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[Display omitted] • The pre-prepared amorphous Si 3 N 4 powder has a unique porous structure. • The growth of α-Si 3 N 4 whiskers follows the vapor–solid mechanism. • The growth of α-Si 3 N 4 granules follows the vapor–solid-solid mechanism. A facile procedure was provided to obtain amorphous Si 3 N 4 powder via a liquid-phase method, followed by a heat treatment process to realize tailorable shapes for α-Si 3 N 4 crystal, while Si 3 N 4 granules were synthesized and whiskers were inhibited. The influence of the temperature on the particle morphology and growth mechanisms were investigated. Characterizations indicated that the crystallization temperature played an important role in controlling the morphology of α-Si 3 N 4 powder, while the relatively lowly temperature (below 1519 °C) was beneficial to forming whiskers. The growth of Si 3 N 4 whiskers, derive from the reaction between SiO(g) and N 2 , follows the vapor–solid (VS) mechanism, which is inhibited by the increase in temperature. Instead, higher temperatures were conducive to forming granular structures. Highly crystalline α-Si 3 N 4 granules were prepared at 1550 °C due to Si*(g) and N*(g) recombining to Si 3 N 4 nuclei via the vapor–solid-solid (VSS) mechanism providing a theoretical basis for inhibiting the production of the whiskers. [ABSTRACT FROM AUTHOR]

Silicon nitride (Si 3 N 4) particles with different morphologies have been used in many fields. In this work, α-Si 3 N 4 whiskers and granular particles with high-phase purity were successfully tailored by the controllable crystallisation process of amorphous Si 3 N 4 powders under different N 2 pressure. Impressively, α-Si 3 N 4 whiskers were prepared by simply heat treating amorphous Si 3 N 4 powders at 1550 °C for 2 h under the low N 2 pressure of 0.2 MPa, whereas equiaxed α-Si 3 N 4 particles with uniform size of ~280 nm were obtained under an elevated N 2 pressure of 2.0 MPa. With the evaluated N 2 pressures and temperatures, large scale α-Si 3 N 4 whiskers or equiaxed α-Si 3 N 4 particles could be produced. The growth mechanisms of the α-Si 3 N 4 particles with distinct morphologies were rationally proposed, and these consist of two main growth processes. First, amorphous Si 3 N 4 powders decomposed into Si(g) and N 2 (g) under high-temperature treatment. Subsequently, N 2 (g) dominated the recombination of the evaporated chemical with the Si 3 N 4 molecule. The initial N 2 concentration, which plays a key role in tailoring the shape and size of products, was controlled by the N 2 pressure. [ABSTRACT FROM AUTHOR]

In the present work, high‐quality MgSiN2 powders were prepared by a combustion synthesis method, based on Mg‐Si3N4‐N2 system. The effects of additive content (NH4Cl or NH4F) and N2 pressure on phase composition and crystal morphology of products were investigated. The results suggested that both NH4Cl and NH4F additives alleviated agglomeration and decreased the grain size of MgSiN2. In addition, NH4Cl did not result in the formation of an extra impurity, while MgF2 residue was found when NH4F was introduced. Therefore, NH4Cl additive was considered as a better choice for the preparation of MgSiN2 powders in comparison with NH4F additive. According to the observation of some MgSiN2 whiskers and notably increased Si content under low N2 pressure, the pressure of N2 over 0.5 MPa was essential to have a complete reaction in the present work. Finally, MgSiN2‐Si3N4 composite powders with quasi‐spherical morphology were prepared by adding excessive Si3N4 and NH4Cl additive in reactant. The formation and growth mechanism of MgSiN2 grains was reasonably speculated based on the experimental results. Because of the remarkable merits, the as‐prepared Si3N4‐MgSiN2 composite powders showed great potential for alternative sintering aid in the sintering of Si3N4‐based ceramics. [ABSTRACT FROM AUTHOR]

In this paper, α-Si 3 N 4 nanobelts assembled by Si 3 N 4 microcrystals on the nanowires were synthesized from amorphous Si 3 N 4 powders that were prepared by thermolysis of silicon diimide powders. By prolonging the heating time at 1550 °C, the amorphous Si 3 N 4 powders were converted to hexagonal phase α-Si 3 N 4 microcrystals which were adhered to the surface of nanowires to obtain α-Si 3 N 4 nanobelts. The obtained α-Si 3 N 4 nanobelts were hollow in the centre with a width of ∼850 nm and a thickness of ∼150 nm. Microcrystals were deposited on the surface of 2–3 parallel nanowires. The diameter of the Si 3 N 4 microcrystals was 30–230 nm, and the diameter of the nanowires was 50–100 nm. Evaporation of the amorphous Si 3 N 4 powders released Si vapour and N vapour, which led to the formation of the Si 3 N 4 microcrystals. The nanowires grew on the surface of the amorphous Si 3 N 4 powders due to the nucleation and diffusion of Si and N atoms of amorphous Si 3 N 4 powders. The growth mechanism of the products with unique morphology can be considered as a combination of vapour-solid mechanism and secondary nucleation process. [ABSTRACT FROM AUTHOR]

In this paper, a new nanomaterial, α-Si 3 N 4 nanobelts assembled by high density nanorods were prepared via the crystallization of amorphous Si 3 N 4 at 1450 °C for 12 h. The as-synthesized nanobelts exhibited a width of ∼900 nm and a thickness of ∼150 nm, and a depressed linear region in the central. α-Si 3 N 4 nanorods on the nanobelts had a width of 8–16 nm and a length of 400–450 nm. The nanorods were single-crystal, growing along (101) plane. The nanobelt appeared to be initiated by α-Si 3 N 4 nanorods constantly growing on the both sides of a nanowire which was caused by secondary nucleation and the growth of Si 3 N 4 microcrystals. Growth mechanism related to the formation of the nanobelts was analyzed in detail on the basis of vapor-solid mechanism and secondary nucleation. The current work would provide new insights into α-Si 3 N 4 nanobelts assembled by nanorods which play an important role in nanoscale devices and show widespread potential applications due to their special morphology. • α-Si 3 N 4 nanobelts assembled by nanorods were prepared from amorphous Si 3 N 4 powders. • The α-Si 3 N 4 nanorods constantly grow on the both sides of a nanowire. • Two emission peaks at 437 nm (2.84 eV) and 462 nm (2.68 eV) was measured in the photoluminescence (PL) spectrum. • The growth mechanisms of the α-Si 3 N 4 nanobelts were vapor-solid mechanism and secondary nucleation. [ABSTRACT FROM AUTHOR]

Abstract Novel polycrystal α-Si 3 N 4 submicron rods have been successfully prepared from amorphous Si 3 N 4 powders. As-synthesized submicron rods have oblate section, with the size of 110–150 nm in height, 150–300 nm in width and 3 µm to tens of micrometers in length. Underlying growth mechanism was rationally proposed based on the results of X-ray diffraction analyses, scanning electron microscopy and transmission electron microscopy observations. It was demonstrated that the nucleation and crystal growth of α-Si 3 N 4 occurred simultaneously during the fabrication process of submicron α-Si 3 N 4 rods. The growth of α-Si 3 N 4 submicron rods follows vapor-solid-solid mechanism. The submicron rods grew from the surface of amorphous Si 3 N 4 powders in N 2 atmosphere without catalysts. The silicon monoxide vapor [(SiO)g], as a reaction intermediate, reacted with nitrogen [N 2 ] to form α-Si 3 N 4 seeds, further growing into the Si 3 N 4 nanowires. Finally, the Si and N atoms deposited on the surface of Si 3 N 4 nanowires and rearranged to form submicron rods. [ABSTRACT FROM AUTHOR]

A simple one-pot colloidal method has been described to engineer ternary CuInS2 nanocrystals with different crystal phases and morphologies, in which dodecanethiol is chosen as the sulfur source and the capping ligands. By a careful choice of the anions in the metal precursors and manipulation of the reaction conditions including the reactant molar ratios and the reaction temperature, CuInS2 nanocrystals with chalcopyrite, zincblende and wurtzite phases have been successfully synthesized. The type of anion in the metal precursors has been found to be essential for determining the crystal phase and morphology of the as-obtained CuInS2 nanocrystals. In particular, the presence of Cl− ions plays an important role in the formation of CuInS2 nanoplates with a wurtzite–zincblende polytypism structure. In addition, the molar ratios of Cu to In precursors have a significant effect on the crystal phase and morphology, and the intermediate Cu2S–CuInS2 heteronanostructures are formed which are critical for the anisotropic growth of CuInS2 nanocrystals. Furthermore, the optical absorption results of the as-obtained CuInS2 nanocrystals exhibit a strong dependence on the crystal phase and size. [ABSTRACT FROM AUTHOR]

Highlights: [•] A simple one-pot colloidal approach has been developed to prepare CuInS2 nanoplates. [•] The as-obtained CuInS2 nanoplates possess a wurtzite phase. [•] Higher temperature and time extension can facilitate the growth of CuInS2 nanoplates through the cation inter-diffusion. [•] The as-obtained CuInS2 nanoplates can self-assemble into face-to-face columns. [ABSTRACT FROM AUTHOR]

Ceramic foams with multi‐scale pores and large specific surface area have received extensive attention due to their unique structure and superior properties. Considering that there are still challenges to synthesize porous ceramics with large specific surface area, a novel ceramic foam material with ultra‐large specific surface area has been prepared using hollow silica mesoporous spheres (HMSSs) as building block in this work. These building blocks were made weakly hydrophobic in order to produce HMSS particle stabilized foams. The foams exhibit a uniform primary macropore structure, which is composed of a three dimensional HMSS‐assembled network, via HMSS‐stabilized foams. The influence of sintering temperature on the microstructure and properties of HMSS foams is investigated. The HMSS foams exhibit highest specific surface area of 1733 m2/g, attributed to the radial mesopores in HMSS shell, when sintered at between 500°C and 800°C. This specific surface area is much higher than that of existing ceramic materials. The uniform pore structure and ultra‐large specific surface area make it a promising lightweight material in potential application fields, including catalyst, adsorption, fire‐resistant thermal insulation, and load and control release system. Hollow mesoporous silica spheres assembled foam material with uniform macro‐pore structure and ultra‐high specific surface area, prepared from HMSSs stabilized colloidal foams [ABSTRACT FROM AUTHOR]

To eliminate the excess grain boundary phase in silicon nitride ceramics prepared through hot pressing and a post-sintering heat treatment, carbon was introduced into the sintered ceramics. This was accomplished in two ways: first, a small amount of carbon was used as a sintering additive; however, this creates large pores in the ceramics, resulting in a decrease in the thermal conductivity and mechanical properties of the samples. Alternatively, the C/Si 3 N 4 power bed could be used instead of the BN/Si 3 N 4 power bed, which obviously eliminates the grain boundary phase. Further, the C/Si 3 N 4 power bed possess excellent mechanical properties while increasing the thermal conductivity of the samples. Therefore, the silicon nitride ceramics with 123 W m−1 K−1 thermal conductivity and 986 MPa bending strength of can be obtained using the C/Si 3 N 4 powder bed during the post-sintering heat treatment. [ABSTRACT FROM AUTHOR]

A novel alternating current-assisted sintering (ACS) technology was applied to prepare Al 2 O 3 ceramics. The results show that, compared to conventional pressureless sintering (PS), the ACS method has remarkable advantages in reducing the sintering temperature and shortening sintering time. Specifically, submicron dense Al 2 O 3 ceramics could be obtained by ACS with a soaking time of 5 min and a sintering temperature of 1400 °C. A higher sintering temperature (1500 °C) and longer soaking time (1 h) were necessary for PS to reach a similar density. The maximum flexural strength and hardness of alternating current-assisted sintered (ACSed) samples were 864 MPa and 19.24 GPa, respectively. For the ACSed and pressureless sintered (PSed) samples whose densities and grain sizes were similar, the flexural strength of the ACSed ones was more than 50% higher than that of the PSed ones. The results suggest that the key to achieving rapid densification is the fast heating rate and mechanical load. In conclusion, the ACS technology, whose cost is lower than spark plasma sintering (SPS), is a viable alternative for preparing high performance Al 2 O 3 ceramics. [ABSTRACT FROM AUTHOR]

Three‐dimensional ceramic nanofiber‐assembled materials with large specific surface area and excellent thermal insulation properties are attracting increasing interests for their unique structure and promising applications. In this paper, we propose a facile methodology to fabricate three‐dimensional silicon nitride nanofiber‐knitted ceramic foams via in situ reactive synthesis from silicon foams. Silicon particle‐stabilized foams are fabricated for the first time using long‐chain surfactant cetyltrimethyl ammonium bromide as a hydrophobic modifier. First, the fabrication and stability of silicon foams are investigated. Based on the stable silicon foams, silicon nitride‐based nanofiber‐knitted ceramic foams are synthesized via in situ reactive sintering in nitrogen atmosphere. The novel ceramic foam materials consist of three‐dimensional nanofiber‐assembled strut wall and nanofiber‐spheres in the pores. The diameter of obtained silicon nitride nanofibers ranges from 15 to 100 nm. The unique nanofiber‐knitted foams may have potential applications in specific fields, including catalysis, adsorption, separation, and thermal insulation. [ABSTRACT FROM AUTHOR]

A novel kind of nanowire sponges, namely Si3N4 nanowire‐weaving microspheres, synthesized from a simple, convenient, high‐efficient approach are proposed here. As the reverse template, three‐dimensional foam skeleton structure with uniform pores and ultrathin pore walls is constructed via the effective particle‐stabilized foam method, where the silica sol and carbon black are chosen as the raw materials, providing the sufficient space for the growth of nanowires during the carbothermal reduction reaction process. The formation mechanism of this novel sponge is studied via multiple characterization methods. Si3N4 nanowires formed microspheres possess uniform and curving morphology due to the stable environment for growing via vapor–solid mechanism, leading to the relatively high specific surface area of 86.77 m2/g. Owing to in‐situ oxidation process, micro‐spherical SiO2 nanowire sponges with similar morphology are synthesized, which present diameter in range of 20‐40 nm and specific surface area of 50.47 m2/g. This work provides insights for the design of high‐performance nanowire sponges with promising applications in the filtration, thermal insulators, and catalyst supports fields. [ABSTRACT FROM AUTHOR]

Abstract The mechanical strength of highly porous ceramic foams shows significant attenuation with the increase of porosity, especially for ceramic foams with very high porosity level above 95%. Ultra-light bulk composite ceramic foams were fabricated via introducing 20 wt% zirconia in alumina-based cell wall, aiming at improving mechanical strength of alumina based ceramic foams. The preparation and stability of alumina and zirconia particles synergistically stabilized foams were investigated. The obtained bulk ceramic foams manifested an exciting combination of both high porosity above 96% and excellent specific compressive strength, due to their closed pore structure, defect-free cell wall with uniform thickness, and uniformly distributed zirconia grains in alumina matrix. The alumina/zirconia composite ceramic foams with porosity of 96.7% exhibited compressive strength of 1.11 ± 0.07 MPa, which was much higher than that of alumina ceramic foams in the case of same porosity. [ABSTRACT FROM AUTHOR]

Abstract: The densification trajectories and mechanical properties of zirconia ceramics obtained by oscillatory pressure sintering (OPS) process were investigated, during the sintering process an oscillatory pressure was applied at three stages. Current results indicated that at intermediate stage the oscillatory pressure revealed a favorable improvement of mechanical properties compared with conventional hot pressing (HP) and pressureless sintering (PS) procedures, while the enhancement was not obvious at initial stage. When the oscillatory pressure was applied at final stage, the OPS specimens exhibited the highest bending strength and hardness of 1455 ± 99MPa and 16.6 ± 0.31GPa compared with the PS and HP specimens. Considering the high elastic modulus and Moiré patterns observed in the OPS specimen, the oscillatory pressure applied at intermediate and final stages was detected to facilitate the sliding of grain boundary, plastic deformation of monolithic grains, the removal of pores and the strengthening of atomic bonds. [ABSTRACT FROM AUTHOR]