摘要
绝大多数的材料都具有热胀冷缩的性质,与之相反的,也存在着少数几种材料,它们会随着温度的上升而体积不变甚至减小,人们将这一类材料称之为零膨胀或负膨胀材料。近几年来,负膨胀材料研究引起了广泛的关注,取得了巨大进展。负膨胀材料一个重要的应用是将其与正膨胀材料复合制备任意膨胀系数的复合材料。
在负膨胀材料中,A2(MO4)3(A为三价正离子,M为W或Mo)系列材料具有很大的各向异性膨胀系数,受到人们的普遍关注。它们的结构与A的离子半径有关,通常有单斜和正交两种结构,而只有具备正交结构的材料才具有负膨胀特性,M—O—M中桥氧原子的横向振动是导致负膨胀效应产生原因。我们首次利用拉曼光谱技术研究了A2(MO4)3(A=Al,Cr,Fe,Y和Yb)系列材料,对于A12(MO4)3我们用Zr和Mg来代替部分或完全代替Al元素来尝式降低该材料的相变温度,结果如下：
(1)利用拉曼光谱研究发现,Y2(MO4)3与Y2(MO4)3在室温下有很强的吸水性质,利用TG分析其失水前后重量的变化可计算出它们的分子式分别为Y2(MoO4)3.3.27H2O、Yb2(MoO4)3.2.84H2O与Y2(WO4)3.3.17H2O、Yb2(WO4)3. 4.27H2O,其中水分子的多少与贮存条件有关。由于结晶水的存在导致其内部振动模式受到抑制,令其失去负膨胀效应。它们在室温下有三个典型的拉曼峰值,随着温度的上升,拉曼峰变得尖锐并发生分裂现象,说明水分子对于振动模式有着直接的影响。
(2)对于Al2(MoO4)3、Cr2(MO4)3与Fe2(MO4)3,通过XRD分析它们在室温下为单斜相,为MoO4四面体与AO8多面体共边结构,这种结构不具备负膨胀效应。而随着温度逐渐升高,材料会发生相变,由单斜相转变为正交相,由MoO4四面体与AO6八面体共享顶角,产生负膨胀效应。通过对材料做变温拉曼发现,单斜相Al2(MoO4)3、Cr2(MO4)3与Fe2(MO4)3的最高频对称伸缩振动峰分别位于1028cm-1、998cm-1和992cm-1,由单斜转变为正交相时,这些单斜相的特征峰值消失。通过DSC热流分析测得三种材料相变温度分别为484K、674K与786K。
(3)对于A2(MO4)3来说,其本身有一个很大的缺陷,除了少数如Sc2(WO4)3等在室温下有负膨胀效应外,其它材料在室温下都不具备这一特性,有的甚至要达到很高温度以上才会有负热膨胀效应。这对于材料应用非常不利。本文在Al2(MoO4)3中加入了一定量的Zr、Mg元素形成(Al2x(ZrMg)1-x)(MoO4)3材料来降低该材料的相变温度。用拉曼光谱观察发现在x≤0.5的时候,(Al2x(ZrMg)1-x)(MoO4)3可以在160K的时候转变为正交结构,在室温下具备低热膨胀性质。通过变温拉曼与DSC分析该材料在153K～573K的温度范围内会保持正交相结构,对于工业应用有重大意义。
Most of the solid crystals expand with the temperature increasing in general; on the contrary, there are several compounds which contract as the temperature rises. This kind of abnormal thermal expansion behavior is called the negative thermal expansion (NTE). In recent years, the study of such materials has attracted significant attention due to scientific curiosity and technological interest. One of the important applications is to composite these materials with other positive thermal expansion materials to tailor the thermal expansion to a desired value.
Of the NTE materials, those with formula A2(MO4)3, where "A" is a trivalent cation and M is W or Mo, being anisotropic in thermal expansion coefficient, have received attention. The A2(MO4)3 compounds may crystallize either in a monoclinic or orthorhombic structure depending on the "A" cation size. Only the orthorhombic ones exhibit significant negative thermal expansion behavior. The orthorhombic structure has an open framework structure with M-O-M'linkages which can accommodate for transverse thermal vibrations responsible for negative thermal expansion. In this thesis, we first study the structure and phase transitions of A2(MoO4)3 compounds by Raman spectroscopy, where A=Al, Cr, Fe, Y and Yb, and then try to decrease the phase transion temperature of Al2(MoO4)3 through partial or complete replace of Al by Zr and Mg. The main results obtained are as follows.
(1) Materials of Y2(MO4)3 and Yb2(MO4)3 are strong hygroscopic at room temperature. Their formulas are determined by thermal analysis to be Y2(MoO4)3.3.27H2O, Yb2(MoO4)3.2.84H2O, Y2(WO4)3.3.17H2O and Yb2(WO4)3.4.27H2O, respectively. Raman spectroscopic study shows that the presence of water species hinders every type of motion of the corner shared polyhedral, making these materials expand. They have three typical Raman band in room temperature. With the temperature increasing, Raman peaks become sharp and split into several bands. This indicates that the water species have a direct interaction with the vibration mode.
(2) Fe2(MoO4)3, Cr2(MoO4)3 and Al2(MoO4)3 crystallize in monoclinic structure. The monoclinic modifications in A2(MO4)3 family have an edge sharing structure, which are more densely packed and cannot accommodate for transverse thermal vibrations. With the temperature increasing, these materials change to orthorhombic. The measured the phase transition temperatures are 484K,674K and 786K for Al2(MoO4)3, Cr2(MoO4)3 and Fe2(Mo04)3, respectively. The characteristic Raman bands at 1028cm-1,998cm-1 and 992cm-1 for monoclinic Al2(MoO4)3, Cr2(MoO4)3 and Fe2(MoO4)3, respectively, disappear after phase transition to orthorhombic.
(3) The high phase transition temperatures (much higher than room temperature) of these materials indicate that they cannot be used as NTE materials for applications. We tried to reduce the phase transition temperature of Al2(MoO4)3 by replacing Al with Zr and Mg. The formula can be written as (Al2X(ZrMg)1-x)(MoO4)3. Raman spectroscopic analyses reveal that (Al2X(ZrMg)1-x)(MoO4)3 crystallize in orthorhombic structure when x≦0.5. The monoclinic structural to orthorhombic phase transition temperature of Al(ZrMg)o.s(Mo04)3 is well below 160K. And the sample could keep stable from 173K to 873K on the basis of temperature dependence of the Raman spectra and DSC analyses.
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