Spark plasma sintering of boron nitride micron tubes reinforced boron carbide ceramics with excellent mechanical property

In this paper, the novel boron nitride micron tubes (BNMTs) were used to reinforce commercial boron carbide (B₄C) ceramics prepared via spark plasma sintering technology. The effects of the sintering parameters, sintering temperature, the holding time, and the BNMTs content on the microstructure and mechanical properties of B₄C/BNMTs composite ceramics were studied. The results indicated that adding a proper amount of BNMTs could inhibit the grain growth of B₄C and improve the fracture toughness of the B₄C/BNMTs composite ceramics. The prepared composite ceramic sample with 5 wt% BNMTs at 1850°C, 8 min and 30 MPa displayed the best mechanical properties. The relative density, hardness, fracture toughness, and bending strength of the samples were 99.7% ± .1%, 35.62 ± .43 GPa, 6.23 ± .2 MPa m^1/2, and 517 ± 7.8 MPa, respectively. Therein, the corresponding value of hardness, fracture toughness, and bending strength was increased by 10.3%, 43.59%, and 61.5%, respectively, than that of the B₄C/BNMTs composite ceramic without BNMTs. It was proved that the high interface binding energy and bridging effect between boron carbide and BNMTs were the toughening principle of BNMTs.     

The effect of boron nitride nanosheets on the mechanical and thermal properties of aluminum nitride ceramics

The boron nitride nanosheets (BNNSs)/aluminum nitride (AlN) composites were prepared by hot press sintering at 1600°C. The microstructure, mechanical properties, and thermal conductivity of the samples were measured, and the effect of adding BNNSs to AlN ceramics on the properties was studied. It is found that the addition of BNNSs can effectively improve the mechanical properties of AlN. When the additional amount is 1 wt%, the bending strength of the sample reaches the maximum value of 456.6 MPa, which is 23.1% higher than that of the AlN sample without BNNSs. The fracture toughness of the sample is 4.47 MPa m^1/2, a 68.7% improvement over the sample without BNNSs. The composites obtained in the experiment have brilliant mechanical properties.     

ROMP‐based boron nitride composites

The present work focuses on the investigation of the thermal and dielectric properties of composites obtained by surface‐modified hexagonal boron nitride (hBN) and ring‐opening metathesis polymerization (ROMP) based polymer. A new kind of high performance composites was developed based on using amino silane functionalized hBN (AS‐hBN) and bromine functional group possessing homo and copolymers synthesized via ROMP pathway. Aminosilane capped boron nitride (BN) and bromine bearing polymer backbone enhance the interaction between filler and the polymer chains. The effects of surface‐modified BN (AS‐hBN) and its content on the dielectric properties, and thermal resistance of composites, are systematically investigated and discussed. The resultant composites possess high electrical break over voltages. While all of the ROMP‐based films exhibit low ?′ value in a wide frequency range, in the case of the composite with 20% AS‐hBN and poly(bromooxanorbornene‐co‐cyclooctadiene) (ROMP‐BN‐6) displays very low dielectric constants in around 1.5 up to 1 MHz at 20 °C. This value is significantly lower than that of common polymer dielectrics, which is usually in the range of 3–6. Besides the lowest dielectric constant of ROMP‐BN‐6, it has also the smallest dielectric loss tangent even at high temperatures. Tan δ of ROMP‐BN‐6 is 0.003 and 0.0067 at 10 Hz–1 MHz at 20 °C, respectively. Thermal stability of polymers was also improved by introducing surface‐modified hBN. Polymers bearing 20% AS‐hBN are highly thermally stable up to ～350 °C and gave 25% char yield at 800 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45658.     

Fabrication and mechanical properties of boron nitride nanotube reinforced silicon nitride ceramics

In this study, silicon nitride (Si₃N₄) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated by hot-pressing method. The influence of sintering temperature and BNNTs content on the microstructures and mechanical properties of Si₃N₄ ceramics were investigated. It was found that both flexural strength and fracture toughness of Si₃N₄ were improved when sintering temperature increases. Moreover, α-Si₃N₄ phase could transform into β-Si₃N₄ phase completely when sintering temperature rises to 1800 °C and above. BNNTs can enhance the fracture toughness of Si₃N₄ dramatically, which increases from 7.2 MPa m^1/2 (no BNNTs) to 10.4 MPa m^1/2 (0.8 wt% BNNTs). However, excessive addition of BNNTs would reduce the fracture toughness of Si₃N₄. Meanwhile, the flexural strength and relative density of Si₃N₄ decreased slightly when BNNTs were added. The related toughening mechanism was also discussed.     

Thermal properties of polymer-derived ceramic reinforced with boron nitride nanotubes

We report the thermal properties of boron nitride nanotube (BNNT) reinforced ceramic composites using the polymer derived ceramic (PDC) processing route. The nano-composites had a BNNT loading of up to 35.4 vol.%. TGA results showed that nano-composites have good thermal stability up to 900°C in air. BNNTs in nano-composites survived in an oxidizing environment up to 900°C, revealing that nano-composites can be used for high temperature applications. Thermal conductivity of PDC reinforced with 35.4 vol.% BNNT was measured as 4.123 W/(m·K) at room temperature, which is a 2100 % increase compared to that of pristine PDC. The thermal conductivity value increases with the increase of BNNT content. A thermal conductivity percolation phenomenon appeared when the BNNT content increased to 36 ± 5 vol.%. The results of this study showed that BNNTs could effectively improve the thermal conductivity of PDC materials. BNNT reinforced PDC could be used as thermal structural materials in a harsh environment at temperatures up to 900°C.     

先驱体法制备氮化硼纤维的研究

以三聚氰胺和硼酸为原料,用有机化学法合成先驱体,制备氮化硼纤维。采用中和滴定法、红外吸收光谱、X射线衍射及扫描电镜进行氮化硼纤维的氮含量的测定及结构分析。结果表明:合成的先驱体是结晶体,晶体发育良好;制得的氮化硼纤维具有B-N键、B-N六元环的特征吸收;随着氮化处理温度的提高, 氮化硼纤维的氮含量增加;用扫描电镜观察1 700℃制得的氮化硼纤维的直径为2-5μm,长径比为20- 100,氮含量为53．46％。     

Metal‐free boron nitride adsorbent for ultra‐deep desulfurization

Activated metal‐free boron nitride (BN) adsorbent has been prepared by a surfactant assisted regulation strategy. By tuning the variety of surfactants (such as P123, PVP, F127), usage and reaction temperature, the adsorptive performance of activated BN was optimized. The optimized BN‐P123 adsorbent displays porous structure with a high surface area about 1185 m²/g and exhibits excellent adsorptive desulfurization activity for dibenzothiophene (45.7 mg S/g adsorbent for 500 ppmw sulfur model oil), which is comparable or even superior to the state‐of‐the‐art adsorbent. Additionally, this activated BN‐P123 could realize the ultra‐deep desulfurization through adsorptive process to reach the latest international standard (less than 10 ppmw). Considering the nontoxic metal‐free feature and the excellent adsorption performance, the obtained activated BN‐P123 may be a powerful candidate to meet the requirements of potential industrial applications. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3463–3469, 2017     

Synthesis of nanocrystalline boron nitride by combustion process

Nanocrystalline boron nitride powders were synthesized by combustion process using urea as a fuel. Experiments were carried out by heating boric acid and urea in an N₂ atmosphere at 850°C. Boric acid was used as a source of boron while urea, as a source of nitrogen. The reactions were carried out in an autoclave with provisions for purging with nitrogen gas. The samples were characterized by powder X-ray diffraction, Fourier transform IR spectroscopy (FT-IR), FT-Raman spectroscopy, UV-VIS spectroscopy, and SEM. The article is published in the original.     

Physical and mechanical properties of hexagonal boron nitride ceramic fabricated by pressureless sintering without additive

Hexagonal boron nitride hBN ceramic was successfully fabricated by pressureless sintering at 2100C using submicrometre hBN powders without any sintering additive. The as-prepared hBN ceramic showed a room temperature flexural strength of 30.7MPa. Its flexural strength increased with the increment of temperature in N₂ atmosphere, and it retained a strength of 57.2MPa nearly two times of the room temperature strength at 1600C due to clean grain boundaries with no glassy phase. Additionally, the as-prepared hBN ceramic showed a high thermal conductivity of 31.76Wm¹k¹ and a good thermal shock resistance, which retained a relatively high residual flexural strength of 22.6MPa 73.5 of the original flexural strength at T800C. The as-prepared hBN ceramic presents a good application prospect at high temperature.     

Extension‐induced mechanical reinforcement in melt‐spun fibers of polyamide 66/multiwalled carbon nanotube composites

In this work, polyamide 66 (PA66) and its composites with multiwalled carbon nanotubes (MWNTs) were melt spun into fibers at different draw ratios. PA66 fibers at high draw ratio demonstrate a 40% increase in tensile strength, 66% increase in modulus and a considerable increase in toughness. It is demonstrated that this reinforcement can be mainly attributed to high‐draw‐ratio‐induced good dispersion and orientation of MWNTs, particularly the enhanced interfacial adhesion between MWNT and matrix thanks to interfacial crystallization. Our work provides a simple but efficient method to achieve good dispersion and strong interfacial interaction through melt spinning. Copyright © 2011 Society of Chemical Industry     

Wet-chemical coating of silicon carbide fibers with hexagonal boron nitride layers

Hexagonal BN fiber coatings and BN powders were prepared by pyrolysis of the raw materials boric acid and urea in an atmosphere consisting of hydrogen and nitrogen. The powders were used to determine the appropriate mixing ratio of the raw materials to produce BN with the desired composition and crystal structure. The pyrolysis of boric acid and urea in a molar mixing ratio of 1:2 resulted in a BN that was hexagonal and had a near-stoichiometric composition.To prepare a solution for the coating of fibers, boric acid and urea were dissolved in an ethanol-water mixture. The coating was then applied to SiC filaments using a continuous roll-to-roll dip-coating process. It could be shown by SEM/EDS that BN layers were applied to the fibers. No significant bridging in the fiber bundle was found. Furthermore, it could be demonstrated by grazing incidence x-ray diffraction that the layers were crystalline.     

High hardness cubic boron nitride with nanograin microstructure produced by high‐energy milling

High‐energy shaker milling of hexagonal boron nitride (hBN) powders was used to produce powders rich in sp³ bonding. The powders contained up to 68% sp³ bonding and were found to nucleate nanosize cBN grains during consolidation at 5.5 GPa and 1400°C. The effect of hBN starting particle size, milling time, and powder‐to‐milling ball ratio were studied. The amount of sp³ bonding for milled hBN powders was determined, using ¹¹B solid‐state NMR. The milled material was also analyzed by XRD, Raman spectroscopy, and HRTEM. The results indicate that the material has a nanosized microstructure comprised of a disordered hBN matrix and cBN nuclei in the form of sp³‐rich domains. Eight different milled powders were produced and consolidated at pressures of either 5.5 or 6.5 GPa and temperatures of either 1400°C or 1450°C into 12 mm diameter and 5 mm thick pellets. Consolidated pellets formed from milled hBN with 68% sp³ bonding had Vickers hardness of 42 ± 1 GPa and fracture toughness 3.8 ± 0.1 MPa.m^1/2. Vickers hardness of 49 ± 1 GPa and fracture toughness of 4.6 ± 0.1 MPa.m^1/2 was achieved with a precursor that contained milled hBN and 50 vol. % of 0.5 μm diameter cBN crystals.     

Enhanced thermal conductivity of boron nitride epoxy‐matrix composite through multi‐modal particle size mixing

Castable particulate‐filled epoxy resins exhibiting excellent thermal conductivity have been prepared using hexagonal boron nitride (hBN) and cubic boron nitride (cBN) as fillers. The thermal conductivity of boron nitride filled epoxy matrix composites was enhanced up to 217% through silane surface treatment of fillers and multi‐modal particle size mixing (two different hBN particle sizes and one cBN particle size) prior to fabricating the composite. The measurements and interpretation of the curing kinetics of anhydride cured epoxies as continuous matrix, loaded with BN having multi‐modal particle size distribution, as heat conductive fillers, are highlighted. This study evidences the importance of surface engineering and multi‐modal mixing distribution applied in inorganic fillered epoxy‐matrix composite. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Processing and properties of polycrystalline cubic boron nitride reinforced by SiC whiskers

Dense polycrystalline cBN (PcBN)–SiCw composites were fabricated by a two-step method: First, SiO₂ was coated on the surface of cubic boron nitride (cBN) particles by the sol-gel method. Then, silicon carbide whisker (SiC_w)- coated cBN powder was prepared by carbon thermal reaction between SiO₂ and carbon powders at 1500°C for 2 hour. Then, cBN–SiC_w complex powders were sintered by high-pressure and high-temperature sintering technology using Al, B, and C as sintering additives. The phase compositions and microstructures of cBN–SiC_w composites were investigated by X-ray diffraction and scanning electron microscopy, respectively. It was found that the SiC_w and Al₃BC₃ had been fabricated by in situ reaction, which cannot only promote densification but also improve mechanical properties. The relative density of PcBN composites increased from 96.3% to 99.4% with increasing SiC_w contents from 5 to 20 wt%. Meanwhile, the Vickers hardness, fracture toughness and flexural strength of as-obtained composites exhibited a similar trend as that of relative density. The composite contained 20 wt% of SiC_w exhibited the highest Vickers hardness and fracture toughness of 42.7 ± 1.9 GPa and 6.52 ± 0.21 MPa•m^1/2, respectively. At the same time, the flexural strength reached 406 ± 21 MPa.     

Large-scale fabrication of boron nitride nanotubes with high purity via solid-state reaction method

An effective solid-state reaction method is reported for synthesizing boron nitride nanotubes (BNNTs) in large scale and with high purity by annealing amorphous boron powder and ferric chloride (FeCl₃) catalyst in ammonia atmosphere at elevated temperatures. FeCl₃ that has rarely been utilized before is introduced not only as a catalyst but also as an efficient transforming agent which converts boron powder into boron chloride (BCl₃) vapor in situ. The nanotubes are bamboo in shape and have an average diameter of about 90 nm. The effect of synthetic temperatures on nanotube morphology and yield is investigated. The photoluminescence (PL) measurement shows emission bands of the nanotubes at 354, 423, 467, and 666 nm. A combined growth mechanism of vapor–liquid-solid (VLS) and solid–liquid-solid (SLS) model is proposed for the formation of the BNNTs.     

Dielectric properties of polymer-derived ceramic reinforced with boron nitride nanotubes

The electrical and dielectric properties of boron nitride nanotubes (BNNTs) reinforced ceramic composites using the polymer-derived ceramic (PDC) processing route were investigated in this work. The electrical resistivity of the pristine PDC increases from 10⁶ to 10⁸ Ω m after the addition of BNNTs. When the BNNT loading was increased to 5 wt%, the average real relative permittivity of the PDC decreased from 2.94 to 2.80, while the quality factor (Q) of the PDC increased from 134.40 to 176.77. The BNNTs can increase the Q factor of the PDC due to the reduction in the porosity cause by the introduction of the BNNTs. Further increasing the BNNT content decreases the real relative permittivity of the nanocomposites and increases the Q factor at high frequency. The average real relative permittivity decreases to 2.29, while the average Q factor increases to 208.60 when the BNNT content is increased to 30 wt%. The dielectric loss after the addition of high fraction of BNNTs can be explained by the Lorentz resonance relaxation process. Results of this work showed that PDC-BNNT nanocomposites are satisfactory electromagnetic transparent materials when the BNNT fraction is less than 10 wt%.     

Microwave‐assisted in situ polymerization of polycaprolactone/boron nitride composites with enhanced thermal conductivity and mechanical properties

Polycaprolactone/boron nitride (PCL/BN) composites were prepared by microwave‐assisted ring‐opening polymerization of ε‐caprolactone (ε‐CL). In order to improve the dispersibility and interfacial interaction between BN fillers and PCL matrix, hydroxyl functional BN (mBN) was first prepared to be used as a macroinitiator for ε‐CL. Then BN grafted PCL (BN‐g‐PCL) copolymers were obtained via the in situ method, which acted as in situ compatibilizers in the PCL/BN composites. Various techniques were applied to characterize the mBN and PCL/BN composites. The Fourier transform infrared spectroscopy results confirm the structure of the BN‐g‐PCL copolymer. Field emission SEM graphs exhibit that, for the PCL/mBN composites, the mBN presents a homogeneous dispersion in the matrix and interfacial adhesion between the PCL and mBN is improved. These are beneficial for enhancing the thermal conductivity of the PCL/mBN composites. Notably, the PCL/mBN composite with 5 wt% mBN loading achieves the highest thermal conductivity of 0.55 W m^?1 K^?1, which is 2.75 times higher than that of pure PCL, 0.20 W m^?1 K^?1. This indicates that the excellent dispersion and interfacial adhesion could lead to the construction of continuous thermal conductive paths at a low BN loading and reduce the heat loss caused by phonon scattering in the interface. Furthermore, mBN could help to improve the mechanical properties of the composite. On adding 5 wt% mBN, the tensile strength and tensile modulus of the composite are 1.58 and 2.05 times higher, respectively, than those of PCL. © 2020 Society of Chemical Industry     

Effects of acid dyes on crystallization and mechanical properties of melt‐reprocessed nylon 66

Nylon fibers dyed with different types of acid dyes were melt reprocessed using a compression‐molding machine. The crystalline structure and mechanical properties of the melt‐reprocessed nylon were experimentally evaluated. It was found that metal complex acid dyes showed much more distinct effects on the structure and mechanical properties of melt‐reprocessed nylon than nonmetallized acid dyes. They decreased the crystallization rate of the molten nylon and reduced its crystallinity. They also reduced the imperfect form I structure in the crystalline region. Compression‐molded nylon samples showed inferior mechanical properties in the presence of metal complex acid dyes. The coordinate bonds between the Cr³⁺ ions and amide groups are possibly formed in melt‐reprocessed nylon, which could be the reason for the changes in the structure and properties of melt‐reprocessed nylon. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2386–2396, 1999     

Thermal conductivity and mechanical properties of thermally conductive composites based on multifunctional epoxyorganosiloxanes and hexagonal boron nitride

As electronics become portable and compact with concomitant thermal issues, the demand for high-performance thermal interface materials has increased. However, the low thermal conductivity of polymers and the poor dispersion of fillers impede the realization of high filler loading composites, and this in turn limits the increase in thermal conductivity. To overcome this, multifunctional epoxyorganosiloxanes (MEOSs) were synthesized and used to fabricate thermally conductive composites in this study. In the first part of this study, the effect of the molecular weights of MEOSs on the curing behaviors of the MEOSs/trimethylolpropane tris(3-mercaptopropioante)/1-methyl imidazole systems was investigated by a DSC analysis. Both the nonisothermal and isothermal curing of the epoxy compositions (ECs) verified that the reaction rate of EC-1 containing MEOS-1 with lower molecular weight was faster than that of EC-2. In addition, mechanical properties of the cured EC-1 were superior to those of its counterpart because of a higher density in crosslinking. In the second part, EC-1 was admixed with h-BN to fabricate thermally conductive (TC) composites. Owing to the low viscosity (1.6 Pa s at 0.1 Hz) of EC-1, a TC-3 composite containing 45 wt% h-BN fillers was obtained, and the in-plane and through-plane thermal conductivity of the cured TC-3 composite reached 3.55 ± 0.29 Wm^?1K^?1 and 1.08 ± 0.08 Wm^?1K^?1, respectively. Furthermore, the tensile modulus of the cured TC-3 was measured as 76.3 ± 6.1 MPa, which was 9.1 times higher than that of the cured EC-1. Both the high thermal conductivity and good mechanical properties of the cured TC-3 composite were ascribed to the percolation of h-BN networks stemming from the high filler loading.     

In-situ X-ray observation of phase transformation of rhombohedral boron nitride under static high pressure and high temperature

In-situ X-ray diffraction study for phase transformation of rhombohedral boron nitride (rBN) to a denser phase was performed under static high pressure (HP) and high temperature (HT) up to 9 GPa and 1600 °C. It was found that the layer stacking sequence of rBN structure began to change at less than 1 GPa, and the phase transformation to wurtzite structure (wBN) was observed at 6–7 GPa and room temperature. After conversion to wBN, further transformation to the zincblend type cubic structure (cubic BN) at 8 GPa and 1400 °C was observed, which is quenchable and the P-T conditions yielding cBN form were similar to that from hexagonal boron nitride. The observed behavior of the phase transformation of rBN by using in-situ X-ray diffraction study is well consistent with the results obtained from the quenching experiment from HP/HT by using belt type HP apparatus.

No structural change was observed at 600°C isothermal compression up t0 8GPa, while wBN formation was observed at room temperature compression at 7 GPa. This variation of the transformation behavior under HT isothermal compression may essentially be caused by the reduction of shear stress which affects the rotation and/or slip of hexagonal plane of rBN under HP.