Sigemaro Nagakura

Study of Metallic Carbides by Electron Diffraction Part I. Formation and Decomposition of Nickel Carbide

Thin nickel films formed by evaporation were carburized in CO gas stream at temperatures from 250°C to 500°C and were studied by electron diffraction. Only Ni 3 C was found to be formed. This has c.p.h. arrangement of nickel atoms with lattice constants a h =2.628 A and c h =4.306 A, but showed super-lattice reflections due to a hexagonal lattice of \(a{=}\sqrt{3}a_{h}\) and c =3 c h , which is considered to be caused by a regular arrangement of carbon atoms. The carbide was formed below 450°C in CO gas with slow rate and decomposed rapidly above 430°C in vacuum. The rate of the carbide formation was highest at about 350°C, although it depended very much on the texture of nickel films. Hydrogen atmosphere did not alter essentially the stability of the carbide. It was found that nickel carbide is formed from metallic nickel with an epitaxial relation (00.1) Ni 3 C //(111) Ni and [10.0] Ni 3 C //[10\bar1] Ni (Ni 3 C: c.p.h. indices) at the nucleation stage. A mechanism of the lattice transformation from met...

Study of Metallic Carbides by Electron Diffraction Part III. Iron Carbides

Thin iron films formed by evaporation were carburized in CO gas stream. The carbides e, χ and θ were found to be formed in the temperature ranges (i) below 250°C, (ii) from 250° to 350°C and (iii) above 350°C, respectively. The e-carbide is hexagonal with lattice constants a = 4.767A and c = 4.354A, and its structure belongs to the space group P 6 3 22, iron atoms occupying 6 g positions with the parameter x = 1/3 and carbon atoms 2 d positions. Irreversible phase transitions from e to χ and from χ to θ take place at 380°∼400°C and at about 550°C, respectively. By the transitions changes in the chemical compositions may be very small. Observation on the crystal growth suggests that a mechanism of the e-carbide formation is different from that of the χ- and θ- carbide formations.

Study of Metallic Carbides by Electron Diffraction Part IV. Cobalt Carbides

Thin cobalt films formed by evaporation were carburized in town gas at temperatures below 550°C for several hours. Electron diffraction studies of these films confirmed the existence of two kinds of carbide, Co 2 C and Co 3 C. Co 2 C is an orthorhombic crystal with lattice constants a =2.910, b =4.469 and c =-4.426 A, and has a distorted c.p.h. structure. The lattice relation between Co 2 C and Co is (001) Co 2 C //(00.1) α-Co //(111) β-Co and [100] Co 2 C //[10.0] α-Co //[10\bar1] β-Co . Co 2 C is formed at temperatures in a range 450°∼500°C, and decomposes gradually to α-Co containing stacking faults and graphitic carbon at temperatures in a range 450°∼470°C. Co 3 C is an orthorhombic crystal with lattice constants a =4.483, b =5.033 and c =6.731 A and is isomorphous with cementite (Fe 3 C). The lattice relation is (1\bar21) Co 3 C //(10.1) α-Co //(100) β-Co , and [111] Co 3 C //[0\bar1.0] α-Co //[001] β-Co . Co 3 C is formed at temperatures in a range 450°∼500°C and decomposes gradually to α-Co contain...

Crystallographic Study of the Tempering of Martensitic Carbon Steel by Electron Microscopy and Diffraction

Structural changes taking place during the tempering proceed in four stages. At the preliminary stage taking place below 370 K, interstitial carbon atom clusters are formed below 270 K, rearrange to make a modulated structure between 270 and 360 K, and disappear at about 370 K. A long period ordered phase with respect to the interstitial carbon atoms or interstitial vacancies also appears between 330 and 350 K. With the disappearance of these structures, the first stage takes place between 370 and 470 K, where η-Fe2C is formed in the matrix of low carbon martensite. The second stage occurs around 550 K with the retained austenite decomposing to θ-Fe3C and α iron. At the succession of the first stage, the third stage appears in a temperature range from 470 to 900 K, where θ-Fe3C, χ-Fe5C2 and also higher carbides θn-Fe2n+1Cn intergrow microsyntactically in the particles precipitated below 720 K, but only θ-Fe3C is formed above this temperature.

Study of Metallic Carbides by Electron Diffraction Part II. Crystal Structure Analysis of Nickel Carbide

The crystal structure of nickel carbide, Ni 3 C, was studied by the electron diffraction method. The observed intensity showed considerable primary extinction and this was corrected, using Wilsons intensity statistics combined with the theory of primary extinction (S. Nagakura, Acta Cryst. 10 (1957) 601). The actual procedure was described in detail and it was concluded that the structure of nickel carbide belongs to the space group R \bar3 c and nickel atoms occupy 18 e positions with the parameter x =1/3 and carbon atoms 6b positions. It was found also that the carbon atoms take a kind of one-dimensionally disordered arrangement in the carbide lattice.

X-Ray Topography of Magnetic Domains in Iron Whisker Crystals

Magnetic domains in well annealed [001] whisker crystals have been studied by X-ray topography. The three-dimensional shape of the rectangular domain observed by the powder pattern technique was found to be a cuboid. Qualitative explanation is given to the image contrast of 90° domain walls by the dynamical theory of X-ray diffraction in distorted crystals. The lattice distortion around the line of intersection of 90° and 180° domain walls was determined by the analysis of butterfly images. It was also found that the 180° domain wall is made visible by the application of magnetic field. Fringe patterns appeared when the whiskers took the closure domain structure across the axis and also were elastically bent to produce the serrated domain wall. Dynamical calculation proves that they are the moire fringes produced by the double diffraction in the adjacent domains. Some observations about whisker tips and kinked whiskers are also described.

High Resolution Electron Microscopy of Crystal and Defect Structures of the High-Tc Superconductor Ba2YCu3O7-δ

Structural study of the superconducting oxide Ba2YCu3O7-δ has been made at room temperature by high resolution electron microscopy. The orthorhombic structure was confirmed by the direct imaging of metal atoms. Planar defects with stacking sequence of metal-oxygen planes in the La2CuO4 type of structure were observed. Depending on the selected arca in electron diffraction, the axial ratio b/a was found to vary in a range from 1.003 to 1.018. Black and white contrasts appearing alternately along the twin boundaries are discussed in relation to the lattice distortion and the oxygen deficiency in the Cu-O layer.

Growth and crystallographic features of titanium carbide whiskers

TiCx whiskers were produced by the reaction of methane, TiCl4 and hydrogen on a carbon-alumina coated tungsten wire heated to 1200–1350 °C. Effects of the hydrogen and methane flow rates were investigated and the optimum growth condition was established. Many whiskers grow radially from a suitable site on the substrate surface. The sizes are 0.2–250 μm in diameter and 3 μm–6 mm in length. The growth axes are [100], [110] and [111]. The [100] and [110] whiskers have octagonal cross sections surrounded by the side faces {100} and {210} and by {100}, {110} and {111}, respectively, while the [111] whiskers have a dodecagonal cross section surrounded by {123}. X-ray examination revealed that all the whiskers are single crystals with a carbon to titanium atomic ratio of x = 0.7. The growth mechanism is also discussed.

Modulated Structure of High-Tc Superconductor Bi-Ca-Sr-Cu-O Studied by High-Resolution Electron Microscopy and Electron Diffraction

An incommensurate atomic displacive modulation has been clearly observed in high-resolution electron micrographs of the superconductive oxide Bi-Ca-Sr-Cu-O in addition to a concentration modulation in Bi layers. A commensurate form of the orthorhombic modulated structure has cell dimensions a=0.54, b=2.7 and c=3.08 nm. Characteristic features of electron diffraction patterns can be explained when transverse and longitudinal displacement waves for metal atoms along [010] are taken into consideration.

Electron Diffraction and Microscopy of the Structure of La-Ba(Sr)-Cu Oxides at Room Temperature

Structural study of the La-Ba(Sr)-Cu oxides was made by means of electron diffraction and microscopy at room temperature. In the orthorhombic La1.9Ba0.1CuO4 weak extra reflections are found and the space group was determined as Pccm. On the other hand, no extra reflections are found for the tetragonal La1.8Ba0.2CuO4 with onset Tc=28 K and La1.8Sr0.2CuO4 with onset Tc=36 K. High resolution reveals the metal atom arrangement in these compounds.