pH值对沉淀法制备Al_2O_3相转变的影响

为获得Al2O3粉体,以Al(NO3)3为原料,NH3.H2O为沉淀剂,采用液相化学沉淀法,在不同pH值下制备了Al(OH)3前驱体,并利用TEM,XRD,TGA等分析手段对前驱体进行了表征。pH=5时前驱体的相转变主要是由非晶Al(OH)3→非晶Al2O3→α-Al2O3;pH=9及pH=11前驱体的相转变主要是由Al(OH)3→γ-Al2O3→θ-Al2O3→α-Al2O3。XRD结果表明,低pH值时制备的Al(OH)3更容易转化为α-Al2O3稳定相。pH=5时前驱体在1 100℃就可转变为α-Al2O3稳定相,而pH=9和pH=11时制备的前驱体则分别需在1 150℃和1 200℃才能转变为α-AlO。研究表明,pH值对制备的前驱体的物相、形貌、相转变都有很大影响。     

液相沉淀法制备ZrO2/Al2O3纳米复合粉体

以NH4Al(SO4)2·12H2O,ZrOCl2·8H2O,Y(NO)3为原材料,用NH4HCO3作沉淀剂,控制滴定速度小于5 mL/min,采用液相沉淀法制备了超细3Y-ZrO2/Al2O3前驱体.前驱体为分散的碱式碳酸盐,在1 200℃煅烧得到了分散性良好,平均粒径为20 nm的t-ZrO2和α-Al2O3两相分布均匀的纳米复合粉体.X射线衍射分析显示前驱体在煅烧过程中无中间相γ-Al2O3,θ-Al2O3生成.所制备的粉体具有高的烧结活性.在1 450℃烧结后烧结体相对致密度可达97.4%.     

溶胶-凝胶法制备α-Al2O3微晶陶瓷的研究

以一水氧化铝(AlOOH)为原料,采用溶胶-凝胶法,经搅拌分散、胶溶、凝胶以及凝胶干燥过程制备了α-Al2O3微晶陶瓷的前驱体,热处理得到α-Al2O3微晶陶瓷.通过TG(DTG)-DTA和XRD等分析测试手段,详细考察了胶溶剂、pH值、固含量等工艺参数对α-Al2O3微晶陶瓷性能的影响,得到了合适的工艺参数.     

薄水铝石粒度对煅烧形成α-Al2O3粉体的影响

研究了薄水铝石粒度对其煅烧形成α-Al2O3粉体的影响.先用水热法制备出均匀分散的纳米、亚微米及几个微米的薄水铝石前驱体,用x射线衍射仪和电子显微镜分析了薄水铝石在不同温度煅烧所得产物的相结构及形貌.结果表明,粒度30～100 nm的薄水铝石在1200℃煅烧1 h转变为α-Al2O3,为蠕虫状的烧结颗粒;粒度0.4～0.6 μm的薄水铝石在1 250℃煅烧1 h可转变为α-Al2O3,颗粒尺寸变化不大,仍在0.4～0.6 μm范围内;粒度1 μm左右的薄水铝石在1350℃下煅烧2 h尚不能完全转变为α相,并已出现明显烧结.因此,以水热法制备的亚微米级薄水铝石晶体作为前驱体,经直接煅烧可以制备出分散性较好的亚微米级α-Al2O3粉体.     

纳米α-Al2O3的合成及其对混凝土力学性能的影响

采用溶胶凝胶法合成α-Al2O3纳米粉,利用DSC-TG、XRD及TEM对α-Al2O3纳米粉的煅烧温度、晶相及微观形貌进行表征.将该纳米粉掺入水泥混凝土,借助万能试验机研究纳米α-Al2O3粉对混凝土力学性能的影响.结果 表明:干凝胶前驱体经1050℃煅烧得到结晶良好的α-Al2O3纳米粉;纳米α-Al2O3粉的适量添加使混凝土的抗压强度、抗折强度及劈裂抗拉强度均有所提高,对劈裂抗拉强度的提高最为显著;纳米α-Al2O3粉的合理掺量在2.0％ ～2.5％之间;纳米α-Al2O3粉的添加有利于降低混凝土的脆性.     

前导物粒径对x-Al2O3至α-Al2O3相转换过程的影响

观察氧化铝(Al2O3)过渡相χ-Al2O3及χ-Al2O3 κ-Al2O3混合相微粒粉末相变特征,目的在了解同一过渡相晶径组成的粉末粒体大小对最终所得α相生成及数量的关系.样品以热处理方式由Gibbsite相变先得χ-Al2O3,将其研磨(Pearl mill)降低其凝聚状态,再以重力分级(Gravity settling)处理得到3种不同粒径(90,165和265 nm)的χ-Al2O3粉末,χ和κ混合相粉末3样品(粒径3.9,4.2和4.5 μm),再将此3种χ-Al2O3粉分别热处理获得.研究结果显示由χ或κ相生成α相的相变温度与起始原料粒体大小有关,χ相组成粒体粒径越小,其相变温度越低且越明显.     

溶胶-凝胶法低温合成亚微米α-Al_2O_3的制备与表征

以硝酸铝、硝酸铜、钛酸丁酯为原料,采用溶胶-凝胶法低温合成亚微米氧化铝粉体,并对不同温度煅烧后粉体的物相与形貌进行表征。通过在硝酸铝的无水乙醇溶胶中引入少量CuO-TiO2氧化物助剂前驱体,在80℃水浴中长时间静置后获得浅蓝色凝胶,在不同温度下煅烧该凝胶,结果发现:煅烧温度低于950℃时,所获得的粉体主要为无定形态,仅含有少量的γ-Al2O3;将凝胶在1000℃煅烧2h后,全部变成粒径在80～100nm之间、粉体颗粒呈球形的α-Al2O3;当煅烧温度升高到1050℃时,α-Al2O3的颗粒直径长大到200～400nm。CuO-TiO2氧化物助剂前驱体的引入,不但降低了α-Al2O3的晶相合成温度,同时促进了α-Al2O3粉体颗粒的生长。     

γ-FeNi包裹Al2O3复合微球的制备

利用非均相沉淀包裹技术,在室温于水溶液中,以球形α-Al2O3、硫酸镍、硫酸亚铁和碳酸氢铵为原料,先制备了无定形碱式碳酸镍和水合氧化铁包裹氧化铝球形微粉前驱体.然后,前驱体在600℃经氢气还原2 h制备表面光滑、致密的γ-FeNi包裹Al2O3复合微球.利用扫描电镜、能量散射分光仪、X射线衍射和热重分析仪等表征了复合微球前驱体及还原产物的表面和断面形貌、成分以及前驱体热分解过程.对被包裹氧化铝含量、加料速度、搅拌速度、反应时间及表面活性剂等影响因素进行了分析.初步得到优化制备条件,即15 g/L α-Al2O3,加料速度为5 mL/min,搅拌速度为1 000 r/min,反应时间为1 h,表面活性剂添加量为5 mL/L.γ-FeNi的热膨胀系数接近于氧化铝,因而使两者形成了较好的界面结合.这种γ-FeNi包裹Al2O3复合微球可用作新型吸波材料.     

γ-Al2O3中孔陶瓷膜的制备及表征

以粒径为0.3～0.4 μm的α-Al2O3为原料,通过悬浮液真空抽吸法,制备出平均孔径约为70 nm的完整无缺陷的片状α-Al2O3支撑体;以仲丁醇铝为前驱体,采用颗粒溶胶路线制备出稳定的Boehmite溶胶,以此溶胶采用浸浆法,在制备的α-Al2O3支撑体上制备出完整无缺陷的γ-Al2O3中孔膜,并考察了烧成温度对γ-Al2O3中孔膜性能的影响.结果表明,本文制备出的γ-Al2O3膜的孔径约为3 nm,对PEG的截留分子量为2800～5300,纯水渗透通量为11.5～25.9 L·m-2·h-1[7.6×105 Pa,(14±1)℃].说明在孔径为70 nm左右的载体上直接制备孔径为3 nm的完整的中孔膜是可行的.     

SiO_2-Al_2O_3复合氧化物作为Pt负载型NO氧化催化剂载体的性能

采用共沉淀法制备Si O2-Al2O3复合氧化物载体,研究制备过程中陈化时间、Si O2与Al2O3物质的量比、焙烧温度以及硅前驱体对载体物化性能的影响,采用N2物理吸附仪、激光粒度仪、X射线衍射、NO程序升温脱附和扫描电镜对样品进行表征。分别在800℃和1 000℃条件下对载体进行高温老化,对比老化前后载体性能。采用浸渍法负载载体质量分数0.3%的Pt,制得Pt负载型NO氧化催化剂,对催化剂活性进行评价。结果表明,硅铝物质的量比为20∶80,陈化12 h,以硅溶胶为硅前驱体,500℃焙烧得到的载体负载Pt后达到预期的NO氧化效果,可以作为NO氧化催化剂用载体。     

优质超滤ZrO2复合膜制备研究

用无机盐氧氯化锆（ZrOCl3·8H2O）为前驱体,CaO为晶型稳定剂,六次甲基四胺为水解促进剂,在超声场作用下采用溶胶-凝胶（Sol-Gel）法在圆形Al2O3片基底上成功地制备了钙稳定的ZrO2陶瓷超滤膜,所用基底的最可几孔径为280nm。重点考察了超声波、涂膜次数及添加剂对膜性能的影响。并借助TG—DTA、SEM、XRD及气体渗透等测试手段,对ZrO2超滤膜的表面形貌、晶型稳定性及气体在其中的渗透特性等进行表征和分析。研究结果表明：该法制备的ZrO2超滤膜,表面无裂纹缺陷,孔径分布窄,最可几孔径为20nm,膜厚度为5μm。     

Transient liquid phase sintering of high-purity mullite for high-temperature structural ceramics

High-purity mullite ceramics, promising engineering ceramics for high-temperature applications, were fabricated using transient liquid phase sintering to improve their high-temperature mechanical properties. Small amounts of ultrafine alumina or silica powders were uniformly mixed with the mullite precursor depending on the silica-alumina ratio of the resulting ceramics to allow for the formation of a transient liquid phase during sintering, thus, enhancing densification at the early stage of sintering and mullite formation by the reaction between additional alumina and the residual glassy phase (mullitization) at the final stage of sintering. The addition of alumina powder to the silica-rich mullite precursor resulted in a reaction between the glassy silica and alumina phases during sintering, thereby forming a mullite phase without inhibiting densification. The addition of fine silica powder to the mullite single-phase precursor led to densification with an abnormal grain growth of mullite, whereas some of the added silica remained as a glassy phase after sintering. The resulting mullite ceramics prepared using different powder compositions showed different sintering behaviors, depending on the amount of alumina added. Upon selecting an optimum process and the amount of alumina to be added, the pure mullite ceramics obtained via transient liquid phase sintering exhibited high density (approximately 99%) and excellent high-temperature flexural strength (approximately 320 MPa) at 1500 °C in air. These results clearly demonstrate that pure mullite ceramics fabricated via transient liquid phase sintering with compositions close to those of stoichiometric mullite could be a promising process for the fabrication of high-temperature structural ceramics used in an ambient atmosphere. The transient liquid phase sintering process proposed in this study could be a powerful processing tool that allows for the preparation of superior high-temperature structural ceramics used in the ambient processing atmosphere.     

Epitactic Nucleation of Spinel in Aluminosilicate Gels and Its Effect on Mullite Crystallization

Control over the structure of hybrid (colloidal + molecular) aluminosilicate gels was utilized to demonstrate that precursor chemistry has a direct and controllable effect on the ∼1000°C crystallization of spinel and mullite in molecular precursor systems. Synthesis or preparation conditions leading to the development of a cubic, transition alumina result in the epitactic nucleation of spinel at ∼1000°C in gels that otherwise crystallize directly to mullite at ∼1000°C. Thus, the preference for spinel nucleation in gels derived from solution precursor systems whose chemistries promote formation of transition alumina readily explains the reported inability to obtain substantial mullite yields at ∼1000°C. Isothermal transformation kinetics of colloidal and hybrid gels show that in the absence of direct mullite formation at ∼1000°C, the release of alumina from the spinel-type crystal structure becomes the rate-controlling step in the transformation. This necessitates higher temperatures for mullite formation and limits the kinetic enhancement possible with extrinsic increases in mullite nucleation frequency.     

Mullite-based cellular ceramics obtained by a combination of direct foaming and reaction bonding

Mullite-based cellular ceramics were prepared via the polymer precursor route using poly(silsesquioxane) in combination with particulate alumina or alumina/aluminum mixtures. The multi-functional preceramic polymer was used as pore-forming agent by employing a self-foaming process during the polymer cross-linking step, as well as a precursor for reactive silica, one of the reagents in mullite formation. The size of filler particulates was found to strongly affect foaming of the polymer/filler mixtures, with coarser particles facilitating an improved foaming performance. Thermal conversion in air at 1600 °C resulted in the formation of cellular ceramics with high mullite contents. The partial substitution of alumina with aluminum in the initial mixtures resulted in improved mechanical properties at comparable porosities, resulting in compressive strengths of 0.3 MPa at total porosities of 93%. A correlation between thermal analysis data and crystalline phase development during the thermal treatment allowed for the clarification of processes taking place during heat treatment, yielding information for a future process optimization approaches.     

Comparison between two ultrasonic methods in their ability to monitor the setting process of cement pastes

This paper presents the comparison between ultrasonic wave transmission (USWT) method and ultrasonic wave reflection (USWR) method in their ability to monitor the setting process of cement pastes. The velocity of ultrasonic longitudinal waves and shear wave reflection coefficient were measured simultaneously on cement pastes with different hydration kinetics. Even though both methods are able to reliably monitor the hydration process and formation of structure of an arbitrary cement paste, they monitor the setting process in different ways. The relationship between the velocity of longitudinal waves and shear wave reflection coefficient can be simplified into three characteristic phases and the end of the first phase can be used to define the beginning of the setting process of cement paste.     

基于超声衰减和声速动态监测石蜡的相变过程

为研究石蜡在相变过程中内部结构和状态的改变特性,用中心频率为5 MHz的脉冲式超声波动态测量石蜡的相变过程,采集并分析不同升降温速率下的声速和声衰减信号的变化规律,结果与差示扫描量热仪(DSC)测量的热力学性质比较,并拍摄石蜡溶解过程的图像为辅助,探讨了二者由于测量原理不同导致的差异和特点。结果表明,两种方法均得到约50℃的初凝点,且二者信号反映的相变规律一致,表明利用声衰减和声速能够较好的表征石蜡在相变过程中的声学特性。超声可能成为一种新的蜡化物性质原位测量手段。     

大比表面积高牢固度堇青石蜂窝涂层的制备

以拟薄水铝石粉为原料,分别采用铝溶胶法、活性Al_2O_3浆料法和混合浆液法,对堇青石蜂窝基体进行涂层处理。通过黏度、SEM、BET和超声振荡等分析测试手段,着重考察了涂层浆液固含量和黏度对蜂窝涂层量、比表面积和牢固度的影响,比较了采用不同方法进行涂层的效果。结果表明,铝溶胶的涂层具有牢固度高的特点,缺点是比表面积小;Al_2O_3浆料的涂层比表面积较大,但牢固度较差。而混合浆液法利用铝溶胶作为Al_2O_3浆料的胶联剂,结合上述两种涂层方法的优势,涂层后的蜂窝既具备大的比表面积(达59m~2·g~(-1)),又保持了较高的涂层牢固度,而且涂层工艺简单可控,有利于工业化应用。     

沉降过程超声成像传感器制作及其声场测量

在沉降过程超声成像系统中,工作于液态环境传感器的设计与制作质量直接决定投影数据的采集精度.根据超声波传播理论、压电陶瓷材料的谐振特性和圆形检测场的要求,制作了柱面形且尺寸满足成像要求的压电传感器.根据位相跟踪法,测得传感器的谐振频率在200kHz附近,谐振等效电阻在460Ω左右；发射传感器的发射角度达62°；接收传感器的接收电压达到百毫伏数量级.实验结果表明：接收传感器灵敏度高；发射传感器的发射角度大,声场范围充满了圆形检测场；通过合理控制激励电路的发射功率,声场中无反射波产生,使测量数据具有良好的稳定性.     

聚丙烯腈原丝超声波水洗工艺

将超声波技术应用到聚丙烯(PAN)原丝的水洗工艺中,研究了超声波功率、水洗温度、水洗时间对PAN原丝中残留二甲基亚砜(DMSO)含量的影响。结果表明,超声波功率为200 W,洗涤温度50℃,洗涤时间10 min,洗涤后的PAN原丝中的DMSO含量大幅降低,且超声波水洗PAN原丝的洗涤效果比普通水洗好。     

赤藻糖醇/甘露醇过冷特性分析

制备了以赤藻糖醇/甘露醇为基液的相变材料,期间通过添加纳米二氧化钛为相变材料的凝固提供了结晶附着力,促进晶体生长结晶的形成;通过超声波处理使得相变材料局部产生高温高压,促使晶核产生和生长。本文分析了材料制备过程中的纳米颗粒含量、超声时间、超声功率对赤藻糖醇/甘露醇过冷度和凝固过程的影响。实验结果表明,纳米颗粒含量的增加、超声时间的延长都会导致材料的过冷度降低,而随着超声功率的增加,过冷度则先降低后升高。纳米颗粒质量分数、超声时间、超声功率分别作为单一变量时,过冷度最低为11.3℃、16.2℃、10.8℃,对应凝固时间缩短最高为38.0%、18.4%、16.4%。