Calcium carbonate, CaCO_3, occurs in six different forms: three crystalline polymorphs (calcite, aragonite, and vaterite), two hydrate phases, and amorphous calcium carbonate (ACC). These polymorphs are important both in the earth science, as rock-forming minerals, and in chemical technology, as raw materials for industrial processes. Furthermore, the occurrence of CaCO_3 in living organisms has received considerable attention. Therefore formation process of these polymorphs has been extensively investigated, however our knowledge is far from complete. In the present paper, we review how the formation process of CaCO_3 has been described in literatures, including a discovery that crystallization occurs through non-classical mechanism such as stable prenucletaion cluster aggregation (Gebauer et al.: Science, 322, 1819), which give a new picture of the early stages of calcium carbonate growth.
The nucleation and growth of two crystal phases were observed in situ in Sr(NO_3)_2-H_2O peritectic system. The high temperature phase, Sr(NO_3)_2, appeared as an initial phase in almost all the conditions studied although the low temperature phase, Sr(NO_3)_<2.>4H_2O, grew faster than the high temperature phase. These relations in the nucleation and growth were necessary for the evolution of the peritectic structure in which the high temperature phase was enclosed by the low temperature phase.
Polymorphism is a general phenomenon occurring widely in organic and inorganic systems. Its transition occurs in solid state as well as in solution phase. In the present article our understanding of the phenomena of solid state transition based on our experiments is reviewed which cover organic and inorganic materials with and without the addition of mechanical energies. Although transition to more stable phases is generally accepted without doubt, experimental results seem to show difficulties to define and predict stable phases to which transition proceeds. Details are discussed for theophylline.
Disaccharides are expected to be the natural cryoprotecting materials for living bodies and cells. However, since they are too large to permeate the biomembrane, the cryopreservation mechanism has not been revealed. The microscopic-scale observations on the freezing process of dissacharide solutions provided new insight of this mechanism. Two kinds of experiments, the transmission electron microscopic observations of freeze-fractured replica films and the powder x-ray diffraction measurements, on the quenched trehalose solutions revealed that the crystal growth of ice Ih crystallites was inhibited to grow by the non-crystalline trehalose phase. In addition, it was found that the quasi-stable phase of cubic ice (Ic) were formed in the large concentration samples. Since the stability of ice Ic was extended as high as approximately 243 K, it was considered that the meso-pore space stabilizing ice Ic was maintained until the trehalose molecules broke off their hydrogen-bonded network at the vitrification temperature. Based on the formation and anomalous stability of ice Ic, the compartment-like structure formed by the disaccaride molecules in the original solution would inhibit the growth of ice Ih, which results in the cryoprotective effect.
Food is generally non-equilibrated system, i.e., it will change its physical properties such as melting point, viscoelasticity, molecular structure etc. by polymorphic transformation or phase separation as time course. Here we will focus on the crystallization of fats; triacylglycerols. Fats are explained at first and, secondly, polymorphism and polymorphic transformations will be described. Thirdly, the effects for crystallization and for kinetics of polymorphic crystallization will be shown by using our recent publish. Finally, the effect of external effect, especially shear stress and additive effect for acceleration of polymorphic crystallization of chocolate will be shown.
Silicon carbide forms polytypes due to different stacking arrangements of Si and C. For the use of semiconductor materials, it is necessary to fabricate a single polytype material. In this paper, firstly, determination factors of polytypes are introduced from both experimental and theoretical viewpoints, and the difficulty of the polytype control based on thermal stability is shown. Next, the actual control techniques in sublimation growth method and CVD method are shown to be based on the concept that the polytype sequence of seed crystal continues to the grown crystal. Finally, we propose the kinetic polytype control method based on the growth speed difference among polytypes in SiC solution growth.