Modification of BN nanosheets and their thermal conducting properties in nanocomposite film with polysiloxane according to the orientation of BN

A facile technique was developed to modify boron nitride (BN) nanosheets with iron oxides in order to fabricate highly-oriented polysiloxane/BN nanosheet composite films and their thermal properties were evaluated according to the orientation of BN. The surfaces of the BN nanosheets were modified with iron oxide nano particles by chemical vapor deposition, and their one-dimensional arrangement with variation of BN content was controlled under a magnetic field. The homogeneous suspension of BN nanosheets and pre-polymers of polysiloxane was cast on a glass spacer, and subjected to a magnetic field before the mixture was crosslinked. X-ray diffraction, transmission electron microscopy, and superconducting quantum interference device measurements were employed to identify the phases and amounts of iron oxide nano particles deposited on the BN nanosheets. The results revealed that the modified BN nanosheets were aligned either horizontally or vertically to the film plane, depending on the direction of magnetic flux with high anisotropy. The transmittance and thermal conductivity of the nano composite films were improved due to the orientation of the BN nanosheets inside the polymer matrix.     

Facile preparation of a polysiloxane-based hybrid composite with highly-oriented boron nitride nanosheets and an unmodified surface

A facile technique was developed to fabricate polysiloxane-based hybrid composite films containing boron nitride (BN) nanosheets using a nanopulse-width electric field. BN nanosheets assumed anisotrophic alignment under the electric field, without requiring surface coating with metallic nano particles despite the wide band gap. BN was dispersed by sonication in a pre-polymer polysiloxane mixture. The homogeneous suspension was cast on a glass spacer and subjected to either a DC electric field or a nanopulse-width electric field before the mixture was cured through polymerization. X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that BN nanosheets in the polysiloxane matrix were aligned with high anisotropy to the electric field direction, which was perpendicular to the film plane. The transmittance of the film samples, measured by UV–visible spectrometry, indicated that the composite, prepared using a nanopulse-width electric field manifested a significantly improved transmittance, compared with composites prepared without using the electric field.     

Epoxy resin-based nanocomposite films with highly oriented BN nanosheets prepared using a nanosecond-pulse electric field

Facile orientation of boron nitride (BN) with high anisotropy in epoxy resin-based nanocomposite films was performed in a polyepoxide matrix using a nanosecond-pulse electric field to generate a high electric flux. Control of the BN anisotropy was achieved in the polymer without damaging the composite films or requiring surface modification of the BN. The degree of BN orientation perpendicular to the nanocomposite film plane, which was parallel to the electric flux, could be controlled by applying the nanosecond pulse for different lengths of time before cross-linking. The resulting composite films with oriented BN nanosheets manifested improved thermal diffusivity compared to a composite prepared without orientation.     

Self-assemblies of linearly aligned diamond fillers in polysiloxane/diamond composite films with enhanced thermal conductivity

The relocation of diamond fillers was performed in polysiloxane-based composite films under different electric fields. The microscale diamond filler particles were dispersed by sonication in a prepolymer mixture of polysiloxane, followed by high-speed mixing. The homogeneous suspension was cast onto a polyamide spacer of microscale thickness and subjected to three different electric fields: AC, DC, and switched DC, before the mixture became cross-linked. Analysis revealed that self-assemblies of linearly aligned diamond fillers (LADFs) were fabricated in the composite film, connecting the film planes as bridges with different thicknesses depending on the applied electric field. Composites with assemblies of LADFs exhibited enhanced thermal conductivity and electrical insulation, and are attractive for application as thermal interface materials in the semiconductor industry.     

Desolvation-Triggered Versatile Transfer-Printing of Pure BN Films with Thermal–Optical Dual Functionality

Although hexagonal boron nitride (BN) nanostructures have recently received significant attention due to their unique physical and chemical properties, their applications have been limited by a lack of processability and poor film quality. In this study, a versatile method to transfer-print high-quality BN films composed of densely stacked BN nanosheets based on a desolvation-induced adhesion switching (DIAS) mechanism is developed. It is shown that edge functionalization of BN sheets and rational selection of membrane surface energy combined with systematic control of solvation and desolvation status enable extensive tunability of interfacial interactions at BN–BN, BN–membrane, and BN–substrate boundaries. Therefore, without incorporating any additives in the BN film and applying any surface treatment on target substrates, DIAS achieves a near 100% transfer yield of pure BN films on diverse substrates, including substrates containing significant surface irregularities. The printed BNs demonstrate high optical transparency (>90%) and excellent thermal conductivity (>167 W m⁻¹ K⁻¹) for few-micrometer-thick films due to their dense and well-ordered microstructures. In addition to outstanding heat dissipation capability, substantial optical enhancement effects are confirmed for light-emitting, photoluminescent, and photovoltaic devices, demonstrating their remarkable promise for next-generation optoelectronic device platforms.     

Field nanoemitters: ultrathin BN nanosheets protruding from Si3N4 nanowires

Field emitters in nanoscale are important in micro/nanoelectronic devices. Here, we report a large scale synthesis and effective field emission of field nanoemitters. The integrated nanostructures of ultrathin BN nanosheets aligned on Si3N4 nanowires are prepared through a two-stage process. Si3N4 nanowires were previously synthesized through heating Si powder at 1500 degrees C under a N2 atmosphere. Ultrathin BN nanosheets were then deposited on Si3N4 nanowires by heating a homemade B-N-O precursor under a N2/NH3 atmosphere. The as-prepared nanofilaments act as cold electron emitters displaying excellent field emission performance owing to the untrathin and sharp edges of the protruding BN nanosheets.     

PECVD法淀积氮化硼薄膜场发射特性研究

用 Ｂ２Ｈ６和 ＳｉＨ４作反应气体,通过射频等离子体增强化学气相淀积（ＲＦ－ＰＥＣＶＤ）方法,在 Ｓｉ（１００）面上沉积生长ＢＮ薄膜,用Ｓ－５２０扫描电子显微镜对所得薄膜进行观测,并用红外透射光谱测试分析了膜的成分。在室温、压力为 ８ × １０－４ Ｐａ条件下,对 ＢＮ薄膜的电流一电压特性进行测量,并得到了 Ｆｏｗｌｅｒ－Ｎｏｒｄｈｅｉｍ特性曲线,ＢＮ膜的场发射开启电场为９ Ｖ／μｍ,在电场３７．５ Ｖ／μｍ时,电流密度达到２４．８ ｍＡ／ｃｍ２。     

Pt浓度对磁控溅射制备Co1-xPtx:C薄膜的影响

采用组合靶,利用磁控共溅射技术制备了Co1-xPtx:C复合纳米颗粒薄膜,研究发现CoPt粒子取向和磁性能与CoPt:C薄膜中的Pt浓度有密切关系,在较高Pt浓度的CoPt：C薄膜中观察到垂直各向异性现象。通过改变Pt浓度,可以获得粒子粒径小于lOnm、矫顽力可控、高的垂直磁晶各向异性能的薄膜。综合考虑,薄膜的最佳成分为Co47Pt53:C,此时矫顽力达到最大。     

Magnetic and optical properties of amorphous NdFeCo thin films by pulsed laser deposition technique

Rare earth and transition metal doped (NdFeCo) thin films were fabricated on Si (100) substrate by pulsed laser deposition technique keeping the substrate at constant temperature of 300 °C. A KrF Excimer laser (248 nm, 20 ns) was used as an energy source for the deposition. Thin films were deposited without and under the influence of transverse magnetic field applied across the plume. The applied magnetic field was varied from 3 to 6 kOe. The deposited films were characterized by XRD, FESEM, VSM and SE (Spectroscopic Ellipsometry). The deposited films were amorphous in nature. All the films regardless of the applied magnetic field exhibit perpendicular magnetic anisotropy. The thickness of the thin films was found to increase monotonically from 166 to 266 nm with the increase in the applied external magnetic field. The saturation magnetization has a maximum value of 1682 emu/cc for the film deposited under 4.5 kOe magnetic field. The value of optical band gap energy for the same film is found to have a maximum value of 3.1 eV. The values of both the saturation magnetization and the band gap energy were decreased with the increase in the applied magnetic field.     

Chemical vapour deposition of thin films of BN onto fused silica and sapphire

Thin films of BN were prepared by chemical vapour deposition onto fused silica and sapphire using the reaction of BCl₃ with NH₃. The temperature of the substrate was varied between 600 and 1100°C. Transparent and smooth films of BN were obtained on fused silica and sapphire at substrate temperatures of 1000–1100°C. The growth rate of the film on sapphire was about 1 μm h^?1, and the growth rate on fused silica was about one-half that on sapphire. The films were chemically inert and adherent to the substrate. The absorption of the BN film was measured at room temperature. In the near-UV region, the main absorption peak was at about 6.2 eV and a sharp drop occurred near 5.8 eV. The sharp drop is attributed to the direct band gap. The photoluminescence of the films was measured at room temperature by excitation with light of wavelength 254 nm. A broad emission with a peak near 360 nm was observed.     

Annealing behavior of magnetic anisotropy in CoNbZr films

The effect of magnetic and nonmagnetic annealing on the magnetic anisotropy in CoNbZr films, formed by a DC opposing-targets sputtering method, was investigated. It was revealed that the origin of the magnetic anisotropy is the directional ordering of the magnetic atoms. The anisotropy fields and the direction of the easy axis obtained when the films are annealed in zero magnetic field are almost the same as those for the magnetic field parallel to the easy axis of the as-deposited films. When the films are annealed in a magnetic field perpendicular to the easy axis, the anisotropy field induced in parallel with the magnetic field, H_k(t), is well represented by the following formula: ln {1-H_k(t )/H_k(∞)∝-√Tt, where H_k(∞) is the thermal equilibrium value of the anisotropy field and D is the diffusion constant. The activation energy of the as-deposited film is 0.86 eV. Annealing the film increases the activation energy which is 2.1 eV when the film is annealed at a temperature of 450°C for 2 h     

Perpendicular magnetization structure of Co-Cr films

In a perpendicular recording system, a Co-Cr film as a medium is capable of storing very high density signals. Lorentz microscopy of 1000 kV TEM was used to observe the structure of recorded magnetizations in Co-Cr films having perpendicular anisotropy. A composite medium of a Co-Cr film with a soft magnetic back layer was shown by Lorentz microscopy to have a horseshoe magnetization structure. The stable antiparallel magnetization of transition in the Co-Cr layer determined the head-on magnetization structure of the soft magnetic back layer, which consists of a new straw-rope domain structure. The perpendicular magnetization structure of the Co-Cr film was found to consist of small domains magnetized through the film thickness which correspond to the columnar microstructure of the film. Since the intrinsic hysteresis loop of a Co-Cr film was shown to essentially have an ideal rectangular shape, it can be concluded that the Co-Cr layer of a composite film can be recorded by an ideal magnetizing process with negligible demagnetizing field at the transition.     

铁磁性纳米片间相互作用对其微波磁性的影响

在铁磁性元素中,交换能、偶极能和各向异性能之间存在复杂的竞争,因此,这种结构化介质的静态和动态性能与构成材料的固有磁特性,各个元素的形状和尺寸等有着密切的关系.这些多个自由度提供了对于通常未构图的磁性薄膜不可达到的新性能.本文通过将所研究的系统划分成立方体网格的三维阵列来对其进行建模,研究具有不同相对位置、纳米片间距、磁各向异性方向的两矩形铁磁性纳米片的微波磁性能.研究发现:与单个矩形铁磁性纳米片相比,具有不同相对位置、纳米片间距的两矩形铁磁性纳米片共振峰频率分布发生变化;当两矩形纳米片磁各向异性方向所呈角度由0°增加到30°时,其磁性质没有明显变化,而从30°到90°时,其磁性质对磁各向异性方向变化比较敏感.通过调控纳米片的相对位置、纳米片间距以及磁各向异性方向可以制备具有良好性能的吸收材料.     

Interface investigation of BN particle and aluminosilicate short fibre hybrid reinforced Al-12Si alloy composite

A BN particle and aluminosilicate short fibre reinforced hybrid Al-12Si composite was fabricated by squeeze casting. The interface microstructures between reinforcements and matrix were investigated by transmission electron microscopy (TEM). The experimental results showed that there is no reaction product formation at fibre/matrix and BN/matrix interface, and the matrix is in intimate contact with fibre and BN. Because of the addition of self-lubricant BN particles and good connection between BN and matrix alloy, the coefficient of friction of the hybrid composite is improved.     

Influence of magnetic annealing on high-frequency magnetic properties of FeCoNd films

FeCoNd thin film with thickness of 166 nm has been fabricated on silicon (1 1 1) substrates by magnetron co-sputtering and annealed for one hour under magnetic field at different temperatures (T_a) from 200 °C to 700 °C. The As-deposited and annealed FeCoNd film samples at T_a ≤ 500 °C were amorphous while the ones obtained at T_a ≥ 600 °C were crystallized. We found that the perpendicular anisotropy field gradually decreases as the annealing temperature increases from room temperature to 300 °C. A well induced in-plane uniaxial anisotropy is achieved at the annealing temperature between 400 and 600 °C. The variation of the dynamic magnetic properties of annealed FeCoNd films can be well explained by the Landau-Lifshitz equation with the variation of the anisotropy field re-distribution and the damping constant upon magnetic annealing. The magnetic annealing might be a powerful post treatment method for high frequency application of magnetic thin films.     

Effect of working pressure on the properties of BaTiO3-CoFe2O4 composite films deposited on STO (100) by PLD

The BaTiO₃-CoFe₂O₄ (BTO-CFO) composite films were grown on SrTiO₃ (STO) (100) substrates at 750 °C under various working pressures by pulsed laser deposition. The composite film grew into a supersaturated single phase at the working pressure of 10 mTorr, BTO and CFO (00 l) oriented hetero-epitaxial films on STO (100) at 100 mTorr, and a polycrystalline composite film at 500 mTorr. The slow growth rate at high working pressure led to the phase separation in the composite film. The CFO was compressively strained along out-of-plane due to the lattice mismatch with the BTO matrix phase. The BTO-CFO composite film grown at 100 mTorr showed reversible switching of ferroelectric polarization and magnetic hysteresis with strong magnetic anisotropy.     

Synthesis of vertically aligned boron nitride nanosheets using CVD method

Boron nitride nanosheets (BNNSs) protruding from boron nitride (BN) films were synthesized on silicon substrates by chemical vapor deposition technique from a gas mixture of BCl₃–NH₃–H₂–N₂. Parts of the as-grown nanosheets were vertically aligned on the BN films. The morphology and structure of the synthesized BNNSs were characterized by scanning electron microscopy, transmission electron microscopy, and Fourier transformation infrared spectroscopy. The chemical composition was studied by energy dispersive spectroscopy and X-ray photoelectron spectroscopy. Cathodoluminescence spectra revealed that the product emitted strong UV light with a broad band ranging from 250 to 400 nm. Field-emission characteristic of the product shows a low turn-on field of 6.5 V μm^?1.     

铁磁层/导电层/铁磁层多层膜中巨磁阻抗效应理论*

采用经典电磁理论,对铁磁层／导电层／铁磁层（M／C／M）多层膜中出现的巨磁阻抗效应进行了理论分析。对于单轴横向磁各向异性多层膜,理论计算结果表明：高频阻抗在某一外加磁场（近似等于等效各向异性场）下出现最大值,铁磁层和导电层电阻率相关较大的多层膜中将出现较强的巨磁阻抗效应。多层膜在1MHz附近即可出现远大于单层膜的阻抗变化比。多层膜理论计算与实验结果能够较好地符合。     

Study of FePt films prepared by reactive sputtering

In the present study, ⁵⁷FePt films are prepared with reactive ion beam sputtering using mixture of argon and nitrogen gases. Energy-dispersive X-ray reflectivity is used to estimate the thickness of the as-deposited films. Structural and magnetic properties of the as-deposited and annealed films are studied using grazing incidence X-ray diffraction (GIXRD), magneto-optical Kerr effect (MOKE) and conversion electron Mossbauer spectroscopy (CEMS). Significant difference in structural and magnetic properties i.e., formation of ordered L1₀ phase and perpendicular magnetic anisotropy are observed for the films prepared with mixture of nitrogen and argon as compared to the film prepared with argon only. From the GIXRD, peaks corresponding to the ordered face-centred tetragonal FePt phase are observed for the films prepared with mixture gas. The results of CEMS clearly show the perpendicular magnetic anisotropy (PMA) for the films prepared with mixture of nitrogen and argon. The observed enhanced chemical ordering and the development of PMA in the films prepared with mixture gas is due to the role played by the defects created as a consequence of nitrogen escape in the films with high temperature annealing.     

Cubic phase content and structure of BN films from an X-ray absorption study

X-ray absorption near-edge structure (XANES) was used to study the cubic boron nitride (c-BN) content in the BN films deposited on various substrates by different physical vapor deposition or plasma-enhanced chemical vapor deposition methods. By fitting the XANES curves of thin-film samples using standard spectra of pure c-BN and sp(2)-bonded BN in the films with suitable weight factors, the c-BN contents at the film's surface region and across the film's thickness have been determined quantitatively. The results agree well with the previous transmission electron microscopic observations. The method is proved to be independent of the optical properties of thin film and provides a possibility to evaluate the cubic content of BN films accurately.