 10–100 µm can be produced having a free silicon content of less than 5 vol%. Components with large dimensions can be prepared as graded or ungraded materials. Graded components are composed of silicon infiltrated SiC base material with diamond–SiC composite layers of 0.1 mm by dip coating technology to several mm in thickness by doubled die pressing in regions with high loading. This creates the possibility of producing low-cost, wear-resistant components of various geometries and dimensions with bending strengths of 400–500 MPa, hardness values of 48 GPa, and fracture toughness levels of 4.5–5 MPa m1/2 for use in extreme wear conditions. Thermal conductivities of up to 500 W/(m K) were obtained, render these materials interesting for heat sinks.

Freeze casting of traditional ceramic suspensions and freeze casting of preceramic polymer solutions were directly compared as methods for processing porous ceramics. Alumina and polymethylsiloxane were freeze cast with four different organic solvents (cyclooctane, cyclohexane, dioxane, and dimethyl carbonate) to obtain ceramics with ∼70% porosity. Median pore sizes were smaller for solution freeze casting than for suspension freeze casting under identical processing conditions. The pore structures, which range from foam-like to lamellar, were correlated to the Jackson α-factor of the solvent; solvents with low α-factors yielded nonfaceted pore structures, while high α-factors produced more faceted structures. Intermediate α-factors resulted in dendritic pore structures and were most sensitive to the processing method. Small suspended particles ahead of a solid–liquid interface are hypothesized to destabilize the dendrite tip in suspension freeze casting resulting in more foam-like structures. Differences in processing details were highlighted, particularly regarding the improved freezing front observation possible with solution-based freeze casting.

In a new design approach, C/C–SiC sandwich structures have been realized via liquid silicon infiltration and in situ joining methods. In the first process step, the carbon fiber reinforced polymer preforms for the planar skin plates as well as for the core structures were manufactured via warm pressing and autoclave technique, using C fiber fabrics preimpregnated with phenolic resin. Thereby, two types of C/C–SiC core structures were used: foldcore and grid core. In the second process step, the sandwich components were pyrolyzed separately, leading to porous C/C preforms and subsequently joined to a C/C sandwich structure, using an adhesive. In the last process step, the C/C structure was infiltrated with molten Si, and the SiC matrix was built up by a chemical reaction of Si and C, leading to a permanently joined C/C–SiC sandwich structure. For mechanical characterization, sandwich specimens were manufactured and tested in 4 point bending.