Dense additive-free bulk boron nitride ceramics developed by self-densification of borazine

Boron nitride (BN) ceramics are promising candidates for high-temperature structural and functional materials. However, it is difficult to sinter dense additive-free bulk BN monoliths because of the covalent bond and flake structure. Here, we report dense bulk BN with relative density higher than 95% without sintering additives achieved via a self-densifying approach of borazine within a wide temperature range from 800 °C to 1800 °C. A polyborazylene with a controlled degree of cross-linking was synthesized and press-molded into shaped green bodies, which were pressureless pyrolyzed into porous frameworks and then densified through repeated borazine infiltration and pyrolysis method. The microstructural and crystalline evolution of the bulk BN ceramics, as well as the corresponded mechanical, thermal and dielectric properties were investigated.     

Micromorphology and structure of pyrolytic boron nitride synthesized by chemical vapor deposition from borazine

Pyrolytic boron nitride (PBN) plates were synthesized by chemical vapor deposition (CVD) with temperatures of 900–1900?°C and total pressures of 50–1000?Pa on graphite by using borazine as the precursor. The effects of temperature and pressure on the micromorphology and crystal structure of the PBN were investigated. The as-deposited PBN possessed three typical types of micromorphologies depending on the deposition condition. PBN with dense and laminated structure (Type A) were deposited at temperatures of 1150–1900?°C with relative low pressures of 50–200?Pa, and PBN with porous and isotropic structure (Type C) was deposited at temperatures above 1100?°C with higher pressures above 250?Pa. PBN with dense and glass-like fracture structure (Type B) was obtained at the other range of the deposition condition. The interlayer spacing (d₍₀₀₂₎) and the preferred orientation (PO) of the crystallite were calculated by using XRD data of the PBN plates. The degree of the preferred orientation tended to be higher with the increase of temperature and decrease of pressure, and higher temperature led to smaller value of d_(002). The crystal growth mechanism of the three types of PBN was discussed.     

Permeability and corrosion resistance of porous SiOC-bonded SiC ceramics prepared by the preceramic polymer

Porous SiC ceramics have been used in high temperature flue gas filtration fields because of their excellent properties such as high strength, high temperature resistance, corrosion resistance, and long service time. This work reports the porous SiOC-bonded SiC ceramics prepared at low temperature. The properties of porous SiC ceramics were first investigated with silicone resin content from 10 to 25 wt%, and then the effects of different pore-forming agent contents on the behaviors of porous SiC ceramics were discussed by adjusting poly (methyl methacrylate) PMMA microbeads from 5 to 20 wt%. The prepared porous SiC ceramics showed apparent porosity from 17.3% to 57.7%, compressive strength from 6 to 216 MPa, and Darcy permeability k₁ ranging from 7.02 × 10⁻¹⁴ to 1.45 × 10⁻¹² m². The corrosion behavior of porous SiC ceramics was investigated in acidic and alkaline media. The porous SiC ceramics showed better corrosion resistance in acidic solutions.     

Fabrication and properties of borazine derived boron nitride bonded porous silicon aluminum oxynitride wave-transparent composite

Boron nitride bonded porous silicon aluminum oxynitride composite was fabricated by gel-casting, precursor infiltration and pyrolysis process, and the composition, microstructure, mechanical and dialectical properties of the composite were characterized. The results show that the composite is comprised of β-SiAlON (z = 3) synthesized from mixed ceramic powders, and continuous h-BN pyrolyzed from borazine, with a relatively high porosity of 24.21%. The flexural strength, elastic modulus and fracture toughness of the composite are 178.58 MPa, 75.51 GPa and 4.54 MPa m^1/2, respectively. The sintering shrinkage of SiAlON ceramics can be greatly decreased by the borazine infiltration and pyrolysis process. The existence of h-BN phase and the high porosity reduce the values of dielectric constant and loss tangent of the composite, which are 3.51–3.69 and 0.9–3.1 × 10⁻³ at the frequency from 7 to 18 GHz with the elevating temperature from 25 to 1200 °C.     

固相反应合成磷酸硼工艺研究

采用固相反应法以硼酸和磷酸二氢铵为原料制备磷酸硼 （BPO4）产品,研究了原料配比（氧化硼与五氧化二磷物质的量比）、反应温度对产物化学组成和结构的影响,确定的最佳工艺条件为：n（氧化硼）/n（五氧化二磷）为1.0～1.05,反应温度为850～1 000 ℃。用红外光谱（IR）、X射线衍射（XRD）研究了产品的物相结构。温度是影响物相结构的主要因素,当固相反应温度达到1 000 ℃时,所得产品晶体结构完整,产品质量稳定。采用热重-差热分析（TG-DTA）和差示扫描量热法（DSC）研究了高温固相反应过程,提出了该工艺的反应机理,并确定了在技术上、经济上可行的工艺路线。     

Bonding of silicon nitride using preceramic polymer and germanium powders for potential fuel cell applications

Abstract

A new method for bonding blocks of Si₃N₄ has been developed that produces bonds whose maximum service temperature is equal to the temperature used during the bonding process. In the present paper a system consisting of blocks of Si₃N₄ coated with a preceramic film containing a fine dispersion of silicon and a thin layer of germanium powder has been investigated to determine the effect of the thickness of the germanium film. The maximum service temperature is not determined by the melting point of the germanium since the germanium forms a higher melting point solid solution with the silicon in the film. Control of the thickness of the germanium film is found to be critical as a thicker layers results in lower strength bonds owing to differences in thermal expansion, and the maximum service temperature is lower owing to the lower liquidus temperature of the leaner Ge–Si solid solution. This technique has potential applications in fuel cells as a result of the small differences in thermal expansion coefficients and firing shrinkage in fuel cell materials, thus allowing successful fabrication and joining of monolithic solid oxide fuel cells (MSOFCs) with few defects.     

Determination of the miscibility of phosphorus additives in polymer matrices by high resolution solid state NMR

Summary The measurement of high resolution solid state NMR proton T_1p values (relaxation time in the rotating frame) provides a method for the study of the compatibility of phosphorus additives with polymer matrices. Results are compared for two commercial phosphorus stabilizers, bis (2,4 di-t-butylphenyl) pentaerythritol diphosphite (Phosphite 1) and tris (2,4 di-t-butylphenyl) phosphite (Phosphite 2), in 5% masterbatches in linear low density polyethylene (LLDPE). Phosphite 1 shows compatibility even when poorly mixed while the Phosphite 2 shows separation into two domains with different relaxation rates and different chemical shifts in the solid state.     

Structure of the boron nitride E-phase: Diamond lattice of B12N12 fullerenes

The molecular and crystal structures of the boron nitride E-phase are determined. Analysis of all calculated for the first time main peaks of X-ray diffraction patterns (6.28, 3.85, 3.28, 2.72, 2.50, 2.22, 2.09, 1.92 Å) in comparison with experimental ones indicates that E-BN has the diamond-type lattice (the cell constant a = 10.877 Å, the space group Fd3m or O_h⁷) formed with T_h symmetry B₁₂N₁₂ molecules copolymerized by the hexagonal faces. Hence we propose to mean “extradiamond” term instead of “explosion” term in the E-phase name. Extradiamond-B₁₂N₁₂ has a framework type of faujasite and can be referred to as [B₁₂N₁₂]-FAU zeolite. Ideal crystal of E-BN has 192 atoms per unit cell and theoretical density of 3.074 g/cm³.     

Atomic structure and electronic state of boron nitride fullerenes and nanotubes

Boron nitride (BN) fullerenes and nanotubes were obtained by applying the arc-melting method to a powder mixture of boron and iron oxide in a nitrogen gas. BN fullerenes of size 0.7 nm were observed. Angular tips of BN nanotubes of diameter 6.6 nm and length 50–100 nm were also observed. When using a powder mixture of boron and gold, BN fullerenes and nanotubes were not obtained. Atomic structure models for BN fullerene and nanotube were proposed, and the (BN)₃₆ cluster was found to be stable in spite of the existence of distorted six- and four-membered rings. The density of states for the cluster was calculated theoretically, and is similar to that of hexagonal BN. The prime novelty of the study is that BN fullerenes and BN nanotubes can be synthesized by a simple arc-melting method, and the stability of their atomic structure and the electronic state were predicted by theoretical calculations.     

Thermal behaviour of a series of poly[B-(methylamino)borazine] for the preparation of boron nitride fibers

The thermal behaviour of a series of poly[B-(methylamino)borazine] prepared at various temperatures ranging from 140 to 200 °C is studied in the present paper as potential BN fiber precursors. It was shown that the softening capability of poly[B-(methylamino)borazine] can be tailored by controlling the temperature at which polymers were prepared to achieve melt-spinning and produce high quality green fibers. Thus as-spun fibers could be next converted into boron nitride fibers using ammonia (25–1000 °C) and nitrogen (25–1800 °C) atmospheres. The quality of boron nitride fibers was shown to depend on the first part of the pyrolysis step (25 and 1000 °C; ammonia atmosphere) in which the great majority of the weight loss necessary for boron nitride production occurs. Ideal poly[B-(methylamino)borazine] as BN fiber precursors are those prepared between 170 and 180 °C. They display appropriate melt-spinnability and ceramic conversion capability.     

Thermal and electrical properties of aluminum nitride/boron nitride filled polyamide 6 hybrid polymer composites

The effects of boron nitride (BN) and aluminum nitride fillers on polyamide 6 (PA6) hybrid polymer composites were investigated. In particular, the thermal and electrical conductivity, thermal transition, thermal degradation, mechanical and morphological properties and chemical bonds characteristic of the materials with crystal structure of BN and aluminum nitride (AlN) filled PA6 prepared at different concentrations were characterized. Thermal conductivity of hybrid systems revealed a 1.6-fold gain compared to neat PA6. The highest thermal conductivity value was obtained for the composite containing 50 vol% additives (1.040 W/m K). A slight improvement in electrical conductive properties of composites appears and the highest value was obtained for the 50 vol% filled composite with only an increase by 3%. The microstructure of these composites revealed a homogeneous dispersion of AlN and BN additives in PA6 matrix. For all composites, one visible melting peak around 220°C related to the α-form crystals of PA6 was detected in correlation with the X-ray diffraction results. An improved thermal stability was obtained for 10 vol% AlN/BN filled PA6 composite (from 405.41°C to 409.68°C). The tensile strength results of all composites were found to be approximately 22% lower than pure PA6.     

29Si solid state NMR investigation of pozzolanic reaction occurring in lime-treated Ca-bentonite

Lime is widely used as additive to improve the mechanical properties of natural soil used in earthworks. However, the physico-chemical mechanisms involved are yet not well understood. In order to develop and optimize this treatment method, a better understanding of the interaction between lime and the minerals of the soils, in particular clay minerals, is required. In this study, Ca-bentonite was treated with 2, 5 and 10 wt.% of lime during 1 to 98 days. Modifications in the Si local environment were then monitored by solid state nuclear magnetic resonance to investigate the pozzolanic reaction. All the soil mineral phases contribute to the release of Si and to the pozzolanic reaction, with a rapid and total consumption of Si-polymorph and an exacerbated dissolution of montmorillonite. Mechanism of C–S–H formation, function of the Ca content in the system, was found to match the sorosilicate-tobermorite model described in cement systems.     

Silica, carbon and boron nitride monoliths with hierarchical porosity prepared by spark plasma sintering process

Silica SBA-15, carbon CMK-3, boron nitride (BN), the latter synthesized from the first two compounds as templates, are mesoporous materials in the form of powders. They have a high specific surface area and an important mesoporous volume. The porosity is organized with the hexagonal symmetric space group p6mm. For selected applications, it could be interesting to preserve these characteristics with materials in a well-defined shape at a macroscopic scale (few millimeters to centimeter). Spark plasma sintering (SPS) is a well-known technique which allows to prepare monoliths with relatively mild conditions. The SPS technique has been used on these mesoporous powders without charge or with a uniaxial charge and at temperatures of 600 °C, 800 °C for silica, 1100 °C, 1300 °C for carbon and 1600 °C, 1700 °C for boron nitride during 1–5 min. The nitrogen adsorption/desorption isotherms reveal that the obtained monoliths present high specific surface area (300–500 m²/g) and important mesoporous volume. The coexistence of interconnected mesoporosity and macroporosity (with volume’s close value) was observed by SEM and TEM, while the XRD and TEM characterization show that the mesoporosity organization is partially preserved.     

Ceramic foams and micro-beads from emulsions of a preceramic polymer

A commercially available solid silicone resin was dissolved in a solvent and emulsified via stirring in the presence of water and surfactant to form three different types of emulsions, namely water-in-oil (w/o), water-in-oil-in-water (w/o/w) and oil-in-water (o/w), by following different preparation procedures. After curing, thermosets possessing different morphologies, ranging from highly porous (monolithic) foams to porous micro-beads and solid micro-beads, formed. The samples kept their shape upon pyrolysis, and resulted in ceramic foams (via w/o) and porous micron sized (∼200 μm) spherical particles (via w/o/w) having more than 80 vol% of total porosity, while with o/w emulsification solid SiOC ceramic particles with an average diameter of ∼100 μm formed. Both surfactant and water altered the IR spectra for emulsion-derived thermoset samples, in comparison to the pure cured resin, but upon pyrolysis similar amorphous ceramics were obtained from all samples.     

Effect of ammonium chloride on the morphology of hexagonal boron nitride prepared by magnesium thermal reduction

Hexagonal boron nitride (h-BN) crystals with cylindrical and nanoplate shape were prepared by the magnesium thermal reduction method. A simple and novel strategy successfully controlled the preferred growth direction of h-BN crystal by adding a small amount of ammonium chloride to the reaction system, realized the transformation of the h-BN crystal growth mode from cylindrical to nanoplate, and the thickness of h-BN crystal nanoplates rapidly decreased from 2 μm to 40 nm. XRD results showed that the ammonium chloride content also significantly promoted the crystallinity of the synthesized h-BN nanoplates, and the G.I. index decreased from 2.4 to 1.9. Raman and TEM results showed that the crystallinity of synthesized h-BN was close to the properties of the single crystal.     

Densification and mechanical properties of boron carbide prepared via spark plasma sintering with cubic boron nitride as an additive

Hexagonal boron nitride (h-BN) can reinforce boron carbide (B₄C) ceramics, but homogeneous dispersion of h-BN is difficult to achieve using conventional methods. Herein, B₄C/h-BN composites were manufactured via the transformation of cubic (c-) BN during spark plasma sintering at 1800 °C. The effects of the c-BN content on the microstructure, densification, and mechanical properties of B₄C/h-BN composites were evaluated. In situ synthesized h-BN platelets were homogeneously dispersed in the B₄C matrix and the growth of B₄C grains was effectively suppressed. Moreover, the c-BN to h-BN phase transformation improved the sinterability of B₄C. The sample with 5 vol.% c-BN exhibited excellent integrated mechanical properties (hardness of 30.5 GPa, bending strength of 470 MPa, and fracture toughness of 3.84 MPa⋅ m^1/2). Higher c-BN contents did not significantly affect the bending strength and fracture toughness but clearly decreased the hardness. The main toughening mechanisms were crack deflection, crack bridging, and pulling out of h-BN.     

Facile obtainment of luminescent glass-ceramics by direct firing of a preceramic polymer and oxide fillers

Rare-earth (RE) doped glass-ceramics represent very interesting luminescent materials. The thermal annealing of the glass precursor causes the controlled precipitation of several crystalline phases, in which RE may be variously distributed, also with different oxidation states, e.g. Eu²⁺ and Eu³⁺. The present investigation demonstrates the feasibility of preparation of RE-doped alumino-boro-silicate glass-ceramics by direct firing in air (at 1000–1200 °C) of a preceramic polymer, filled with nano- and micro-sized particles, as an alternative to glass melting and annealing. In particular BaCO₃ or SrCO₃ micro-particles, mixed with nano-sized γ-Al₂O₃, were found to react with amorphous silica, available from the oxidative decomposition of a commercial silicone, yielding a strontium or a barium alumino-silicate phase. Boric acid micro-particles contributed both to the development of a liquid phase upon firing (promoting ionic interdiffusion) and to the formation of a La-borate phase, by interaction with La₂O₃ micro-particles. The blue and red luminescence of the obtained glass-ceramics is attributed to the incorporation of Eu²⁺ and Eu³⁺ ions, from nano-sized Eu₂O₃ filler, in alumino-silicate and borate phases, respectively.     

The impact of polymer matrix blends on thermal and mechanical properties of boron nitride composites

To improve mechanical and thermal properties of a hexagonal boron nitride platelet filled polymer composites, maleic anhydride was studied as a coupling agent and compatibilizer. Injection molded blends of acrylonitrile butadiene styrene (ABS), high-density polyethylene (HDPE), and maleic anhydride with boron nitride filler were tested for thermal conductivity and impact strength to determine whether adding maleic anhydride improved interfacial interactions between matrix and filler and between the polymers. Adding both HDPE and maleic anhydride to ABS as the matrix of the composite resulted in a 40% improvement in impact strength without a decrease in thermal conductivity when compared to an ABS matrix. The best combination of thermal conductivity and impact strength was using pure HDPE as the matrix material. The effective medium theory model is used to help explain how strong filler alignment helps achieve high thermal conductivity, greater than 5 W/m K for 60 wt % boron nitride. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48661.     

Mechanical,dielectric and thermal properties of porous boron nitride/silicon oxynitride ceramic composites prepared by pressureless sintering

Porous boron nitride/silicon oxynitride (BN/Si₂N₂O) composites were fabricated by pressureless sintering at 1650 °C with Li₂O as sintering aid. The influence of Li₂O and hexagonal boron nitride (h-BN) contents on phase, microstructure, mechanical, dielectric and thermal properties of the resulting porous BN/Si₂N₂O composites was investigated. Increasing Li₂O content facilitated densification and decomposition of Si₂N₂O into Si₃N₄. The apparent porosity of the composites increases with the h-BN content increases and Si₂N₂O grain growth was restrained by the dispersed h-BN particles. The dielectric properties and thermal conductivities (TC) were affected mainly by porosity. Porous BN/Si₂N₂O ceramic composites with 4 mol% Li₂O and 25 mol% BN exhibit both low dielectric constant (3.83) and dielectric loss tangent (0.008) with good mechanical and thermal performance, suggesting possible use as high-temperature structural/functional materials.     

High resolution solid state 13C NMR investigation of the deoxycholic acid / ferrocene inclusion compound

Summary The molecular mobility of deoxycholic acid (DCA) / ferrocene inclusion compound has been investigated by high resolution solid state ¹³C NMR spectroscopy. Compared to the single crystal of ferrocene, the molecular mobility of the ferrocene in the inclusion compound is enhanced because it is included in the large cavity of DCA. It is found that the relaxation time measurement by solid state ¹³C NMR can provide powerful evidence to confirm the formation of the inclusion compound. Received: 19 July 1999/Revised version: 27 September 1999/Accepted: 4 October 1999