摘要:

The lattice thermal expansion of a number of C atoms and graphites has been studied over the temperature range −265 to 2600°C, and is discussed taking account of other diffraction data published recently. It is shown that for the temperature range 0 to 2600°C, the increase in the interlayer spacing for a given temperature rise is not significantly dependent on stacking disorder. Hence the mechanism whereby the expansion takes place is essentially the same for all C atoms, implying that a van der Waals interlayer bonding prevails in all carbon materials. The degree of directional orientation of a particular carbon is shown to have no effect on its thermal expansion behaviour. At sub‐ambient temperatures the thermal expansion is found to be significantly dependent on stacking disorder and hence carbon materials of different degrees of graphitic perfection have different limiting distances of closest approach between the atom layers as the temperature is lowered towards 0°K. At room temperature the coefficient of expansion αcdecreases with increasing stacking order. At higher temperatures the situation is more complex. The variations with temperature of the interlayer binding energies and compressibilities have been calculated. It is shown that C atoms representing the extremities of structural perfection have interlayer binding energies which differ by ~0.76 kcal.mole−1at 0°C. For an increment of 500°C the binding energy change is ~0.33

ISSN:1600-5767    

DOI:10.1107/S0021889871006149

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY

摘要:

A single‐crystal three‐circle X‐ray diffractometer is described in which the specimen is cooled to about 20°K by the cold‐finger t

ISSN:1600-5767    

DOI:10.1107/S0021889871006150

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY

摘要:

Decomposition of β‐silicon carbide whiskers by heating in vacuum results in the formation of highly oriented graphite crystallites with basal planes tangent to the whisker surface but showing no structural relationship to the parent β‐silicon carbide. Doubling of the 00.lreflections of graphite is explained by refraction of the electron beam from faces parallel to the basal planes in a direction normal to the whisker

ISSN:1600-5767    

DOI:10.1107/S0021889871006162

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY

摘要:

Ann‐component substitutional solid solution in a perfect crystal can be characterized uniquely byn−1 independent composition functions. There aren(n−1)/2 distinct short‐range‐order parameters αij(i,j= 1,2,...n−1) corresponding to these (n−1) independent compositions. The SRO parameters are linearly but not quadratically independent: for example, in ternary systems, the following equation holds Σ[(1 −_{1}) (1 −2)α11α22 −1 2α12α21] = 0 in which1and2are the average compositions pertaining to atomic species 1 and 2, the summation extending over all significant correlation ranges in the crystal. This relation can be used to reduce the uncertainty relative to the interpretation of diffraction experiments realized with less than the opt

ISSN:1600-5767    

DOI:10.1107/S0021889871006174

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY

摘要:

A brief outline of the essential features of the geometry of X‐ray fibre diffraction patterns is given and leads to a discussion of the beginnings of a systematic study, using optical diffraction patterns, of the correspondence between various features of the structure and effects in the X‐ray scattering patt

ISSN:1600-5767    

DOI:10.1107/S0021889871006186

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY

摘要:

A procedure is presented for the separation of the local‐order intensity scattered by a binary body‐centred cubic alloy along three lines in reciprocal space. Linear Fourier coefficients are defined for the distribution of the local‐order intensity along these lines and these are related to the Warren local‐order coefficients, αi. The local‐order coefficients have been obtained by this so‐called linear analysis for Fe‐3.9 at.% Mo and Fe‐6.1 at.% Mo, and are compared with local‐order coefficients derived by Fourier analysis of the local‐order intensity in its m

ISSN:1600-5767    

DOI:10.1107/S0021889871006198

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY

摘要:

The effects of axial and equatorial divergences, of beam penetration, and of misalignment on the integrated Bragg reflections as well as on diffuse scattering and specimen fluorescence are calculated for the reflection and transmission methods of powder diffractometry. The formulation encompasses both the θ0–2θ case with stationary specimen and (wide) receiving slit and the usual θ–2θ scan with a narrow receiving slit. To a good approximation, the effects of the various aberrations may be separated and simple formulas which allow the corrections to be calculated to an accuracy of 0.1% are presented. Although almost all practical cases are included, the formulation is illustrated in detail for the important case of Bragg scattering in reflection geometry. The largest effects are in the low angle region where, for the θ–2θ scan, all the aberrations increase the observed intensity. For a realistic case of a low density sample near 20° 2θ, the total correction may be well above 5%, but the results depend strongly on absorption coefficient and on scattering angle. The effects of the most serious aberrations are considerably reduced in the θ0–2θ case; for the above example the total is less than 2%. In the transmission method, within the useful range, the only significant aberrations arise from the deviation of the specimen center from the diffractometer axis and from axial divergences. In the case of diffuse scattering or specimen fluorescence, in either transmission or reflection geometries, the intensity is affected by the equatorial aberrations even more than in the Bragg case, but the effects of axial aberrations are u

ISSN:1600-5767    

DOI:10.1107/S0021889871006204

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY

摘要:

The variance of the X‐ray spectral distribution in the CuKα multiplet has been redetermined with improved accuracy and over a wider range than previously. This investigation is parallel to that of Edwards&Toman on iron radiation, with which comparison is made on the following points: (i) the effect of using a β‐filter and (ii) the effect of interpolating the line profile under theKα satellite group. Both definitions of the variance are considered and three linear regions can generally be distinguished in the variance–range function. The choice of definition and the usefulness of the different linear regions in any practical application are also d

ISSN:1600-5767    

DOI:10.1107/S0021889871006216

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY

摘要:

Two different small‐angle X‐ray scattering methods have been used to determine the electron density of the particles in a colloidal silica suspension. In the first method, the electron density of the particles was calculated from the measured change of the zero‐angle scattered intensity caused by the addition of known quantities of sucrose to increase the solvent electron density. In the second method, the scattering from the suspension was compared with the scattering from a nearly ideal gas. Within an experimental uncertainty of about 5%, the electron density of the silica particles was found by both methods to have a value corresponding to pure SiO2particles with the mass density 2.28 g.cm−3which is listed in the manufacturer's literature describing the suspensions. This determination of the electron density will enable these suspensions to be used as standard samples in determinations of the ratio of the scattered intensity to the incident

ISSN:1600-5767    

DOI:10.1107/S0021889871006228

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY

摘要:

Preferred orientation produced by heavily drawing high‐density polyethylene sheet has been studied by means of computer drawn pole figures, using a Schulz texture goniometer. A resolving power of the order of one degree was obtained, and this revealed that the heavy drawing caused a (100) [001] texture, and twinning on (310). Annealing at temperatures near the melting point produced relaxation of the residual twinning stresses and restoration of the (100) [001]textur

ISSN:1600-5767    

DOI:10.1107/S002188987100623X

出版商:International Union of Crystallography    

年代:1971

数据来源: WILEY