雪晶成核和生长机理研究

在自然界,雪晶的形成是一个非常复杂的过程,要经历成核、生长、融并等阶段.探索雪晶的成核和生长机理对人工造雪技术的发展具有重要的作用.雪晶的形态是由冰晶的晶格结构和雪晶的生长环境所决定的.通过对均相成核和异相成核过程中自由能变化的分析,确定了晶核形成所需的临界半径和成核势垒,为研究晶核的形成过程提供了理论依据.依据雪晶质量生长率与水蒸汽通量之间的关系,从理论上明确了各种雪晶形态与生长速率之间的规律.     

内混式造雪机雪晶粒径分布实验研究

为研究人工造雪的雪晶生长规律，利用激光粒度仪测量模拟环境室内人造雪的雪晶粒径，分析了雪晶粒径随落雪高度的变化情况以及气水压力比和环境温度对雪晶生长的影响。实验结果表明：雪晶粒径随落雪高度的增加而增大，且增大趋势逐渐变缓；在落雪高度为80 cm处，晶核生长至成熟雪晶状态，索特平均直径为57～73μm;此高度时雪晶粒径接近正态分布，气水压力比和环境温度对其影响较大：气水压力比为0.5 MPa∶0.4 MPa时，成雪效果好，雪晶粒径较大且分布更均匀；低温环境也能加快雪晶生长，随着环境温度的降低，小粒径占比减少，大粒径占比增多。     

金刚石微粉末的气相合成

引言在金刚石的气相合成中,根据合成条件的不同,通常是在基板上析出具有各种自形面微粒状的金刚石。随着时间的增长,可以看到晶粒的生长。这时,一般认为是晶核在基板上产生,然后生长出晶粒。生成的晶粒形成由基板,将粒子切取成一半的形状。基板对晶核产生的     

“蒸发成核法”研究KDP晶核大小及形状

提出一种有望用于研究临界或近临界晶核大小及形状的新方法,借助于原子力显微镜(AFM),对磷酸二氢钾(KDP)晶核的大小以及形状进行了研究。结果表明,应用"蒸发成核法"可得到纳米尺寸的晶核,随溶液浓度的增大,晶核直径从25.8nm增大至59.1nm。其中最小尺寸的晶核,与低过饱和度下的理论预测值较接近。一般情况下,晶核呈球缺型,与经典理论假设形状相符。但蒸发温度低时,晶核可呈矩形(柱状),与KDP晶体的宏观形状更为接近。在蒸发速度较快时,晶核难以形成,只会出现溶质的堆积体。     

具有立方晶核ZnSe四足纳米晶的制备与表征(英文)

纳米材料的本征性质与其结构密切相关,在纳米尺度操控材料并表征其结构是纳米科学与技术的关键.采用热蒸发法制备了一种四足结构ZnSe纳米晶,通过高分辨透射电子显微镜对这种四足ZnSe纳米晶的晶体结构进行了表征.该ZnSe纳米晶由一个四面体的立方晶核和四个沿[001]方向生长的六方相分枝构成.本研究对这种ZnSe纳米晶的形貌和结构进行了讨论,证明了在ZnSe纳米晶内两种晶相的共存.根据ZnSe的结晶学特性和晶相的温度稳定性,解释了这种四足结构纳米晶的生长机制:ZnSe的四面体立方晶核在高温区域形成后,ZnSe蒸汽在低温区继续沉积在晶核上形成四个六方相的分支足,最终形成了具有立方晶核的ZnSe四足纳米晶.     

环戊烷水合物生长过程实验研究

实验研究了环戊烷水合物形成过程,实验结果表明水合物相变过程包含降温、晶核诱导、水合物快速生长和水合物缓慢生长几个过程,其中晶核形成诱导期和生长期呈现出随机性的特点。水合物的形成首先出现水-环戊烷的的界面处,并向下生长。在水合物形成的最后阶段,水合物沿着铜管爬升生长,充满整个蓄冷器。     

BaMoO4薄膜在电化学法制备中生长基元的实验研究

在采用恒电流电化学技术研究BaMoO4薄膜的初期生长特性实验中,发现了若干很重要的实验现象:薄膜生长初期形成的晶核首先具有白钨矿结构的骨架;晶核和刚开始长大生成的晶粒都是疏松的,晶粒都明显显示有蜂窝状空隙存在;随着制备时间的增加,白钨矿结构骨架原先疏松的程度逐渐减弱,经过一定时间,晶粒趋于饱满,晶粒表面基本光滑;在薄膜形成的过程中,新生成的晶核也具有类似情况.结合BaMoO4薄膜电化学制备机制分析,作者认为:利用电化学技术制备BaMoO4晶态薄膜时,在生长初期基体Mo以[MoO4]2-的形式构成负离子配位多面体生长基元,这些生长基元优先选择在基体缺陷处作为白钨矿结构的晶核堆砌和生长,并进而与溶液中的[Ba]2 相连接,键合成BaMoO4晶粒,再逐渐长大.显然,该发现和研究对于丰富晶体生长动力学知识及指导利用电化学技术制备晶态薄膜都具有重要意义.     

AZ91D镁合金锌系磷化膜成膜机理和生长过程的研究

以磷酸盐化学转化膜为研究体系,采用SEM、XRD、OCP等分析方法及检测手段,研究AZ91D镁合金系磷酸盐化学转化膜的成膜机理、膜层结构及生长过程。研究发现AZ91D镁合金在磷化液中成膜过程分5个阶段:初始成核(1～5s)、基体快速溶解(5～60s)、晶体快速生长(1～2min)、膜层稳态生长(2～10min)和膜层沉积溶解平衡阶段(10min以后)。AZ91D镁合金表面的磷酸盐晶核的形成并非在金属进入溶液的最初时刻一次形成,是分批形成。最先形成的晶核逐渐长大,新的晶核不断生成,磷酸盐晶粒对其表面的覆盖度逐渐增大,直至各个晶粒逐渐长大相互接界,将其表面完全覆盖,结晶过程结束。晶核的形成未优先发生在基体金属的晶界上,随着晶核的生长和外延而形成磷化膜。     

CVD金刚石的形核和生长

应用自制的热解丝CVD装置,研究了在金刚石沉积过程中改变甲烷浓度对其形核和生长的影响。结果表明,金刚石形核后,增加甲烷浓度,仍然可以在硅基底表面继续形成新的晶核。但是甲烷浓度由0．6％逐渐增加到1．2％时,所得最终形核密度比一开始就将甲烷浓度设为1．2％的形核密度低。新晶核比先形成的晶核具有较大的长大速度,随后所有晶核尺寸逐渐趋向相同。     

微混效应对化学法制备超细铜粉的影响分析

采用化学还原法制备了粒度在200nm左右、形貌均匀的球形超细铜粉.研究结果表明,将硫酸铜滴加到水合肼和PVP的混合溶液中,形成的溶液体系局部过饱和度高,化学反应速率和形核速率都非常快,在极短时间内可形成大量的铜晶核.高速搅拌时,溶液体系的微混状况良好,在剪切搅拌抑制机制与断裂破碎机制的作用下会生成粒度较小、形貌均一的铜粉颗粒;低的搅拌速度下,微混作用较弱,PVP分子在铜晶核表面产生不完全吸附,导致铜晶核的二次成核和定向生长,生成长方体的条形颗粒.     

Microstructural resistance of snow following first wetting

One of the main causes for the formation of wet snow avalanches is the decrease in strength following the introduction of liquid water. The influence of liquid water on initially dry snow was investigated in twelve field experiments in the Eastern Swiss Alps. For this, a variety of different snow types were artificially wetted and liquid water content (using the Snow Fork), microstructural penetration resistance (Snow Micro Penetrometer, SMP) and snow density were measured. Measurements were repeated up to eight times resulting in a total 1100 liquid water content, 355 penetration resistance, and 737 snow density measurements. Following wetting, microstructural penetration resistance decreased in layers consisting of faceted crystals or depth hoar at low liquid water content. Layers consisting of precipitation particles or small round grains, on the other hand, showed no significant trend at a similar water content. The results indicate that first wetting of faceted crystals and depth hoar layers has an impact on snow strength, and consequently contributes to a decrease in snow stability.     

A numerical study of mechanisms of accretion for rotating snow sleeves evolving on conductor rods

It is the purpose of this paper to construct a mathematical/computational model for the accretion of snow on rotating snow sleeves attached to conductor rods, and to consider some of the limitations in the use of the assumption of perfect cylindrical-sleeve growth, as currently employed to provide estimates of snow-loading intensity. For purposes of illustration, three rotating snow accretion regimes are considered, in which the direction of the airflow is always taken as perpendicular to the axis of the conductor rod. Firstly, a horizontal conductor rod is free to rotate and is initially bare. Rotation of the system (conductor rod and accreted snow) is initiated by snow torque, due to eccentric snow loading on the windward side of the conductor, and is driven by aerodynamic torque generated by the windflow past the evolving, rotating, asymmetrical sleeve growth. Secondly, a vertical conductor rod is free to rotate and the bare rod is given an initial small angular velocity; the rotation of the accreting system is then driven by aerodynamic torque alone. Thirdly, a horizontal rod is fixed and is initially covered by an annulus of snow, supported by a thin layer of (unfrozen) water existing between the conductor and the snow sleeve. In this case, the rotation of the snow sleeve is due to the combined snow and aerodynamic torques and is now retarded by the viscous torque due to the rotating thin film of water. Asymptotic solutions obtained for small accretion time supplement numerical results derived from the computational models. Numerical results on these rotating non-periodic snow accretion regimes are presented as a function of the wind speed, snow precipitation rate and diameter of the conductor rod.     

雪晶形成过程与人工造雪机技术

本文从自然降要过程出发，介绍了自然雪晶的形成过程、条件和成长机理，进而阐明了实现人工造雪过程所需要的基本条件和要求，对目前存在的人工造雪机进行了分类机理说明，简要介绍了国外目前的技术发展和产品情况。     

边坡坡度对路堤风吹雪影响研究

路堤边坡坡度是影响风吹雪区域路堤积雪的重要因素,利用现场模型试验以及流场数值模拟的方法,研究边坡坡度对路堤积雪影响规律,分析积雪形成机理。结果表明:路堤周围积雪范围与流场风速减弱区域存在较好的对应关系,但并非定量对应,用流场中风速减弱区域简单地定量判断积雪范围的方法误差较大;迎风边坡坡度的改变对路堤周围积雪分布影响较大,较缓的迎风边坡坡度将有效地减小路堤周围积雪量和积雪范围,路面更不易形成积雪,可以减小风雪流对路堤工程的危害;背风边坡坡度的改变对路堤周围积雪影响较小,路堤设计时可作为次要因素考虑;根据试验和模拟结果得出路面不易积雪的临界边坡坡度在40°和45°之间,保守起见,建议工程应用中取40°为路面不易积雪的临界边坡坡度。     

Pneumatic Conveying for Snow Storage Plants

In Japan, the number of snow storage plants has been increasing for the purpose of storing agricultural products and for the air-conditioning of buildings in summer. In order to gather snow into the snow storage room, a rotary snow plow is often used, but it is difficult to pile up snow within the room from inside out. To overcome this limitation, a pneumatic snow conveying system was developed in order to transport snow to the snow storage plant. The system consists of a blower, snow feeder, cyclone separator, and pipeline. The pressure type of the conveying system incorporates both negative and positive pressures, with a capacity to convey wet snow at 10 t/h a distance of 40 m. For snow with free water content below 15%, the system conveyed wet snow without sticking to the inside walls of the pipeline.     

气相转移法制备多孔双相沸石复合物

采用气相转移法制备了同时含有Y和ZSM-5沸石的双相沸石复合物。采用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、N₂吸附-脱附和骨架红外(FT-IR)等手段对合成的样品进行了表征。结果表明, 合成的样品受Y型沸石的添加量、干胶制备条件和气相转移法转化条件的影响; 水热预处理后的ZSM-5沸石前驱体中具有ZSM-5沸石的骨架特征振动峰, 归结于ZSM-5沸石晶核或微晶的形成, 这有利于在气相转移法转化过程中引导或促使体系向ZSM-5沸石相的转变, 并有利于抑制ZSM-35杂晶的形成; Y型沸石在蒸汽处理过程中的脱铝过程导致合成的沸石复合物具有介孔结构。     

水热法制备A型沸石粉体中的聚合生长

通过水热法制备A型沸石粉体实验和粉体表征,对水热条件下粉体晶粒的生长过程进行较深入的研究,提出了水热体系中A型沸石晶粒同时存在聚集生长和聚合生长。聚集生长是物粒从小尺寸粒子向大尺寸粒子输运的重结晶过程,而聚合生长则是小晶粒之间通过聚合作用形成粒度更大的晶粒的作用过程。两者的热力学驱动力都是由于晶粒平均粒度的增大降低了体系总的表面自由能。水热体系A型沸石晶粒的形成经历了前驱物溶解成核并形成纳米小粒子晶粒的聚合生长晶粒的聚集生长4个阶段,最后得到的晶粒以聚合生长为主,同时伴随着聚集生长的综合结果。     

Direct synthesis of ZnS nanoribbons, micro-sheets and tetrapods

ZnS nano and micro structures such as nanoribbons, large sheets and tetrapod shaped crystals were fabricated by direct thermal evaporation of ZnS powder without using any catalyst. Formation of the one dimensional structures such as nanoribbons and micron order sheets was attributed to the vapor-solid growth mechanism. The formation of octahedron nucleus with cubic crystal structures was proposed as the growth unit of the wurtzite crystal structured tetrapods. Appearance of the periodic stacking faults or twining planes in between alternate cubic and hexagonal crystal structured zones along the growth direction of the ribbons provided secondary growth sites for the octahedron nucleus and subsequent crystal growth resulted in to the formation of the tetrapod arrays. These nano/micro structures of ZnS exhibited a green emission band at room temperature.     

Some concepts concerning twinning and cellular growth of bulk barium metaborate (BBO) crystals

Bulk single β-BaB₂O₄ (BBO) crystals have been grown by the TSSG method in Czochralski variant, using Na₂O and NaF as the solvents. It was found that formation of twins (electric type) or cellular substructures of different scales is their specific growth feature. We believe that their formation is governed by changes in the composition and structure of the double-electric layer (DEL) at the interface of crystal growth. In Z-axis crystals only microtwins structures were revealed whose number and localization are determined by the composition of used solvent. The cellular growth of these crystals at a particular stage is a result of the impossibility of frontal formation of an antipolar structure (macrotwin) under the conditions of increasing charge density in the DEL. In the Y(X-)-axis crystals the conditions for formation of one or three (five, etc.) macrotwin boundaries and, hence, noncellular growth are always realized. Model concepts, characterizing seeding and growth of Y(X-)- and Z-axis BBO crystals are discussed.