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Phase Transformations in Amorphous Solids

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Phase Transformations in Amorphous Solids Phase Transformations in Amorphous Solids L. Stoch and I. Waclawska University of Mining and Metallurgy 30-059 Cracow, Poland ABSTRACT exactly defined, and it comprises substances of different ordering degree of internal structure, varying micro- This paper presents the results of investigations of and macrohomogeneity and varying origin. amorphous substances formed as a result of thermal Amorphous substances are formed as a result of decomposition of inorganic crystalline solid bodies by different processes. Gels are obtained through preci- way of their structural rearrangement in a solid phase pitation from solutions in which the parent compound as an order-disorder transformation. The investigations is in a state of molecular dispersion. Glasses are most were conducted on model substances represented by often obtained by supercooling a liquid, but also by hydrated borates of alkalis and alkaline earths as well means of the deposition from a vapour and by as silica-rich zeolites. We examined the glass transition sputtering in a vacuum system. phenomenon as a mode of cancellation of internal The amorphous state can also be formed through stresses induced by the state of structure randomness, destruction of a crystalline structure under the influence and next the recrystallization process of thermally of energy supplied from the outside in various forms, amorphized substances. Recrystallization processes such as heat, pressure or radiation. proceed usually in several stages with their structural The degree of the randomness and inhomogeneity mechanism changing with temperature. of the internal structure of amorphous substances The formation of the amorphous states and their depends to a high extent on the degree of the dispersion recrystallization represent a cycle of phase transition of the primary substance. The highest homogeneity is realised by means of continuous rearrangement of the demonstrated, for example, by glass obtained by con- internal structure. densation of vapours or from solutions (in the sol-gel These processes may be utilized as an alternate method) when the parent substance was in the state of method of obtaining vitreous and glass-ceramic molecular dispersion. materials of various relations between the crystalline Amorphous substances formed through destruction phase and the glassy matrix. of the crystalline structure of the precursor in a solid state, at low temperature, preserve to a considerable degree the elements of the primary structure. An 1. INTRODUCTION example here are the metamictic minerals rendered amorphous by radiation. Solid bodies may occur in two forms differing in Depending on the mode of their formation and their internal structure, i.e. as crystalline substances of chemical composition, the degree of randomness of the ordered internal structure, and as amorphous bodies of internal structure of a solid body may vary. It may random structure. The amorphous state has not yet been overcome only the long range ordering, manifested by 181 Vol. 13, No. 3, 1994 Phase Transformation in Amorphous Solids the geometry of the crystal lattice of the given amorphous state, on the other hand, does not introduce substance, i.e. long range order with preserved medium any essential changes in the nature of chemical bonds, range order, for example, such as the assemblage of or such changes are relatively small. The coordination coordination polyhedra. Examples are the amorphous number of some cations, however, is occasionally substances formed by thermal decomposition of changed. These factors, together with the degree of inorganic polymers (silicates, borates, phosphates) and randomness of the internal structure, determine the certain glasses of complex composition. differences in the properties of the amorphous and In some amorphous substances there occurs only crystalline substances. short-range ordering limited to the particular coordina- The subject of the present study is the chemical and tion polyhedra. Glasses of simple composition are structural transformations occurring in amorphous examples of this. inorganic polymers obtained by heat treatment of The randomness may also extend to a first coordina- crystalline substances. tion zone, i.e. the nearest surroundings of atoms As already shown earlier 111, it is possible to obtain forming the amorphous structure. This takes place in an amorphous substance by heat treatment in cases the case of elements in a glassy form or certain glassy where the destruction rate of the primary crystalline metals. structure is greater than the recrystallization rate of the It has been assumed in this paper that the internal amorphous matrix which is then formed. structure order in the amorphous substances has been Thermal amorphization of crystalline solids can be distorted or destroyed to a degree excluding the obtained by (1) thermal dissociation of compounds appearance of the X-ray diffraction lines characteristic (dehydration of some hydrous silicates, borates, of their crystalline forms. This means that the phosphates, etc.), (2) destruction or distortion of the disappearance of long-range ordering in the structure is crystal structure of a solid under prolonged heat a sufficient reason to regard the substance as an treatment (quartz). amorphous one; the state of randomness in X-ray The investigations were carried out on a few amorphous substances may, obviously, have a wider selected borates and zeolites rich in silica. The range of occurrence and include even the first occurring phase transitions, consisting in rearrange- coordination sphere. ment of the internal structure under the influence of The state of randomness in the area of the first temperature, were determined and the observed coordination sphere may change the nature of the inter- phenomena compared with those occurring in other atomic interactions and thus affect the electrical and substances rendered amorphous by heat treatment and magnetic properties of the substance. These states are in glasses. the subject of investigations in solid state physics. The aim of the investigations was to recognize the To investigate the amorphous substances, especially regularities controlling the phase transformations by those in which the short- and medium-range ordering the structural rearrangement of amorphous solids. has been retained, attempts are being made to apply An attempt has also been made to define the mutual methods used in structural chemistry and crystallo- relations between substances rendered amorphous by chemistry. With respect to their chemical and physical heat treatment and glasses. properties, these substances show considerable Thermal amorphization appears to be a promising similarity to the affinated crystalline substances. They method of obtaining some glasses and glass-ceramic contain the same type of coordination polyhedra as the materials, as well as ceramic powders, especially those corresponding crystalline substances. Occasionally, the that cannot be obtained by the sol-gel method. transition into the amorphous state cancels the Moreover, the examined processes correspond to deformations of coordination polyhedra to which they those occurring during vitrification and crystallization are subjected in a crystal structure. This takes place, for of xero-gels in the sol-gel method of obtaining glasses example, in the case of Si04 tetrahedra in vitrified and glass-ceramic materials. framework silicates III. The transition into the 182 L. Stoch and /. Waclawska High Temperature Materials and Processes 2. MATERIALS Zeiss Ur-10 spectrometer and with a Digilab FTS-60V Fourier Transform Infrared Spectrometer. Samples The model substances selected for the examinations were prepared as KBr pellets. were the borates of alkalis and metals of alkaline earths In the case of some substances, of which there were which have different complex borate anions. They have sufficiently large amounts available, the changes in their equivalents among the borate glasses. Moreover, density and microporosity induced by phase transitions these borates belong to substances of low melting tem- were also determined. perature. Coarse-crystalline, pure borate minerals from Measurements of the specific surface (BET), pore the collection of the Mineralogical Museum of the volume and size were conducted, taking advantage of Wroclaw University were examined. They included: the phenomena of adsorption and desorption of pure colemanite Ca2B608(0H)6-2H20, nitrogen at the temperature of liquid nitrogen by means pandermite Ca2B508(0H)3-2H20, of a computer-controlled ASAP 2000 apparatus kaliborite HKMg2B12Oi6(OH)i0-4H2O), (Micromeritics). Density measurements were performed ulexite NaCaB506(0H)6-5H20, in an Accu-Pyc 1330 helium pycnometer tincalconite Na2B40j(0H)4-3H20, and (Micromeritics). kernite Na2B406(0H)2-3H20. Synthetic borax was also examined. Synthetic zeolites: ZSM-5 zeolite of high Si0 2 4. RESULTS content and NaA zeolite containing aluminium in its structure were examined too. They belong to framework A Phase Transformations in Amorphous Borates silicates and are built of Si04 and A104 tetrahedra. They may be regarded as substances corresponding to Results of the thermal analysis (Fig. 1), X-ray minerals of the Si02 groups and aluminium-doped patterns (Fig. 2) as well as IR spectra (Figs. 3, 4, 5) quartz glass on account of their chemical composition indicate the range of amorphous structure stability and and the kind of short-range order of internal structure. the course of its reciystallization. They depend
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