Abrupt irreversible transformation of rhombohedral BN to a dense form in uniaxial compression of CVD material

The behavior of rhombohedral BN has been investigated under uniaxial compression along the [001]_r texture axis of a highly textured CVD rBN material (thin plate sample) in a diamond anvil cell without a gasket. An abrupt martensitic transformation of the graphite-like rBN structure to a dense cubic BN form accompanied by a distinct acoustic emission signal has been recorded at the sample center at a pressure somewhat above 5.6 GPa, which is an order of magnitude below the threshold pressure of the transformation under hydrostatic compression.     

Effect of BN content on microstructures,mechanical and dielectric properties of porous BN/Si3N4 composite ceramics prepared by gel casting

Porous BN/Si₃N₄ composite ceramics with different BN contents have been fabricated by gel casting. The rheological behaviors of the suspensions, microstructure, mechanical properties, dielectric properties and critical temperature difference of thermal shock (ΔT_C) of porous BN/Si₃N₄ composite ceramics with different BN contents were investigated. With BN contents increasing, the mechanical properties of the porous BN/Si₃N₄ composite ceramics were partially declined, but the dielectric properties and thermal shock resistances were enhanced at the same time. For the porous Si₃N₄ ceramic without BN addition, the porosity, flexural strength, dielectric constant and critical temperature difference were 48.1%, 128 MPa, 4.1 and 395 °C, while for the 10 vol% BN/Si₃N₄ porous composite ceramics, they were 49.4%, 106.6 MPa, 3.8, and 445 °C, respectively. The overall performance of the obtained porous BN/Si₃N₄ composite ceramics indicated that it could be one of the ideal candidates for high-temperature wave-transparent applications.     

Ordering structure of barium magnesium niobate ceramic with A-site substitution

The ordering behaviour of Ba(Mg_1/3Nb_2/3)O₃ ceramics (BMN) substituted by La³⁺, Na⁺, K⁺ was investigated using X-ray powder diffraction and transmission electron microscopy. The 1:2 ordered structure of BMN can be transformed to 1:1 ordered structure by substituting a small amount of La cation ion into the A-site. Moreover, the degree of ordering was increased with La content in the compound, and reached almost unity at [La] = 50 mol%. When the La ion in (Ba_1−xLa_x)(Mg_(1+x)/3Nb_(2−x)/3)O₃ (BLMN) was substituted by Na or K ions, the 1:1 ordered structure of BLMN was transformed into the 1:2 ordered structure. The degree of 1:2 ordering was influenced by the sintering temperature and the size difference between the A and B site ions.     

Large Q factor variation within dense,highly ordered Ba(Zn1/3Ta2/3)O3 system

The large amount of Q factor variation within dense, highly ordered region of Ba(Zn_1/3Ta_2/3)O₃ (BZT) system were studied by means of crystal structure analysis, micro-structural analysis and electrical measurements using samples around stoichiometric BZT. Presence of small amount of secondary phase (e.g., less than 1%) affects Q factor decrements even in the dense (density = 7.15–7.78 g/cm³), highly ordered (estimated ordering ratio around 70–80%) samples. This suggests that the structural order and the presence of the secondary phase are playing an important role on Q factor in BZT system. Therefore, suppressions of small amount of secondary phase (e.g., less than 1%) by strict composition control can provide further Q factor improvements. Single phase ordered perovskite obtained in the vicinity of stoichiometric BZT showed an improvement in Q factor (Q × f = 133,000 GHz) by extended sintering up to 400 h at 1400 °C.     

Novel carbon diamond-like phases LA5, LA7 and LA8

In this work, the structure and properties of seven diamond-like carbon phases obtained by linking the graphene layers were calculated using DFT and PM3 methods. The LA5 (Cmca), LA7 (Cmcm), and LA8 (I4₁/amd) diamond-like phases were predicted and studied in this work for the first time. Values of the unit cell parameters of the predicted phases are: a = 4.337 Å, b = 5.024 Å, and c = 4.349 Å for LA5 phase; a = 4.942 Å, b = 4.808 Å, and c = 4.390 Å for LA7 phase; and a = 4.906 Å and c = 4.960 Å for LA8 phase. For these LA5, LA7, and LA8 phases, various structure characteristics, densities, cohesive energies, bulk moduli, electronic densities of states and X-ray patterns were calculated. The comparative analysis showed that the diamond-like phase properties depend on the extent of their structure deformation relative to the cubic diamond structure.     

BN analogues of graphene: On the formation mechanism of boronitrene layers — solids with extreme structural anisotropy

Boron nitride is an extremely useful material for applications in material sciences and appears in a manifold of crystalline modifications, with hexagonal and cubic boron nitride as most prominent substances. In hexagonal boron nitride, threefold coordinated boron and nitrogen atoms form two-dimensional layers, which resemble the boron nitride analogue of graphene layers, and we refer to this two-dimensional network as boronitrene layers. So far, there is little knowledge about the elementary growth reactions of these boronitrene layers.Here we show that it is possible to regenerate a previously prepared 14 × 14 BN/13 × 13 Rh(111) superstructure [obtained by CVD of borazine (B₃N₃H₆)] after an oxidation step from an unordered monolayer of a boron–oxygen compound by chemical reactions induced by ammonia as nitrification agent on the surface. The individual steps of the experiment were controlled by a LEED- and XPS- analysis and indicate that the original BN superstructure can be recovered without traceable amounts of oxygen in the film. The presented results indicate that highly mobile species from the B–N–O–H system can be considered as surfactants in the elementary formation and degradation reactions of highly ordered boronitrene layers. An understanding of these reactions is a fundamental issue in tuning the crystal growth and quality of the BN phases.     

Effect of small amount of cobalt substitution on structure and microwave dielectric properties of barium magnesium niobate ceramics

We investigated effects of substituting cobalt for magnesium on microstructures and microwave dielectric properties of BaMg_1/3Nb_2/3O₃ ceramics. Nucleation and growth of 1:2 ordered domains dominate microstructural variations for Ba(Mg_1?xCo_x)_1/3Nb_2/3O₃ ceramics. Microstructures such as grains sizes and domain sizes significantly dominate variations of Q × f value. Less ordered domains are nucleated when substituting small amount of cobalt (x < 0.1). Accordingly, domains and grains significantly grow with increased cobalt substitution. The 1:2 ordering degree is subsequently increased, and Q × f achieves a maximum value at x = 0.05. Nevertheless, while cobalt substitution exceeds x = 0.1, more ordered nuclei occur and consequently affect the domain growth and the grain growth. The Q × f value remarkably decreases at x = 0.1, and varies due to different ordering degrees and compositions. The Q × f value of specimens at x = 0.05 becomes as high as 43,000, and is similar to that of specimens at x = 0.5.     

Theoretical study of noncovalent functionalization of BN nanotubes by various aromatic molecules

The unsatisfactory stability of CNT under high temperature or subject to strong oxidants has limited the potential use of p- and n-type field-effect transistors (FETs). Promisingly, boron nitride (BN) nanotubes are known to have the excellent resistance to oxidant and thermal stability at high temperature as well as the uniform electronic properties, which rends their possible application as alternative FET device. In this paper, through the theoretical computation of the noncovalent functionalization of BN nanotube by various aromatic molecules (including C₁₀H₈, C₁₄H₁₀, porphyrin, DDQ, and TCNQ molecules), we for the first time evaluate its possibility as a candidate of stable FET device. We find that: (i) these aromatic molecules can be stably adsorbed on the studied BN with the adsorption energy ranging from ? 0.22 (C₁₀H₈) to ? 0.42 eV (porphyrin); (ii) the adsorption of electrophilic molecules on the outer sidewall of BN nanotube realizes a p-type semiconductor with a smaller band gap (~ 0.90 eV); (iii) exoherdral adsorption of nucleophilic aromatic molecules leads to an n-type semiconductor. This novel BN nanotube-based material offers great promise for molecule electronics in terms of their good stability.     

Growth of cubic and hexagonal BN particles by using BBr3, NH4Br and metallic Na as reactants

Hexagonal BN (h-BN) particles were prepared by using BBr₃, NH₄Br and metallic Na as reactants in stainless steel autoclaves at 450 ºC for 24 h; When the target temperature was set in the range of 500–600 ºC, cubic BN (c-BN) particles that co-existed with h-BN were obtained. It is found that 600 ºC and a molar ratio of BBr₃ to NH₄Br of 1:3 are the optimal reaction parameters for the highest yield production of c-BN particles. c-BN powder with controllable content has been synthesized through a one-step method in the absence of solvents. The as-obtained samples that contain pure h-BN and the mixture of c-BN and h-BN have high thermal stabilities in ambient atmosphere below 920 and 1100 ºC, respectively.     

Fabrication and properties of ZrB2–SiC–BN machinable ceramics

ZrB₂–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 °C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB₂–SiC–BN (40 vol%ZrB₂–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m^1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB₂–SiC (80 vol%ZrB₂–20 vol%SiC). Noticeably, the hardness decreased tremendously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB₂–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB₂–SiC–BN composites from 1100 to 1500 °C in air atmosphere showed that the weight gain of the 80 vol%ZrB₂–20 vol%SiC and 43.1 vol%ZrB₂–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 °C for 5 h were 0.0714 and 0.0268 g/cm², respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB₂–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 °C and a compact film of zirconium silicate above 1300 °C. The formed borosilicate and zirconium silicate on the surface of ZrB₂–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB₂–SiC–BN.     

Preparation of a novel BN/SiC composite porous structure

A kind of BN/SiC open cell ceramic foams were fabricated from complex co-polymeric precursors of polycarbosilane and tris(methylamino)borane [B(NHCH₃)₃] using a high pressure pyrolysis foaming technique. The as-fabricated foams exhibit cell sizes ranging from 1 to 5 mm with bulk densities varying from 0.44 to 0.73 g/cm³, depending on the proportion of the starting materials. Studies on microstructure and properties of the porous material shown that addition of BN into SiC can improve dramatically its oxidation resistance during 800–1100 °C and compression strength which was generally about a 5–10 times higher than that of a pure SiC foam.     

Abnormal variation of microwave dielectric properties in A/B site co-substituted (Ca1−0.3xLa0.2x)[(Mg1/3Ta2/3)1−xTix]O3 complex perovskite ceramics

A/B site co-substituted (Ca_1?0.3xLa_0.2x)[(Mg_1/3Ta_2/3)_1?xTi_x]O₃ ceramics (0.1 ≤ x ≤ 0.5) were prepared by solid state reaction and the structures, microstructures and dielectric properties were investigated. B site 1:2 cation ordering and oxygen octahedra tilting lead to monoclinic symmetry with space group P2₁/c for x = 0.1. For x above 0.1, the ordering was destroyed and the crystal structure became orthorhombic with space group Pbnm. The B site 1:2 cation ordering tended to be destroyed to form 1:1 ordering by the A site La³⁺ substitution. The dielectric constant increased linearly with increasing content of Ti⁴⁺ as the increasing second Jahn–Teller distortion enhanced the B site cation rattling. The temperature coefficient of resonant frequency and Qf values showed abnormal variations, which were refined to be caused by the increasing A site cation vacancy and diffused distribution of small size ordering domains respectively. Good combination of microwave dielectric properties was obtained at x = 0.5, where ?_r = 48, Qf = 21,000 GHz and τ_f = 2.2 ppm/°C.     

Comparison of the wear particle size distribution of different a-C coatings deposited by vacuum arc

For biomedical application in the field of artificial hip joints diamond-like carbon (DLC) coatings have been widely studied due to their tribological properties. The wear particles as the main factor limiting the life expectancy of hip joints have attracted more and more interest, not only the number of them, but also the distribution of their size. In this study we have deposited DLC coatings on stainless steel (P2000) by a vacuum arc adjustable from anodic to cathodic operation mode, with the anode–cathode diameter ratio of d_a/d_c = 3/1 at a DC bias of − 250 V to − 1000 V. To improve the adhesion of the DLC coating on P2000, titanium as a metallic interlayer was deposited by cathodic vacuum arc evaporation. The internal structure of the coating was investigated by the visible Raman spectroscopy with the four-Gaussian curve fitting method. Comparing the results with the previous work (coatings deposited with d_a/d_c = 1/1), it was found that the anode–cathode diameter ratio has an effect on the structure (e.g. I_D/I_G) as well as the wear particle size distribution. It was shown that the maximum of the frequency distribution e.g. at − 1000 V bias can be shifted to below 1 μm with increasing d_a/d_c.     

A rare 2D coordination polymer of graphite-like structure extended by infinite silver–oxygen–silver bonds

The reaction of AgNO₃ and H₃btb (H₃btb = 1,2,3-benzenetricarboxylic acid) leads to the isolation of {[Ag(μ₂-H₂O)(μ₂-H₂btb)]}_n · 1.5nH₂O (1), which crystallizes in the orthorhombic space group Fddd with a = 11.436(2) Å, b = 6.876(1) Å, c = 58.837(1) Å, V = 4626.5(2) Å³, Z = 32. Silver ions are connected by both strong Ag?Ag interaction and μ₂-H₂O bridges to form hexagonal Ag₆ subunits, and are further assembled into a high ordered 2D grid, presenting an interesting graphite-like 6³ net. Both the fluorescence and the crystal transformation after dehydration are studied.     

Cd-composition induced effects on structure,optical and electrical properties of sputtered Zn1−xCdxO films

Zn_1−xCd_xO films with different Cd contents (0≤x≤1) were successfully deposited on quartz substrates by the direct current reactive magnetron sputtering and post-annealing techniques. It was found that structures, band gaps and electrical properties of the films can be tuned by changing Cd contents x. The Zn_1−xCd_xO film consists of wurtzite phase with highly (002)-preferred orientation at x from 0 to 0.2, mixture of wurtzite and cubic phases at x=0.5, and cubic phase with highly (200)-preferred orientation at x≥0.8. The band gap decreases from 3.25 eV at x=0 to 2.75 eV at x=0.2 for the wurtzite Zn_1−xCd_xO, and decreases from 2.52 eV at x=0.8 to 2.42 eV at x=1, which has a little change for cubic Zn_1−xCd_xO. In addition, Hall measurement results indicate that the influence of Cd content on the conduction behavior of Zn_1−xCd_xO films is significant. The chemical compositions and the bonding states of Zn_1−xCd_xO films were examined by X-ray photoelectron spectroscopy analysis.     

A correlation between piezoelectric response and crystallographic structural parameter observed in lead-free (1-x)(Bi0.5Na0.5)TiO3–xSrTiO3 piezoelectrics

The structure of lead-free (1-x)(Bi_0.5Na_0.5)TiO₃-xSrTiO₃ (BNT-STx) ceramics was analyzed by the Rietveld method, using X-ray diffraction and neutron scattering data. The structural refinement results suggest that the crystal structure successively changes with SrTiO₃ concentration, x, from the rhombohedral phase (x = 0.00) to rhombohedral and tetragonal (x = 0.10–0.30), tetragonal and cubic (x = 0.40–0.60), and finally cubic (x = 0.80–1.00) phases. Correlation between the charge sensor constant (d₃₃) and the weighted off-center value (d_w) was observed, which may be attributed to the increased dipole motion in the unit cell due to an increased tendency to respond to external stimulation. Furthermore, an improved charge sensor constant (d₃₃) of 140 pC/N was observed for BNT-ST0.20, and a large strain of 0.25% and a d₃₃^* value of 443 pm/V were observed from x = 0.30.     

Low dielectric constant porous BN/SiCO made by pyrolysis of filled gels

BN/SiCO composite ceramics having a dielectric constant, ɛ_r, as low as 1.9 have been made by pyrolysis of filled gels at temperature ≥1000 °C. Such a low value of ɛ_r is supposed to be due to a combination of factors: the low ɛ_r of BN and of SiCO itself and the high amount of residual porosity present in the samples. The porous microstructure – and the related dielectric properties – obtained at 1000 °C show a very good stability up to 1400 °C. This result has been ascribed to the high viscosity of the SiCO glass and to the platelet shape particles of BN. Both factors hinder the sintering and prevent the closure of the pores.     

Improved thermal conductivity of sintered reaction-bonded silicon nitride using a BN/graphite powder bed

Sintered reaction-bonded silicon nitride (SRBSN) with improved thermal conductivity was achieved after the green compact of submicron Si powder containing 4.22 wt% impurity oxygen and Y₂O₃-MgO additives was nitrided at 1400 °C for 6 h and then post-sintered at 1900 °C for 12 h using a BN/graphite powder bed. During nitridation, the BN/10 wt% C powder bed altered the chemistry of secondary phase by promoting the removal of SiO₂, which led to the formation of larger, purer and more elongated Si₃N₄ grains in RBSN sample. Moreover, it also enhanced the elimination of SiO₂ and residual Y₂Si₃O₃N₄ secondary phase during post-sintering, and thus induced larger elongated grains, decreased lattice oxygen content and increased Si₃N₄-Si₃N₄ contiguity in final SRBSN product. These characteristics enabled SRBSN to obtain significant increase (∼40.7%) in thermal conductivity from 86 to 121 W ∙ m⁻¹ ∙ K⁻¹ without obvious decrease in electrical resistivity after the use of BN/graphite instead of BN as powder bed.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba(Co1/3Nb2/3)O3 ceramics

The effect of B-site cation deficiency on the structure and microwave dielectric properties of Ba(Co_1/3Nb_2/3)O₃ (BCN) was investigated. Stoichiometric and co-deficient compositions based on Ba(Co_1/3−xNb_2/3)O₃ [x = 0.0, 0.01, 0.02, 0.03 and 0.04] were prepared using the conventional mixed oxide route. Small amounts of V₂O₅ (0.1 wt%) were added to promote densification. The dielectric loss is very sensitive to the composition; it was found that co-deficiency degraded the microwave dielectric properties. The stoichiometric formulation (x = 0) exhibited the best microwave properties. The improvements in the microwave dielectric properties were achieved by increasing the degree of 1:2 cation ordering. The highly ordered, stoichiometric BCN ceramics showed a relative permittivity (ɛ_r) of 32, quality factor (Q × f) of 66,500 GHz and a negative temperature coefficient of resonant frequency (τ_f) of −10 ppm/°C at 4 GHz.     

Electron field emission from filtered arc deposited diamond-like carbon films using Au and Ti layers

In this work, tetrahedral diamond-like carbon (DLC) films are deposited on Si, Ti/Si and Au/Si substrates by a new plasma deposition technique — filtered arc deposition (FAD). Their electron field emission characteristics and fluorescent displays of the films are tested using a diode structure. It is shown that the substrate can markedly influence the emission behavior of DLC films. An emission current of 0.1 μA is detected at electric field E_DLC/Si=5.6 V/μm, E_DLC/Au/Si=14.3 V/μm, and E_DLC/Ti/Si=5.2 V/μm, respectively. At 14.3 V/μm, an emission current density J_DLC/Si=15.2 μA/cm², J_DLC/Au/Si=0.4 μA/cm², and J_DLC/Ti/Si=175 μA/cm² is achieved, respectively. It is believed that a thin TiC transition layer exists in the interface between the DLC film and Ti/Si substrate.