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Si3N4 ceramics with improved thermal conductivity and mechanical properties were prepared by carbothermal reduction and two‐step sintering. The addition of 2 wt% C led to the development of an intergranular phase YNSiO2 with reduced oxygen content. Notably, setting the pre‐sintering temperature at 1450°C promoted the formation of a rough bimodal morphology within the Si3N4 ceramics. Furthermore, an extended holding period at 1850°C resulted in additional grain growth and the elimination of intergranular phases. The Si3N4 specimens produced at 1450°C for 3 h followed by 1850°C for 8 h, incorporating 2 wt% C, exhibited a notable continuity between Si3N4 grains. The thickness of the grain boundary layer was only about 1 nm, and the N/O atomic ratio of the intergranular phase was measured to be 0.42. Consequently, the resulting thermal conductivity, flexural strength, and fracture toughness of Si3N4 specimens were found to be 101 W·m−1·K−1, 712 ± 29 MPa, and 7.7 ± 0.2 MPa·m1/2, respectively. Compared with samples without C additive and directly sintered at 1850°C for 8 h, the Si3N4 specimens produced in this work showed a significant increase of more than 50% in thermal conductivity and about 17% in fracture toughness. [ABSTRACT FROM AUTHOR]

Boron nitride (BN) powders with near‐spherical particles of more than 10 µm were synthesized by templated carbothermal reduction and nitridation. The effects of processing factors, such as raw material mixing methods, C/B2O3 molar ratios, and carbon templates, were systematically studied, and the reaction mechanism was discussed. The experimental results showed that the integrity and morphology of the carbon templates was usually destroyed during ball milling or grinding, but preserved during magnetic stirring. The morphology of synthesized BN particles depended on the C/B2O3 molar ratio in starting reactant mixtures and gradually changed from sheet‐like to near‐spherical shape with decreasing C/B2O3 molar ratio. The morphology and size of synthesized BN particles agreed with that of the carbon templates, which means that the morphology and size of synthesized BN particles can be tailored by controlling that of the carbon template. The BN near‐spherical particles synthesized in this work may be used as thermally conductive fillers for electronic packaging. [ABSTRACT FROM AUTHOR]

When a small atom is induced into a larger lattice, new phonon scattering modes are aroused and thermal conductivity is significantly reduced by increased anharmonicity. This effect, called "rattling," has been reported in pyrochlores and some cage compounds, but not in α‐SiAlON. In this study, we reveal that in (Yb+Ca) co‐doped α‐SiAlON ceramics (YbaCabSi7.5Al4.5O1.5N14.5, x = a/[a+b]), the rattling effect plays a crucial role in reducing thermal conductivity. Samples with different Yb/Ca ratios (x = 0, 0.1, 0.2, 0.3..., 1.0) were sintered by spark plasma sintering (SPS) at 1600°C and their thermal diffusivity/conductivity were measured by the laser‐flash method. We found that substituting Ca2+ with smaller Yb3+ cations in Ca‐α‐SiAlON caused significant reduction in thermal conductivity. On the contrary, substituting Yb3+ with larger Ca2+ cations in Yb‐α‐SiAlON caused only slight reduction in thermal conductivity. A lowest thermal conductivity of 3.3 W/(m·K) was achieved, when x = 0.3 rather than x = 0.5 or 0.7. Further analysis of phonon scattering intensity confirmed that both intrinsic and extrinsic scatterings are enhanced in the sample of x = 0.3. This study demonstrates that by utilization of the rattling effect, α‐SiAlON, well‐known for its excellent mechanical properties, can be tuned to exhibit low thermal conductivity comparable to that of La2Zr2O7 and 8YSZ. [ABSTRACT FROM AUTHOR]

Highly dense α‐silicon nitride (Si3N4) ceramics were densified at a lower temperature of 1450°C under a moderate mechanical pressure of 50 MPa from commercial α‐Si3N4 powder and with conventional Al2O3–Y2O3 sintering additives. The fabricated α‐Si3N4 ceramics exhibited a hardness of 19 GPa, which is higher than that of the β‐Si3N4 ceramics usually obtained. The effect of sintering temperature and sintering pressure in the liquid phase sintering of the α‐Si3N4 ceramics is studied. The role of the dissolution–reprecipitation process and the particle rearrangement during densification of the α‐Si3N4 ceramics is discussed. [ABSTRACT FROM AUTHOR]

β‐Si3N4 powder with well‐dispersed spherical particles was synthesized by liquid‐phase modified carbothermal reduction and nitridation. The influence of the type and content of additives on the particle morphology was investigated. The liquid phase formed at the reaction temperature modifies the morphology of the Si3N4 powder. The Si3N4 products precipitated along the liquid‐solid interface, thus forming a spherical morphology and promoting the transformation from α‐Si3N4 to β‐Si3N4. The type of additives has a strong influence on the formation temperature and viscosity of the liquid phase and hence affects the product morphology. In the samples with CaF2 and CaO additives, sufficient liquid phase occurred and spherical particles were obtained. In the sample with Y2O3, however, the liquid phase was insufficient due to the higher formation temperature, thus resulting in an irregular particle morphology. [ABSTRACT FROM AUTHOR]

Si3N4 ceramics with tailored gradient in color and microstructure were prepared by a rapid cost‐effective one‐step approach. The gradient microstructure was obtained by the manipulation of the dissolution‐reprecipitation process, by controlling the sintering temperature and sintering additive content. In the Si3N4 ceramics, the β‐phase content gradually changed from 84% to 11%. The Si3N4 ceramics exhibited white color on one side and showed a hardness of 19 GPa and fracture toughness of 7 MPa·m1/2 and may be suitable for bio‐implantation applications. [ABSTRACT FROM AUTHOR]

α‐SiAlON is well known for its excellent mechanical properties, but its thermal properties are less investigated. In this work, we investigate the thermal conductivity of Ca α‐SiAlON ceramics with varying m and n values and prepared samples by spark plasma sintering (SPS) at 1600°C or gas pressure sintering (GPS) at 1800°C and 1900°C. The prepared Ca α‐SiAlON ceramics showed thermal conductivities ranging from 4.6 to 10.7 W/(mK) at 25°C. It was found that the thermal conductivity of the α‐SiAlON samples could be reduced by increasing either m or n or both. Besides, the samples prepared by SPS had lower thermal conductivity compared with those by GPS due to their smaller grain size, and the effect of m and n would be minor after m reached 2. These results provide basic knowledge for tuning α‐SiAlON's thermal conductivity for applications concerning thermal management. [ABSTRACT FROM AUTHOR]

Hexagonal boron nitride (h‐BN) has received considerable attention, due to its high thermal conductivity and electrical insulation. However, the intrinsic platelike structure with the strong anisotropic property restricts its applications, and it is necessary to synthesize isotropic spherical h‐BN particles (SP‐BNs) with submicron size. Till now, methods to prepare (SP‐BN) still exist problems, such as high oxygen impurities and pollution, generated by the ammonia and pyrolysis of precursors. Here, a relatively green reaction between the restricted template of carbon nanospheres and boron trioxide (B2O3) under elevated temperature is conducted, and the SP‐BNs with an average diameter of 200–300 nm (named Nano‐BN‐s) have been successfully synthesized. Comprehensive scanning electron microscopy, transmission electron microscopy, and X‐ray diffraction characterizations confirm the obtained products are spherical boron nitride. With the analysis of X‐ray photoelectron spectroscopy and Fourier transform infrared, the reaction mechanism is briefly discussed. These results indicate the reaction occurs on the restricted template of carbon nanospheres, and the C atoms are substituted by B and N atoms as the reaction progress, forming the Nano‐BN‐s. What is more, the restricted template method plays a key role in the design and improves h‐BN‐based materials in the future and may also be extended to form other novel materials. [ABSTRACT FROM AUTHOR]

α‐SiAlON ceramics with different stabilizing ions of Yb, Dy, Nd, Y, Ca, and binary stabilizing ions of (Yb + Ca) and (Yb + Nd) were prepared by spark plasma sintering at 1600°C and gas pressure sintering at 1800 and 1900°C, and their thermal conductivity was investigated. It was found that α‐SiAlON ceramics with larger and heavier stabilizing ions had lower thermal conductivity and the thermal conductivity could be further reduced by using binary stabilizing ions, which can be explained by phonon scattering from point defects. At the same time, the samples prepared at lower sintering temperatures showed smaller grain sizes and lower thermal conductivity. The relationship between the thermal diffusivity of samples and temperature was studied, where the dependence of inverse thermal diffusivity on temperature was better fitted by a quadratic fitting function than the usual linear one over a wide temperature range from 25 to 800°C. [ABSTRACT FROM AUTHOR]

Spherical Si@Si3N4 granules were first prepared by in situ combustion synthesis. The inside of the granules is dense without cracks, and the two phases at the interface are well‐bonded. More significantly, no binder was used between the Si3N4 grains on the surface layer, thereby avoiding phonons scattering of metal oxides. In addition, crystallographic orientation relationships between Si and Si3N4 were identified by coupling transmission electron microscopy with selected area electron diffraction technology, and much more potential matching planes are also predicted according to the edge‐to‐edge model. It was found that the minimization of the interfacial energy will promote the formation of a transition layer at the interface, leading to the well‐bonded between core and shell. On the basis of the observations, the underlying growth mechanism of the Si@Si3N4 was comprehensively analyzed and put forward. Benefiting from the remarkable merits, the as‐synthesized Si@Si3N4 granules showed great potential for alternative fillers in solving heat dissipation problems for electronic devices. [ABSTRACT FROM AUTHOR]

In this study, we successfully prepared red‐emitting AlN:Mn2+ spherical phosphors by using nitrogen pressure–assisted carbothermal reduction–nitridation method. The optimum preparation parameters were explored by changing nitrogen pressure, CaF2 content, reaction temperature, and the amount of MnCO3. It was confirmed that uniform spherical shape, large particle size, less impurity content, and suitable Mn2+ concentrations could effectively enhance the luminous intensity of AlN:Mn2+ phosphors. The structural compositions of the AlN:Mn2+ phosphors were characterized by X‐ray diffraction and X‐ray photoelectron spectroscopy, indicating that Mn2+ ions were successfully doped into AlN lattice by substitution of Al3+ ions, thus providing the red‐emitting luminescence centers. Based on the experimental results, we further proposed the underlying growth mechanism of spherical AlN phosphors. It was suggested that an adequate amount of liquid Ca‐aluminates was essential to achieve uniform spherical morphology and high fluorescence efficiency. [ABSTRACT FROM AUTHOR]

A novel deep‐red‐emitting phosphor Ca2ScNbO6:Mn4+ is prepared via a high‐temperature solid‐state reaction and its luminescent properties are systematically investigated. The results show that Mn4+‐activated Ca2ScNbO6 phosphors have broad absorption in ultraviolet region, and show bright deep‐red emission at 692 nm. The optimal doping concentration, crystal‐field strength, internal quantum efficiency, and mechanism of concentration and thermal quenching effects are discussed in detail. Moreover, NaF flux is screened out to improve both luminescent intensity and morphology of the phosphor. Finally, a red light‐emitting diode (LED) lamp is fabricated with as‐prepared Ca2ScNbO6:Mn4+ phosphors and a 365 nm LED chip. The electroluminescence spectra show a good overlapping with phytochrome PR and PFR absorbance. The results provided the as‐synthesized Ca2ScNbO6:Mn4+ phosphors a great potential in plant growth lighting. [ABSTRACT FROM AUTHOR]

Mn4+‐activated double‐perovskite type Ca2ScSbO6 (CSS) phosphors were synthesized via a high‐temperature solid‐state reaction. The phase purity and crystal structure of obtained samples were investigated by powder X‐ray diffraction (XRD). The successful incorporation of Mn4+ ions into CSS lattice was confirmed by the combination of energy‐dispersive X‐ray spectra (EDX) and X‐ray photoelectron spectra (XPS) results. The luminescent properties of the CSS phosphors, including the photoluminescence (PL) and PL excitation (PLE) spectra, commission international de l'clairage (CIE) chromaticity coordinates, fluorescence decay curves, quantum yields and temperature‐dependent PL spectra, were investigated in detail. Under 310‐nm excitation, the optimized CSS:0.3%Mn4+ phosphor exhibited bright deep‐red emission covering a narrow band from 640 nm to 720 nm, which overlaps with the absorbance of phytochromes. The Racah parameters B, C, and local crystal strength Dq were calculated to be 870, 2703, and 1984 cm–1, respectively. Particularly, the emission intensity of CSS:0.3%Mn4+ still remained 61.4% at 423 K compared with that at room temperature. Therefore, all these outstanding luminescent properties provided the as‐synthesized phosphors a great potential in plant growth lighting. [ABSTRACT FROM AUTHOR]

Under the urgent demand of high‐performance silicon nitride substrate (Si3N4), this paper reports a new gas transporting self‐propagating high‐temperature synthesis technology (GTST) to form Si3N4 coating on silicon carbide (SiC) substrate with the addition of NH4Cl, which may partly be a candidate of high‐performance Si3N4 substrate. The effect of NH4Cl on the formation of the Si3N4 coating is investigated, and the reaction mechanism with the participation of NH4Cl is discussed. The addition of the NH4Cl promotes the formation of the compact Si3N4 coating with a thickness of 20–80 μm on both sides of SiC substrate. During the reaction, NH4Cl acts as a carrier to convert solid and liquid Si particles into gas phase SiCl4, and transport it to SiC substrate. Finally, SiCl4 is nitride into Si3N4 coating under the higher temperature generated by the combustion wave. Moreover, the research mechanism can provide a guide for other complex ceramic coatings by GTST. [ABSTRACT FROM AUTHOR]

β‐SiC powders with different particle morphologies were prepared by combustion synthesis in an N2 atmosphere, with carbon black, carbon fiber, graphene, and graphite as carbon sources. With carbon black as the carbon source, single‐phase β‐SiC nanopowder with an average particle size around 20 nm was obtained. From carbon fiber and graphene, SiC fibers and sheets were produced, which were polycrystalline and consisted of lots of nanocrystals with random orientations. It was proposed that the synthesis of SiC occurs by gas–solid reaction and the growth of SiC is confined to the carbon particles. In the controlled growth of SiC, the morphology of the precursor carbon is preserved, offering an opportunity to control the particle morphology of SiC by changing carbon sources. [ABSTRACT FROM AUTHOR]

Coarse‐grained β‐Si3 N4 powders were prepared by combustion synthesis under N2 pressure of 6 MPa, with a low diluent content of not more than 10 wt.% and high reaction temperature of >1900°C. β‐Si3 N4 was obtained as the major phase in the products, except for a small amount of residual Si. The addition of carbon black was effective to reduce the residual Si, but resulted in the formation of β‐SiC when too much carbon black was used. The coarse‐grained β‐Si3 N4 powders consisted of β‐Si3 N4 crystals with an average thickness of more than 10 µm, and some crystals were thicker than 20 µm. The growth mechanism of the coarse β‐Si3 N4 crystals was discussed, associated with the particular reaction conditions in combustion synthesis. [ABSTRACT FROM AUTHOR]

In this study, blue‐emitting AlN:Eu2+ spherical phosphors were successfully synthesized for the first time by the carbothermal reduction nitridation (CRN) method, assisted with high nitrogen pressure, appropriate synthesis temperature, and the addition of CaF2. The influence of typical experimental parameters, such as N2 pressure, heating temperature, CaF2 content and Eu2+ concentration on the morphologies and luminescence properties of AlN phosphors were comprehensively investigated. The formation mechanism of spherical morphology were significantly proffered, indicating that sufficient liquid Ca‐aluminates during the AlN growth stage were essential for the spheroidization process under the action of surface tension. The synthesized AlN:Eu2+ spherical phosphors presented an intense blue emission band centered in the range of 427‐ 476 nm relative to the reaction temperature. The lifetime of AlN:Eu2+ phosphor was calculated to be around 1.89 μs. The temperature‐dependent PL spectra suggested that the emission band did not shift until 225°C. In addition, the spectral analysis strongly suggested that the luminescence property of AlN:Eu2+ phosphors was significantly enhanced by the large particle size, spherical morphology, reduced impurity content, and appropriate Eu2+ concentration. [ABSTRACT FROM AUTHOR]