Al2O3 hollow nanospheres prepared by fluidized bed chemical vapor deposition and further heat treatment

Hollow-structured Al₂O₃ nanospheres were prepared by chemical vapor deposition (CVD) and further heat-treatment process. For the CVD process, a unique cooling gas loop was designed in the precursor delivery system and amorphous Al₂O₃ nanospheres with incomplete pyrolysis intermediated products were obtained in a vertical fluidized bed at 500℃. The microstructure of the nanospheres tended to hollow after subsequent heat treatment at 1300℃ in air. It was found that the obtained Al₂O₃ hollow nanospheres showed a size of 100-450 nm in diameter and 20-80 nm in shell thickness. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis confirmed that the Al₂O₃ hollow nanospheres were α-Al₂O₃. The possible formation mechanism was proposed based on the results of the thermal gravimetric (TG) measurement connected with a Fourier transform infrared spectrometry (FTIR). The novel method to prepare Al₂O₃ hollow nanospheres can inspire ideas for the design and synthesis of other hollow-structured nanospheres.     

Formation and growth mechanism of porous,amorphous, and fine particles prepared by chemical vapor deposition. Titania from titanium tetraisopropoxide

The formation and growth mechanism of porous, amorphous, and fine particles were investigated. TiO₂ particles were produced in a tubular flow reactor by a chemical vapor deposition technique using titanium tetraisopropoxide as a starting material at low temperatures (573-973 K) and atmospheric pressure. Prepared particles were of submicron size and had large surface area (as large as 270 × 10³ m²/kg). According to the proposed mechanism, reactions begin on the reactor wall and then the primary particles form in the gas phase by chemical reactions. The primary particles collide, coalesce with each other and grow. However, significant experimental deviations from the Brownian collision and coalescence theory imply that other processes, such as the surface reactions on the particle, play an important role in the growth, in addition to coalescence. Intraparticle reactions decreased the surface area by filling the pores.     

Micro‐nanostructured silicone‐carbon composite coatings with superhydrophobicity and photoluminescence prepared by oxidative chemical vapor deposition

Transparent, superhydrophobic, and colored silicone–carbon composite coatings were prepared by oxidative chemical vapor deposition (oCVD) of bulk silicone at ambient pressure. The colors, wettability, morphologies, and transparency of the coatings can be easily varied via changing both the concentration of gaseous oxygen and the deposition temperature. Typically, the black, brown, and yellow silicone–carbon composite coatings with different superhydrophobicity and transparency were achieved under oxygen‐deficient atmospheres. Furthermore, the colored samples showed photoluminescence when they were excited by ultraviolet (UV) light, which is due to the fluorescence of carbons embedded inside the as‐prepared coatings. In addition, more regular papillae and nanofibers with excellent superhydrophobicity were obtained at higher deposition temperatures. Our method was believed to develop a new strategy for fabricating multifunctional silicone–carbon composite coatings. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40400.     

Dual layer SiC coating on matrix graphite sphere prepared by pack cementation and fluidized‐bed chemical vapor deposition

A dual layer silicon carbide (SiC) coating including inner porous SiC (p‐SiC) layer and outer dense SiC (d‐SiC) layer was fabricated on the matrix graphite (MG) spheres of high‐temperature gas‐cooled reactor fuel elements by pack cementation and fluidized‐bed chemical vapor deposition process to improve the oxidation‐resistant property. Microstructure of the coating demonstrates different density and structure of the two SiC layers with no obvious boundaries between them. Weight gain curves of oxidation tests at 1773 K for 200 hours show that the coating could effectively protected the MG sphere by isolating the air infiltration with p‐SiC layer as the main functional layer and d‐SiC layer as the transition layer to improve the bond strength. Due to the transition function of p‐SiC layer, the coated spheres could understand more than 50 times thermal shocking tests from 1773 K to room temperature with no stress cracking.     

化学气相沉积PPy/UHMWPE纤维的研究

采用化学气相沉积制备了聚吡咯/超高相对分子质量聚乙烯(PPy/UHMWPE)纤维,测试了不同氧化剂浓度、不同沉积时间和温度下PPy/UHMWPE纤维的表面剪切强度,用扫描电镜、动态热机械分析仪、傅立叶变换红外光谱仪分析了PPy/UHMWPE纤维的表面形态、热机械性能和复合材料官能团的变化。结果表明:PPy均匀分布在UHMWPE纤维表面,UHMWPE纤维与PPy之间无化学键作用而是分子间作用力;随着氧化剂三氯化铁浓度的增加和吡咯沉积时间的延长,PPy/UHMWPE纤维表面剪切强度先增大后减小;随着处理温度的升高,PPy/UHMWPE纤维表面剪切强度先增大,当处理温度超过85℃时,其剪切强度则减小。     

Structure of copolymer films created by plasma enhanced chemical vapor deposition

The interface structure in copolymer films made using plasma enhanced chemical vapor deposition (PECVD) has been probed for the first time using X-ray reflectivity. Copolymer films made from comonomers benzene (B), octafluorocyclobutane (OFCB), and hexamethyldisiloxane (HMDS) show extremely sharp interfaces and scattering length density depth profiles that are uniform with depth, making them useful for optical applications. The polymer/air interface has an rms roughness (∼5 Å) that is only slightly larger than that of the supporting substrate (∼3 Å). Addition of either benzene or HMDS as a comonomer in the deposition of OFCB alters a transient deposition behavior at the silicon oxide interface that occurs when using only OFCB. For the B-OFCB copolymer films, a facile control of refractive index with monomer feed composition is achieved. A nonlinear variation in the X-ray scattering length density with composition for the HMDS-OFCB copolymer films is consistent with the nonlinear visible light refractive index (632.8 nm) variation reported earlier.     

化学气相沉积工艺制备绳状纳米碳管的研究

用硝酸铁作催化剂,乙炔作碳源气体,高纯氮气作稀释气体,在750℃下化学气相沉积生长了绳状纳米碳管,用高分辨扫描电镜观察了所得绳状纳米碳管的形貌.纳米碳管的直径为100～200nm,长度为10～20 μm.文中还提出了绳状纳米碳管的生长机理.     

Fabrication of SiC/diamond composite coatings by electrophoretic deposition and chemical vapor deposition

In this research, SiC/diamond composite coatings were fabricated by a novel procedure that consisted of the electrophoretic deposition (EPD) of diamond particles onto graphite substrates followed by chemical vapor deposition (CVD) of SiC. Various concentrations of MgCl₂ were employed to increase the deposition rate and uniformity of the deposits during the EPD process by giving a positive charge to diamond particles. The CVD of SiC was found to have a tightly connected diamond‐graphite interface and spherical texture. With higher weight fraction of diamond particles deposits, the wear of steel ball increased, while the wear of SiC coating decreased.     

A novel core-shell structured ultra-coarse WC-Co composite powders prepared by fluidized bed chemical vapor deposition

The ultra-coarse WC-Co composite powders with a core-shell structure were effectively prepared by fluidized bed chemical vapor deposition (FBCVD) using CoCl₂ precursor. An excellent interfacial bonding was formed between WC and the deposited Co. Defluidization was the major barrier to depositing high-Co-content composite powders, which was caused by the adhesion of the deposited Co particles. Decreasing the deposition temperature reduced the cohesion force of the deposited Co particles, which was thus beneficial to preventing the defluidization. Increasing the WC particle size and the gas velocity increased the collision force and benefited the fluidization. The final Co contents were largely dependent on the deposition and fluidization behaviors. For the conditions tested, the optimal deposition temperature was 800°C while the minimum WC particle size suitable for FBCVD was 25 μm.     

Structural study of β‐SiC(001) films on Si(001) by laser chemical vapor deposition

β‐SiC thin films have been epitaxially grown on Si(001) substrates by laser chemical vapor deposition. The epitaxial relationship was β‐SiC(001){111}//Si(001){111}, and multiple twins {111} planes were identified. The maximum deposition rate was 23.6 μm/h, which is 5‐200 times higher than that of conventional chemical vapor deposition methods. The density of twins increased with increasing β‐SiC thickness. The cross section of the films exhibited a columnar structure, containing twins at {111} planes that were tilted 15.8° to the surface of substrate. The growth mechanism of the films was discussed.     

Pure and (zinc or iron) doped titania powders prepared by sol-gel and used as photocatalyst

Pure and doped (zinc and iron) nanocrystalline titania powders were prepared by the sol-gel route. Doping tends to change the existing crystalline phases and their degree of crystallinity, but particle size distribution and morphology of the particles are also affected. In the pure titania system, the main crystalline phase is anatase but rutile is also present. The doped (Zn and Fe) titania crystallizes only as anatase. The undoped titania shows a bimodal distribution of particles size: fine (20-40 nm) and coarse (300-500 nm) grains. The doped TiO₂ powder also exhibits a much more uniform particle size distribution, with all grains under 40 nm.The photocatalytic efficiency of suspended powders was tested on the decolouration of Orange II aqueous solutions under visible artificial light irradiation. The maximum decolouration reached by the pure TiO₂ was 81% at a rate of 3.6 × 10⁻³ min⁻¹. Iron doping decreases the photocatalytic activity; the maximum dye degradation was only 43% at a rate of 1.3 × 10⁻³ min⁻¹. On the contrary, the performance of Zn-doped titania was better, having a decolouration rate of 17.7 × 10⁻³ min⁻¹.     

化学气相沉积的过程监测

介绍了化学气相沉积(CVD)过程监测的内容。根据CVD过程的特点和薄膜生长的要求,在CVD中需要被实时监测的主要过程参数是温度、化学物种和流体流动。对现有监测方法的原理和优缺点进行分析对比。重点介绍了常用监测方法以及原位实验监测的研究进展。     

High-speed epitaxial growth of SrTiO3 films on MgO substrates by laser chemical vapor deposition

Epitaxial (100) and (111) SrTiO₃ films were prepared on (100) and (111) MgO single-crystal substrates, respectively, using laser chemical vapor deposition. The effect of deposition temperature (T_dep) on the orientation and microstructure of the SrTiO₃ films was investigated. On the (100) MgO substrates, SrTiO₃ films showed a (111) orientation at a low T_dep of 1023 K. (100) SrTiO₃ films, which were epitaxially grown at T_dep=1123–1203 K, had dense cross sections and flat surfaces with rectangular-shaped terraces. On the (111) MgO substrates, (111) SrTiO₃ films were epitaxially grown at T_dep=983–1063 K; however, these films' orientations became random at high T_dep of 1063–1113 K. The (111) SrTiO₃ films consisted of columnar grains with triangular pyramidal caps. The deposition rates of the epitaxial (100) and (111) SrTiO₃ films were 13–25 and 18–32 μm h⁻¹, respectively, which is 5–530 times higher than those obtained by MOCVD.     

流化床化学气相沉积法制备近化学计量比的Ti N粉体

传统气-固反应工艺制备Ti N粉体存在难以逾越的内扩散控制过程,导致制备高纯、正化学计量比的Ti N粉体至今存在巨大困难。提出了流态化化学气相沉积工艺(FBCVD)制备高质量TiN粉体,即基于TiCl₄-N₂-H₂体系,在往复运动的TiN种子粉体上沉积新生高质量TiN粉体的新方法。实验发现,当TiN种子粉体粒径大于52.95μm时,即使在1000℃沉积2 h也不会失流,同时在TiN种子粉体上获得了亚微米级的结节状新生TiN颗粒。通过氧氮分析仪和XRD分析发现,新方法显著提升了粉体的氮含量,获得了近化学计量比的TiN0.96,且氧含量下降了约40%。此外,流化床中气相沉积TiN的生长模式为岛状生长模式,为工业中制备高质量TiN粉体提供了一种新的方法。     

Fast preparation of (111)‐oriented β‐SiC films without carbon formation by laser chemical vapor deposition from hexamethyldisilane without H2

(111)‐oriented β‐SiC films were prepared by laser chemical vapor deposition using a diode laser (wavelength: 808 nm) from a single liquid precursor of hexamethyldisilane (Si(CH₃)₃–Si(CH₃)₃, HMDS) without H₂. The effects of laser power (P_L), total pressure (P_tot) and deposition temperature (T_dep) on the microstructure, carbon formation and deposition rate (R_dep) were investigated. β‐SiC films with carbon formation and graphite films were prepared at P_L ≥ 170 W and P_to ≥ 1000 Pa, respectively. Carbon formation strongly inhibited the film growth. β‐SiC films without carbon formation were obtained at P_tot = 400‐800 Pa and P_L = 130‐170 W. The maximum R_dep was about 50 μm·h^?1 at P_L = 170 W, P_tot = 600 Pa and T_dep = 1510 K. The investigation of growth mechanism shows that the photolytic of laser played an important role during the depositions.     

化学气相催化热解法制备螺旋纳米碳纤维

采用酒石酸铜前驱体热分解得到纳米铜粒子作为催化剂,分别对250℃、280℃、310℃分解产生的纳米铜粒子进行测试分析,在3个温度下用化学气相沉积法生长螺旋纳米碳纤维并进行综合热分析。采用X-射线衍射(XRD)分析其物相组成,晶粒大小;用扫描电子显微镜(SEM)观察螺旋纳米纤维的外观形貌。结果表明,310℃生长出的螺旋纳米碳纤维纯度高、外观形貌清晰,热分析质量损失少。     

Facile synthesis and spectroscopic characterization of siliconitride phosphors for white light‐emitting diodes

A modified chemical vapor deposition (CVD) technique is used to synthesize the color‐tunable siliconitride Sr_2‐1.5x‐yCe_xEu_ySi₅N₈ (x = 0.000‐0.016 and y = 0.000‐0.020) phosphors. In comparison with the conventional solid‐state method, the CVD approach successfully improved the crystallinity, particle size distribution, and photoluminescence through the enhanced gas‐solid reaction. Under blue excitation, Sr_1.98Eu_0.02Si₅N₈ exhibited a red emission band at 618 nm. The incorporation of Ce³⁺ ions increased the emission intensity of Eu²⁺ ions by approximately 10% owing to the enhanced absorption and dipole‐dipole energy transfer process from Ce³⁺ to Eu²⁺ ions. It resulted in a shift of the emission colors from yellow to red region. The external and internal quantum efficiencies of Sr_1.906Ce_0.06Eu_0.004Si₅N₈ were calculated as 54% and 70%, respectively. The activation energy of thermal stability for Sr_1.906Ce_0.06Eu_0.004Si₅N₈ was evaluated as 0.31 eV. A white LED with a color rendering index of 80 and a CCT of 4964 K was successfully fabricated with the present phosphors. The current research demonstrated a new series of Sr₂Si₅N₈:Ce³⁺, Eu²⁺ phosphors with color‐tunability for fabricating white LEDs with high color‐rendering index.     

A time‐dependent multiphysics,multiphase modeling framework for carbon nanotube synthesis using chemical vapor deposition

A time‐dependent multiphysics, multiphase model is proposed and fully developed here to describe carbon nanotubes (CNTs) fabrication using chemical vapor deposition (CVD). The fully integrated model accounts for chemical reaction as well as fluid, heat, and mass transport phenomena. The feed components for the CVD process are methane (CH₄), as the primary carbon source, and hydrogen (H₂). Numerous simulations are performed for a wide range of fabrication temperatures (973.15–1273.15 K) as well as different CH₄ (500–1000 sccm) and H₂ (250–750 sccm) flow rates. The effect of temperature, total flow rate, and feed mixture ratio on CNTs growth rate as well as the effect of amorphous carbon formation on the final product are calculated and compared with experimental results. The outcomes from this study provide a fundamental understanding and basis for the design of an efficient CNT fabrication process that is capable of producing a high yield of CNTs, with a minimum amount of amorphous carbon. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Synthesis of oriented nanotube films by chemical vapor deposition

Oriented nanotube films (20-35 μm thick) were synthesised on flat silicon substrates by chemical vapor deposition (CVD) of a gas mixture of acetylene and nitrogen. For the CVD we used metal oxide clusters formed by spin coating an iron(III) nitrate ethanol solution onto a silicon substrate and subsequent heating. The cluster density and its effects on the nanotube density were investigated as a function of the iron(III) nitrate concentration and the synthesis temperature. A high nanotube density was achieved with a high density of iron oxide clusters as nucleation centres for the growth of nanotubes. The cluster density was controlled by the iron(III) concentration of the ethanolic coating solution and by the synthesis temperature. The perpendicular orientation of the nanotubes with respect to the substrate surface is attributed to a high density of nanotubes.