Stability of plasma-deposited amorphous hydrogenated boron films

An investigation of radio-frequency plasma-deposited amorphous hydrogenated boron films was conducted in order to determine the influence of the substrate temperature T_S and d.c. self-bias U_SB on the physical properties and film stability. We found three different regions of stability depending on T_S and U_SB. Films prepared at high d.c. self-bias and substrate temperature are mechanically unstable due to internal stress. They peel off during the deposition process or on first contact with the ambient atmosphere. Films deposited at low self-bias and substrate temperature were found to be chemically unstable. Exposed to the ambient atmosphere, they undergo chemical changes and incorporate large amounts of oxygen and carbon. These two different regions of instability are separated by chemically and mechanically stable films. The chemical composition and the structural properties of chemically stable and unstable films were determined by Fourier transform infrared spectroscopy, X-ray induced photoelectron spectroscopy and ion beam analysis. These measurements show that chemical stability correlates with the boron density. Chemically stable films reveal densities of more than 68% up to 99% of the density of crystalline boron. In general, elevated substrate temperature and ion energy cause densification of the films and increasing internal stress. Densification leads to chemical stability, while internal stress is the reason for mechanical instability.     

Raman characterization of boron doped tetrahedral amorphous carbon films

Boron doped tetrahedral amorphous carbon films, having boron content from 0.59 to 6.04 at.%, have been prepared by a filtered cathodic vacuum arc system using boron mixed graphite targets. The influence of boron on the surface morphologies and microstructures of the films was studied by atomic force microscopy and Raman spectroscopy. The surface images showed that the irregular tops on the surface of the films tended to form larger clusters as boron content increased. The Raman spectra of the films were, respectively, deconvoluted using Gaussian and Breit-Wigner-Fano line shapes. The Raman parameters, including the intensity ratios, peak positions, peak widths and coupling coefficients, obtained from both line shapes were described and compared. It was found that both line shapes could produce consistent results except the peak widths of G bands. The major effect of boron introduction was to increase the clustering of the sp² phase in the films.     

Microstructural study of oxidation of carbon-rich amorphous boron carbide coating

Carbon-rich amorphous boron carbide (B _x C) coatings were annealed at 400°C, 700°C, 1000°C and 1200°C for 2 h in air atmosphere. The microstructure and composition of the as-deposited and annealed coatings were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy and energy dispersive X-ray spectroscopy (EDS). All of the post-anneal characterizations demonstrated the ability of carbon-rich B _x C coatings to protect the graphite substrate against oxidation. Different oxidation modes of the coatings were found at low temperature (400°C), moderate temperature (700°C) and high temperature (1000°C and 1200°C). Finally, the feasibility of the application of carbon-rich BxC instead of pyrolytic carbon (PyC) as a fiber/matrix interlayer in ceramics-matrix composites (CMCs) is discussed here.     

Microstructural study of oxidation of carbon-rich amorphous boron carbide coating

Carbon-rich amorphous boron carbide (B_xC) coatings were annealed at 400°C, 700°C, 1000°C and 1200°C for 2 h in air atmosphere. The microstructure and composition of the as-deposited and annealed coatings were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy and energy dispersive X-ray spectroscopy (EDS). All of the post-anneal characterizations demonstrated the ability of carbon-rich B_xC coatings to protect the graphite substrate against oxidation. Different oxidation modes of the coatings were found at low temperature (400°C), moderate temperature (700°C) and high temperature (1000°C and 1200°C). Finally, the feasibility of the application of carbon-rich B_xC instead of pyrolytic carbon (PyC) as a fiber/matrix interlayer in ceramics-matrix composites (CMCs) is discussed here.     

High pressure synthesis of cubic boron nitride from amorphous state

Cubic-zincblende-type boron nitride was synthesized from amorphous state at pressures higher than 6 GPa and at temperatures higher than 800°C, without any planned addition of catalysts. Also, wurtzite type boron nitride was formed although the amount was small.     

Optical properties and chemical reactivity of hydrogenated amorphous boron thin films

Hydrogenated boron thin films have been deposited at temperatures in the range 50–100 °C with the radiofrequency plasma decomposition of diborane B₂H₆ diluted in hydrogen. The chemical composition of the films has been determined by elastic recoil detection and by the Castaing microprobe techniques. We have found that the as-deposited films contain 10–20 at % H and that they react with the ambient atmosphere within a few days and reach a final composition close to B_0.64H_0.12O_0.12C_0.06N_0.06. The optical properties of the as-deposited films studied by spectroscopic phase-modulated ellipsometry in the range 1.7–5.0 eV are characteristic of the high optical gap semiconductors. The electron diffraction measurements performed onin situ annealed samples show that the films are amorphous up to 950 °C. Infrared spectroscopy investigations performed on the as-deposited films have revealed two hydrogen bonding sites: B-H terminal bonds and B-H-B bridge bonds, along with some B-O-B groups. Upon exposure to the atmosphere, the vanishing of B-H-B and B-O-B bridge bonds and the increase of B-H and B-O-H absorption bands after 1 day, are observed. On leave from Physics Department, CNEA, Avenida Libertador 8250, 1429 Buenos-Aires, Argentina, and CONICET.     

超细高能燃料无定形硼粉的自蔓延制备与表征

采用自蔓延高温合成(SHS)法,用高活性金属Mg粉还原B2O3粉,通过镁热还原反应B2O3 3Mg→2B 3MgO制备出超细高能燃料无定形硼粉,研究镁热还原工艺对还原产物的影响.采用X射线衍射仪对盐酸溶浸后的还原产物进行物相分析.结果表明:还原过程中生成的酸不溶物MgB6、BxO、Mg2SiO4、FeB49是影响硼粉纯度的主要杂质.利用费歇尔粒度测试仪(F.S.S.S)检测无定形硼粉的平均粒径和比表面积.通过优化制备工艺,在B2O3/Mg(质量分数,%)为3.0、反应体系温度为850℃的条件下,可制备出纯度高于94%、比表面平均粒径为0.5～0.7μm、比表面积达4～6m2/g的超细无定形硼粉.     

Facile synthesis of cubic boron nitride nanoparticles from amorphous boron by triple thermal plasma jets at atmospheric pressure

Cubic boron nitride (c-BN) is a superhard material, with hardness value comparable to that of diamond. c-BN is used in a wide range of industrial applications, including tool, abrasives, and refractory. The hardness of c-BN can be improved by decreasing the particle size to the nanoscale; however, the simultaneous application of high pressure (～8 GPa) and temperature (>2,500 K) is required to synthesize the c-BN crystal structure. In this study, we effectively synthesized c-BN nanoparticles from amorphous boron using a triple direct current (DC) thermal plasma jet system at atmospheric pressure. The injection of nitrogen as plasma forming gas generated reactive nitridation species. The average particle size of the synthesized c-BN was 22 nm, and the major crystal structure is the (1 1 1) cubic phase. We carried out a numerical simulation for a thermal fluid, to confirm the high temperature and velocity fields of the plasma jets that formed inside the reactor as the flow rate of plasma forming gas was adjusted. A high production yield of 51% was achieved using amorphous boron at a feed rate of 190 mg/min and the c-BN nanoparticles exhibited high crystallinity without requiring pre-and post-processing.     

Reaction control of TiB2 formation from titanium metal and amorphous boron

TiB₂ powder was synthesized by a controlled formation reaction from titanium metal and amorphous boron. Precursory TiB₂ formed by the pretreatment of the mixed powder (mole ratio: B/Ti=2.0) at 600° C for 60 min in an argon stream. Hollow TiB₂ powder with an average grain size of 15m was obtained by subsequent heat treatment above 900° C for more than 60 min in an argon stream. The formation reaction of TiB₂ powder was further controlled by pretreatment of the mixed powder at 600° C for 60 min in a hydrogen and argon stream and subsequent heat treatment at 1000° C for 360 min in an argon stream, when hollow-free TiB₂ powder was formed by a milder formation reaction between amorphous boron and the reformed titanium metal with hydrogen diffused lattice.     

Crystallization and structure of high boron content iron-boron ultrafine amorphous alloy particles

Amorphous to crystalline transformation of chemically prepared Fe₆₄B₃₆ ultrafine amorphous alloy particles has been investigated by Mössbauer spectroscopy, Brunauer-Emmett-Teller surface area measurements and transmission electron microscopy. Structural relaxation was observed below 350°C, which resulted in narrowing the full width at half maximum for the hyperfine field distribution from 13.0 to 10.6 T, while the average hyperfine field kept unchanged, to be about 20.3 T. Crystallization started on the surface at about 300°C and proceeded into the bulk at about 400°C. Partial crystallization between 400 and 450°C resulted in increasing the average hyperfine field for the remaining Fe-B amorphous matrix to 21.6 T. -Fe and Fe₂B were the only iron containing phases related to bulk crystallization, with the latter as a predominant component, accompanied by the segregation of about 19% boron atoms. Above 500°C, sintering of the particles became very remarkable and a solid state reaction between diffusing iron and boron atoms to form Fe₂B took place making the spectral area ratio for Fe₂B to -Fe components increase accordingly. A locally distorted non-stoichiometric Fe₂B quausicrystalline structure for the high boron content sample was proposed.     

Synthesis of TiB2 powder from a mixture of TiN and amorphous boron

TiB₂ powder was synthesized by solid state reaction using amorphous boron and TiN as a source of titanium. The TiB₂ formation did not occur at all in a nitrogen atmosphere even at 1400° C. TiB₂ formed above 1100° C in argon and hydrogen atmospheres. The only crystalline phase of TiB₂ powder was favourably synthesized at 1400° C for 360 min in an argon atmosphere from a starting powder with a composition containing excess boron (B/Ti = 2.2). The synthesized powder was well dispersed and had a particle size of 0.5 to 2 µm. The powder activity was evaluated by sintering at 4 G Pa and 1300 to 1600° C for 15 min.     

Interaction between amorphous boron and titanium alloys in a gaseous oxygen-containing medium

We study the influence of oxygen partial pressure in the course of diffusion saturation of titanium with the formation of boride phases (TiB, TiB₂) at 900°C. We establish the dependence of the reactivity of saturating medium, consisting of powdered amorphous boron and a vacuum atmosphere, on the oxygen partial pressure. We also present some physical characteristics of boride coatings. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 5, pp. 30–34, September–October, 2006.     

Microstructural characterization of amorphous and nanocrystalline boron nitride prepared by high-energy ball milling

Microstructural parameters like crystallite size, lattice strain, stacking faults and dislocation density were evaluated from the X-ray diffraction data of boron nitride (BN) powder milled in a high-energy vibrational ball mill for different length of time (2-120 h), using different model based approaches like Scherrer analysis, integral breadth method, Williamson-Hall technique and modified Rietveld technique. From diffraction line-broadening analysis of the successive patterns of BN with varying milling time, it was observed that overall line broadening was an operative cause for crystallite size reduction at lower milling time (∼5 h), whereas lattice strains were the prominent cause of line broadening at higher milling times (>19 h). For intermediate milling time (7-19 h), both crystallite size and lattice strain influence the profile broadening although their relative contribution vary with milling time. Microstructural information showed that after long time milling (>19 h) BN becomes mixture of nanocrystalline and amorphous BN. The accumulations of defects cause this crystalline to amorphous transition. It has been found that twin fault (β′) and deformation fault (α) significantly contributed to BN powder as synthesized by a high-energy ball-milling technique. Present study consider only three ball-milled (0, 2 and 3 h) BN powder for faults calculation because fault effected reflections (1 0 1, 1 0 2, 1 0 3) disappear with milling time (>3 h). The morphology and particle size of the BN powders before and after ball milling were also observed in a field emission scanning electron microscope (FESEM).     

Structure and properties of boron/aluminium composites

The influence of heat treatment on the structure and strength of boron/aluminium composites has been studied. Matrix material around a fibre is assumed to be hardened by dispersed particles of boron-containing compounds, presumably magnesium borides. Such a material is expected to have higher yield strength and greater brittleness compared to the original matrix material. A fibre and its surrounding zone is a prospective site of microcracking under loading. These zones grow as heat treatment proceeds. When neighbouring zones become linked to each other, the size of a possible brittle microcrack is doubled and a fall in the composite strength follows. A corresponding microstructural model of the composite failure is constructed.

Without having direct observation of magnesium borides, the model of composite behaviour is supported by (i) dependence of the composite strength on the size of the influence zone; (ii) dependence of the strength on the effective time, the latter being determined by the activation energy of magnesium diffusion in aluminium; (iii) observation of failure surfaces of composites subjected to various heat treatments; and (iv) a correlation of changes of matrix state detected by changes in X-ray diffraction patterns with a hypothetical picture of the development of the influence zones.     

Electrode-limited conduction in amorphous boron films I. D.C. measurements

The conduction mechanism in amorphous boron films sandwiched between plane gold electrodes has been investigated. The films, in the thickness range 3000–8000 Å, were obtained by the vacuum evaporation technique. The conduction process was examined in the temperature range 260–403 K with applied electric fields ranging from 10³ to 10⁵ V cm^-1. The temperature and field dependences of the current flowing through the insulator are understandable in terms of the thermal emission of charge carriers over the Schottky barrier of a blocking contact. The theory of this type of electrode-limited conduction, derived by Simmons, predicts a current-voltage relationship of the type $\text{J～}\text{exp}\text{(β V}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$. A good fit of the experimental J-V curves to this relationship is obtained. Furthermore, a set of parameters has been calculated: the height and the transmission coefficient of the potential barrier are respectively 0.43 eV and 10^-13, and the donor density within the insulator is equal to about 10²¹ cm^-3.     

Formation process of tungsten borides by solid state reaction between tungsten and amorphous boron

Various kinds of tungsten borides were synthesized by solid state reaction between tungsten and amorphous boron powders. The mixed powders with various compositions (B/W = 0.4 to 13.0) were treated at 800 to 1550° C for 0 to 120 min in a stream of argon. Four kinds of boride phases such as W₂B, WB, W₂B₅ and WB₄ were formed, although the boride phase having the composition of the highest boride, WB₁₂, did not appear. The formation of W₂B was initiated approximately at 1000° C in excess of tungsten content. On the other hand, in the excess boron content, the formation of WB, W₂B₅ and WB₄ was initiated approximately at 800, 950 and 1200° C, respectively. The maximum formation amount and crystallinity of WB and W₂B₅ was found in nearly 10 at % excess boron content in their own stoichiometric compositions. The only crystalline phase of WB₄ was prepared with a large excess boron content. However, the formation behaviour of WB₄ showed that WB₄ is metastable above 1400° C. The stability of WB₄ phase could be increased by the presence of excess boron.