纳米结构B4C的合成及其性质的研究

B_4C是继金刚石和立方氮化硼之后自然界中第三硬的超硬材料。然而人们在硬度方面对它的应用却很少。这主要是因为B_4C的自扩散系数很低,很难合成出块体材料的B_4C;其次,B_4C的断裂韧性很低,达不到工业应用的标准,在工业应用中容易出现碎裂。本篇文章利用高温高压法合成了块体材料的B_4C,并且合成的材料具有非常高的致密性。通过硬度测试发现其硬度高于材料的单晶硬度值。利用压痕法测量了样品的断裂韧性,其断裂韧性为4.51 MPa·m~(1/2),这一数值基本接近了工业应用的标准。通过扫面电镜测试发现其具有纳米层状结构。通过原理分析可知,纳米片层结构是导致B_4C具有高硬度和高断裂韧性的原因。     

纳米孪晶超硬材料的高压合成

超硬材料研究有两个重要难题一直备受关注:一是建立晶体宏观性能硬度与微观电子结构参数的定量关联,指导新型超硬晶体的设计;二是发现改进超硬材料综合性能(硬度、韧性和稳定性)的基本原理和技术途径,合成出综合性能更加优异的高性能超硬材料.首先从同时联系晶体硬度和电子结构的化学键出发,提出了共价晶体的压痕硬度为晶体中化学键对压头压入过程的综合阻抗的基本假设,建立了共价晶体硬度的微观模型并推广至多晶共价材料.在多晶硬度模型指导下,在高温高压条件下成功地合成出了纳米孪晶结构的立方氮化硼和金刚石块材,实现了硬度、韧性及热稳定性这三大工具材料性能指标的同时提高.另外,澄清了关于压痕硬度测量的长期争论.本文的研究为研发高性能超硬材料打开了一条新的技术途径,有望带来机械加工业和高压科学领域的新变革.     

纳米结构超硬材料的机遇与挑战

经过几十年的研究和发展，纳米结构金刚石和立方氮化硼已相继被成功制备，其高硬度和强韧性充分表明纳米力学增强机制是制备超强超硬材料的有效途径。目前纳米结构超硬材料的研究仍处于起步阶段，高温高压相转变的路径与机制、复杂中间相的结构与产生的条件、热力学条件对晶粒生长和微结构(孪晶和堆垛层错等)形成的作用，以及超硬材料的纳米结构对力学性能和强化机制的影响等尚未完全揭示出来。为此，文章对近年来在相关领域的研究进行综述，总结了设计与寻找超硬材料的一般策略与原则，概括了典型的纳米微结构对超硬材料力学与热稳定性的影响，归纳了纳米结构超硬材料的高温高压相变与转化机制，并对当前的研究进展和潜在应用进行了归纳与展望。     

TiN/SiC纳米多层膜的生长结构与力学性能

研究了TiN/SiC纳米多层膜中立方SiC（B1cubic SiC）的形成及其对TiN/SiC多层膜力学性能的影响.结果表明：在TiN/SiC多层膜中，非晶态的SiC层在厚度小于0.6nm时形成立方结构并与TiN形成共格外延生长的超晶格柱状晶，使多层膜产生硬度和弹性模量显著升高的超硬效应，最高硬度超过60GPa.SiC随着层厚的增加转变为非晶相，从而阻止了多层膜的共格外延生长，使薄膜呈现TiN纳米晶和SiC非晶组成的层状结构特征，同时多层膜的硬度和弹性模量下降.TiN/SiC纳米多层膜产生的超硬效应与立方 关键词： 立方碳化硅 TiN/SiC纳米多层膜 外延生长 超硬效应     

复合超硬材料的高压合成与研究

复合超硬材料作为一种性能优异的结构材料,被越来越广泛地应用于切削加工、油气钻探等领域.目前大部分复合超硬材料是通过高温高压方法制备.本文主要介绍了近年来复合超硬材料的高压合成与研究取得的成果和进展,重点包括纳米、亚微米、微米聚晶金刚石与立方氮化硼、立方相氮化硅-金刚石超硬复合材料以及金刚石-立方氮化硼超硬合金(复合)材料等,这些新型的复合超硬材料已经被成功合成,各种性能检测表明这些复合超硬材料的硬度、热稳定性等主要性能已明显超越传统超硬材料,可成为有广阔应用前景的新一代复合超硬材料.文中还介绍了近些年研究复合超硬材料出现的一些新的思路、方法与途径,并对复合超硬材料的进一步研究做出了展望.     

"点石成金?"--碳结构超高压物理力学

文章介绍了石墨和碳纳米管在超高压和／或纳米压痕下的层间sp^2-sp^3键转化、软硬相转换、纳米硬度等研究进展．通过量子力学和分子动力学建模分析研究，发现超高压下石墨和碳纳米管存在软相向硬相转变的双相机制，给出了超高压获取石墨和碳纳米管超硬相的条件．理论计算与实验结果吻合很好，并能合理地解释有关石墨和碳纳米管超高压实验中看似矛盾的各种实验现象．提出了碳结构超高压物理力学概念，可为超高压碳相关物质相变、物化性质调控提供理论方法．     

强激光辐照下纳米晶体铜薄膜层裂破坏的实验研究

采用强激光辐照加载技术和激光速度干涉(VISAR)测试技术，对纳米晶体铜薄膜的层裂特性进行实验测量和分析.基于VISAR实测的自由面速度波形，计算得到纳米晶体铜薄膜在超高拉伸应变率下的层裂强度高达3 GPa，明显高于多晶铜的层裂强度, 其原因归咎于纳米晶体材料中存在大量晶界阻碍了位错运动.     

调制结构对c-VC/h-TiB2纳米多层膜的超硬效应的影响

通过磁控溅射法制备了一系列不同调制结构的c-VC/h-TiB2纳米多层膜,采用X射线衍射仪、高分辨透射电子显微镜和纳米力学探针表征了多层膜的微结构和力学性能,研究了纳米多层膜调制结构与超硬效应的关系.基于实验研究结果,建立了立方结构VC和六方结构TiB2组成纳米多层膜时调制结构与硬度的关系图,该图分为四个区域,超硬效应产生于具有明锐界面和共格生长结构的区域中,而在其他区域内调制结构的改变将导致多层膜微结构发生变化,使得硬度相应降低.这一关系图可为类似异结构纳米多层膜获得超硬效应的调制结构设计提供参考.     

AIN/Si3N4 纳米多层膜的外延生长与力学性能

采用射频磁控溅射方法制备单层AlN,Si3N4薄膜和不同调制周期的AlN/S3N4纳米多层膜.采用X射线衍射仪、高分辨透射电子显微镜和纳米压痕仪对薄膜进行表征.结果发现,多层膜中Si3N4层的晶体结构和多层膜的硬度依赖于Si3N4层的厚度.当AlN层厚度为4.0 nm、Si3N4层厚度为0.4nm时,AlN和Si3N4层共格外延生长,多层膜形成穿过若干个调制周期的柱状晶结构,产生硬度升高的超硬效应.随着Si3N4层厚的增加,Si3N4层逐步形成非晶并阻断了多层膜的共格外延生长,多层膜的硬度迅速降低,超硬效应消失.采用材料热力学和弹性力学计算了Si3N4层由晶态向非晶转变的临界厚度.探讨了AlN/Si3N4纳米多层膜出现超硬效应的机理.     

强激光辐照下纳米晶体铜薄膜层裂破坏的实验研究

采用强激光辐照加载技术和激光速度干涉(VISAR)测试技术,对纳米晶体铜薄膜的层裂特性进行实验测量和分析.基于VISAR实测的自由面速度波形,计算得到纳米晶体铜薄膜在超高拉伸应变率下的层裂强度高达3 GPa,明显高于多晶铜的层裂强度, 其原因归咎于纳米晶体材料中存在大量晶界阻碍了位错运动. 关键词： 纳米晶体铜薄膜 层裂 激光辐照     

Superhard cubic BC2N compared to diamond

Recent experiments claimed successful synthesis of cubic boron-carbonitride compounds BC2N with an extreme hardness second only to diamond. In the present Letter, we examine the ideal strength of cubic BC2N using first-principles calculations. Our results reveal that, despite the large elastic parameters, compositional anisotropy and strain dependent bonding character impose limitation on their strength. Consequently, the hardness of the optimal BC2N structure is predicted to be lower than that of cubic BN, the second hardest material known. The measured extreme hardness of BC2N nanocomposites is most likely due to the nanocrystalline size effect and the bonding to the surrounding amorphous carbon matrix. This may prove to be a general rule useful in the quest for new superhard covalent materials.     

高压合成立方氮化硼用的新触媒材料的研究

 本文研究了合成立方氮化硼用新触媒材料Mg₃B₂N₄及Ca₃B₂N₄的制备方法，并对它们的稳定性及其催化作用进行了讨论。氮化硼原料的结晶状态及合成温度、合成时间、气流量等对新触媒的合成有着重要的影响。本文还在高温高压下利用新触媒进行了立方氮化硼的合成实验，结果表明，与碱土金属触媒相比新触媒具有合成压力低、转化率高、合成温度和压力范围宽、产物杂志含量低、破碎强度高等优点，是一种应用前景很大的触媒材料。     

BCxN薄膜的紫外透过光谱研究

利用射频磁控溅射方法以不同的溅射功率(80～130 W)制备出具备紫外增透性能的BCN,BC2N和BC3N薄膜.傅里叶红外吸收光谱和X射线光电子能谱测量发现样品的组成原子之间均实现了原子级化合.溅射功率对薄膜的组分和紫外增透性能有很大影响,其通过改变薄膜组分而影响紫外增透性能,且碳原子数小的样品紫外增透性能好.以110 W条件下制备的BCN薄膜中碳原子数最小,它的紫外增透性能最好,在200～350 nm波段附近平均透过率比玻璃提高了将近40%.     

Atomistic deformation modes in strong covalent solids

We report on a first-principles study of the structural deformation modes in diamond, cubic boron nitride (c-BN), and cubic BC2N. We show that (i) the diamond C-C bonds remain strong up to the breaking point, leading to the large and nearly identical shear and tensile strength, (ii) c-BN exhibits a shear failure mode different from that in diamond and a significant softening in the B-N bonds at large tensile strains long before the bond breaking, and (iii) cubic BC2N displays a large disparity between the shear and tensile strength, contrary to the expectation for the hybrid of diamond and c-BN. We examine the microscopic bond-breaking processes to elucidate the atomistic mechanisms for the deformation modes and the implications for material strength.     

Fluctuations of tensile strength and hardness of c-BC?N crystals induced by difference in atomic configuration

At the atomistic level, the physical properties of a material are determined by its structure such as atomic arrangements. Here first-principles calculations were performed to investigate the effect of atomic configuration on the tensile strength and Vickers hardness of cubic-BC?N (c-BC?N) crystals. Depending on the degree of mixture between diamond and c-BN, the tensile strength of c-BC2N crystals can vary drastically from 27 to 77 GPa. The magnitude of the Vickers hardness fluctuations (~10 GPa) is also comparable to the experimental difference (~14 GPa). Thus, atomic-scale characterization of c-BC?N crystal structures may unveil the discrepancy of the measured Vickers hardness in experiments, and uncover the obvious differences of tensile strength described in theoretical calculations.     

Preparation and Mechanical and Tribological Properties of High-Density Polyethylene/Hydroxyapatite Nanocomposites

High-density polyethylene (HDPE) nanocomposites reinforced with hydroxyapatite nanorods (nHA) were fabricated by means of extrusion and injection molding. The thermal, mechanical, and dry sliding wear properties of HDPE-based nanocomposites filled with nHA loadings up to 20 wt% were investigated. The results of mechanical property characterization showed that nHA additions improved the hardness, elastic modulus, and yield strength of HDPE at the expense of its tensile ductility and impact strength. Thermogravimetric analysis and heat deflection temperature measurements revealed that nHA fillers are very effective to enhance the thermal stability of HDPE. The wear behavior of HDPE/nHA nanocomposites was studied using a pin-on-disk tribometer. nHA fillers of a large aspect ratio improved the wear resistance of HDPE substantially because of their load-bearing effect and the formation of a continuous transfer film on the steel counterface.     

Synthesis and Curing Kinetics of UV-Curable Waterborne Bisphenol-S Epoxy-Acrylate/Polyurethane-Acrylate/Methylacryloylpropyl-POSS Nanocomposites

In order to prepare waterborne UV-curable polyurethane-acrylate (PUA) /epoxyl-acrylate (ERA) nanocomposites, the PUA, bisphenol-S epoxy acrylate (BPSEA) and methylacryloylpropyl polyhedral oligomeric silsesquioxanes (MAP-POSS) were synthesized. UV-curable BPSEA/PUA/MAP-POSS nanocomposites were prepared. The curing process, kinetics, and properties of the nanocomposites were investigated by Fourier transform infrared spectrometer (FTIR), differential scanning calorimeter (DSC) and dynamic mechanical analyzer (DMA). The base-acid resistance ability, adhesive strength, and hardness of coating films were determined. The results showed that these nanocomposites could be cured by both UV-light irradiation and a thermal free radical polymerization. Under the UV-light irradiation, they could be cured basically completely in about 20 min. The thermal free radical curing reaction could be described by a two-parameter autocatalytic ?esták-Berggren (S-B) model. The dynamic mechanical loss peak temperature, T_p, of the cured nanocomposites increased with increasing MAP-POSS content up to 8 wt%, an enhancement of 5.8°C over the pure BPSEA/PUA system, and then decreased. Films of the nanocomposites also had better base-acid resistance ability and hardness than pure BPSEA/PUA.     

First-principles prediction of the hardness of fluorite TiO2

We present first-principles calculations on the elastic constants, ideal tensile and shear strengths of cubic TiO₂ with a fluorite structure (f-TiO₂). The results show that f-TiO₂ is mechanically stable at the ground-state structure. Both shear modulus and value of hardness predicted indicate that the hardness of f-TiO₂ is comparable with TiN but is lower than TiB₂. The ideal shear strength results suggest that the hardness of f-TiO₂ is reduced because of the lower stress on the shear (1 1 1) 〈1 1¯ 0〉 slip system.     

Colossal shear-strength enhancement of low-density cubic BC2N by nanoindentation

Recently synthesized low-density cubic BC2N exhibits surprisingly high shear strength inferred by nanoindentation in stark contrast to its relatively low elastic moduli. We show by first-principles calculation that this intriguing phenomenon can be ascribed to a novel structural hardening mechanism due to the compressive stress beneath the indenter. It significantly strengthens the weak bonds connecting the shear planes, yielding a colossal enhancement in shear strength. The resulting biaxial stress state produces atomistic fracture modes qualitatively different from those under pure shear stress. These results provide the first consistent explanation for a variety of experiments on the low-density cubic BC2N phase across a large range of strain.     

Simple one-step ultrasonic synthesis of anatase titania/polypyrrole nanocomposites

In this work, hybrid nanocomposites based on anatase titania:polypyrrole (TiO₂:PPy) were directly obtained from a simple, one-step, ultrasonic (UT)-assisted synthesis. The properties of these crystalline nanocomposites were compared with those of others fabricated using cold (Cold)-assisted synthesis without any UT assistance, which required a hydrothermal treatment (HT) to yield crystalline anatase titania in the nanocomposite (TiO₂:PPy) at low temperature (130 °C) and in a short time (3 h). The SEM results demonstrated that the UT-assisted synthesis is a feasible method to obtain anatase TiO₂:PPy nanocomposites with controlled morphology using low energy. The Fourier transform infrared (FT-IR) bands of the crystalline nanocomposites exhibited a shift with respect to neat components, which was attributed to the strong interaction between the secondary amine groups (N–H) of PPy and the oxygen from TiO₂. The acceptable absorption in the visible region (λ_max = 670 nm) indicates that these nanocomposites are good candidates for harvesting energy in solar cells. Devices based on these nanocomposites were built to evaluate their electrical properties. An increase in the photocurrent was observed for the devices prepared with the nanocomposites from the UT-assisted synthesis.