The spontaneous growth of tin proper whiskers on electroplated tin films causes a practical problem of short circuit error. In order to solve this problem, factors influencing the whiskers growth as well as the growth mechanism have been investigated by many workers. However, the basic growth mechanism has not yet been wholly explained. In the present paper, some descriptions are given on the factors influencing the whisker growth, such as tin plating brightener, thickness of tin film, metallic substrate, atmosphere, heat treatment of tin film and addition of lead atoms to tin film. Subsequently, a few method to prevent the growth of tin proper whiskers are presented, which are as follows: No use of copper and its alloys as metallic substrate, use of tin plating brightener composed of ketone compound, make tin film more than 10μm thick, and heat treatment (for example, for 3 hr or more at 125℃) of tin film after electroplating. Several experimental results for making the whisker growth mechanism clearer are presented. Whiskers contain not only tin but also copper and zinc atoms of substrate metals. When tin is plated on brass, copper and zinc atoms diffuse into the tin film, then copper atoms form intermetallic compounds and zinc form oxide on the surface of films and whiskers. The tin whiskers easily grow on tin films having irregularly shaped grains.
Growth mechanism of hollow crystals of II-VI compounds is summarized. Hollow ZnO crystals grown by oxidizing ZnSe in the vapour phase are classified into four types: (1) A needle-type hollow crystal is composed of a cluster of c-whiskers. (a) A hollow crystal growing at the top of a long substrate crystal is formed by a combination of the top and normal branch crystals which are composed of c-whisker cluster. (3) A hopper-type hollow crystal is formed by a combination of several top branch crystals grown on short substrate crystals. (4) Twinned hollow crystals with a composition plane (1122) or (1011) are also composed of a cluster of c-whiskers. On the other hand, hollow CdSe and CdS crystals with many striations parallel to the c direction on their side surface were obtained by the sublimation method. Proposed growth mechanisms for these hollow crystals are as follows: (1) Hollow CdSe crystals are composed of many c-needles grown on the lateral surface of an initial c-needle. The c-needles are developed from c-whiskers. These c-needles are united into a wall, leaving a cavity at its center. (2) Hollow CdS crystals are grown by a combination of several Crystal walls extending from previously grown walls. Each wall is developed from an array of c-whiskers. As the result of such combination of crystal walls Several cavities are left in the crystal.
Crystal growth accompanied with the liquid-solid transformations in several materials are observed under high pressure up to 25 kbar at room temperature. Diamond-anvil type high pressure apparatus and optical microscope were used for these observations. Materials studied are: liquid-ice VI-ice VII transformation in H_2O, liquid-solid transformations in carbon tetrachloride (CCI_4), ethyl alcohol (C_2H_5OH), and benzene (C_6H_6). Dissolution of calcite (CaCO_3) and a growth of ikaite (CaCO_3・6H_2O) under high water pressure were also observed. These observations provide us many informations for the structure and the growth mechanism of these crystals.
This investigation is an extension of that reviewed by Uyeda (J. Cryst. Growth 45 (1979) 485). For Mg, details of crystal shapes were studied by a high resolution scanning electron microscope, although no new habits was found. For Zn and Cd, the following habits were found: 1) Various shapes between rough spheres and hexagonal prisms. 2) Hexagonal plates sometimes with indented sides. 3) Thin plates with sides of the 1st and the 2nd kind. 4) Triangular pyramids of the 1st kind and those of the 2nd kind. 5) Hybrids of Nos. 1 and 4. 6) Contact twins and penetration twins.