Quintin Johnson

The crystal and molecular structure of beryllium hydride

Abstract The crystal and molecular structure of BeH 2 has been determined from high-resolution powder diffraction data obtained at a synchrotron radiation source. Computer indexing methods gave the unit cell as body-centered orthorhombic with a = 9.082(4), b = 4.160(2), c = 7.707(3) A V = 291.2(2) A 3 , with systematic absences corresponding to space groups, Ibam or Iba2. Essentially, single crystal methods were used for the structure determination: Patterson synthesis to locate the Be atoms and a “heavy-atom” electron-density synthesis to confirm the location of the H atoms. The crystal structure is based on a network of corner-sharing BeH 4 tetrahedra rather than flat infinite chains containing hydrogen bridges previously assumed. The Be-H bond distances are 1.38(2) A around Be(1) and 1.41(2) A around Be(2). The H-Be-H tetrahedral bond angles range from 107° to 113° and the Be-H-Be bond angle is approximately 128°. The space group in Ibam, and there are 12 BeH 2 molecules in the unit cell. The theoretical density is 0.755 g/cm 3 .

High resolution tomography with chemical specificity

A very fast method of computerized critical absorption tomography featuring approx.10 ..mu..m spatial resolution and high chemical sensitivity is described. Synchrotron radiation is used and the method is especially suited to investigating small samples. From a preliminary experiment it is found that layers of neighboring elements only 0.2 ..mu..m thick can be distinguished at medium atomic numbers.

Energy‐modulated x‐ray microtomography

We describe an instrument for computerized x‐ray tomography which can provide fully three‐dimensional examination of small samples (a few millimeters to a few centimeters) with resolution variable from 5 to 100 μm. Elemental and chemical‐state distributions are obtained with high resolution by modulating the energy of the incident x‐ray beam. An essential component is the monochromator we designed and optimized for chemical microtomography. It has a narrow energy bandpass and a large degree of harmonic suppression, provides a uniform x‐ray beam with small angle of divergence, and is cooled in a way that eliminates thermal and vibrational instabilities.

Nondestructive investigation of damage in composites using x-ray tomographic microscopy (XTM)

X-ray tomographic microscopy (XTM), utilizing intense, highly collimated synchrotron radiation, has been used to nondestructively image materials structures in three dimensions. The spatial resolution of the technique approaches that of conventional optical microscopy, but XTM does not require polished surfaces or serial sections. We present the results of an XTM investigation of a composite material composed of silicon-carbide fibers in an aluminum matrix. The results reveal the aluminum/silicon-carbide interfaces and show microcracks running along many of the interfaces as well as in the matrix. Excellent contrast is observed between the silicon-carbide sheath of the fiber surrounding the graphite core and the coating at the fiber-matrix interface. The sensitivity to small changes in the linear absorption coefficient allows nondestructive imaging of variations in chemistry between graphite and silicon carbide and between silicon carbide and aluminum. The experimentally determined values of the absorption coefficients of these phases are identical to values published in the literature. For the first time, XTM will allow observation of damage accumulation and crack growth {ital during} deformation testing. The availability of such data will greatly improve our understanding of failure in advanced materials.

X‐ray diffraction study of single crystals undergoing shock‐wave compression

Single crystals of LiF, Al, and graphite have been studied by x‐ray diffraction while undergoing shock‐wave compression to about 300 kbar. These studies show that single crystals can transform to the hydrostatically compressed state essentially as single crystals, and crystal orientation is preserved.

X‐ray microtomography on beamline X at SSRL

This article describes a high‐resolution three‐dimensional CT system which has been tested on the new wiggler beamline X‐2 at the Stanford Synchrotron Radiation Laboratory. The present system is a rotate‐only design which uses a virtual phase CCD array camera optically coupled to a high‐resolution phosphor screen. The spatial resolution of this system is presently somewhat better than 10 μm (50 line pairs/mm at 20% contrast). This resolution is limited by the optical elements in the detector, and we are undertaking an effort to improve this resolution to 1 μm. Nevertheless, the spatial resolution offered by this design is sufficient to begin many studies of interest to materials research, such as, supported catalysts, composite materials, and crack growth and propagation.

Flash x‐ray tube for diffraction studies on a two‐stage light‐gas gun

A flash x‐ray tube has been fabricated and has been driven by a Blumlein pulse generator to peak voltages of 60 to 80 kV and peak anode currents of 8 to 10 kA. The pulse width, dependent upon the anode‐cathode spacing, can be made ∼80‐ns wide, and a small spot or line focus is possible. These features make the device useful for flash x‐ray diffraction studies. Current in the diode region exceeds the calculated space‐charge limited values, probably because of neutralization of charge by plasma ejected from the cathode during the initial phase of the discharge.

X-ray tomographic microscopy of fibre-reinforced materials

Aluminium composites containing Al2O3 fibres and precipitates of various intermetallic phases are investigated by high-resolution computerized microtomography. Individual fibres 15 μm in diameter and intermetallic phases forming a network with about 15 μm mesh size have been imaged. The capabilities of the method and its further development down 1 μm and less spatial resolution are discussed.

Elemental and chemical-state imaging using synchrotron radiation.

The near-edge structure in the x-ray absorption coefficient of an element is affected by chemistry and local environment. Experiments demonstrate that this property can be exploited in x-ray imaging both to identify and enhance the detectability of different chemical states of the same element. Chemical contrast images are obtained by digital subtraction of absorption images taken at carefully selected energy values.

Optimization of CCD‐based energy‐modulated x‐ray microtomography

Employing asymmetric Bragg reflection at the monochromator we obtain a wide and practically parallel synchrotron‐radiation beam which, when combined with the use of a CCD detector, enables us to perform chemically specific, high‐speed and high‐resolution tomography. We present recent results obtained with this new method. The actual resolution achieved was determined from the measured line‐spread function of the complete detection system which includes the CCD, the fluorescent screen, and the lens used for optical magnification. The modulation‐transfer function (MTF) calculated from the line‐spread function shows that with only twofold magnification and at 3% contrast detectability about 28 line pairs per mm are resolved. Extrapolating from this result we find that with an optical magnification of 7:1 about 100 line pairs per mm should be resolved. Ways to optimize the method further are discussed.