Quality control of HOPG crystals and Graphite Optics offered to our customers is carried out in our laboratory. It is also used for R&D that Optigraph makes alone and in collaboration with our partners.
For commercial products, the quality control implies measuring of the size and mosaic spread of graphite
crystal or coating.
All forms of well-aligned Pyrolytic Graphite used as reflective layers in Graphite Optics, as well as graphite monochromators, are mosaic crystals. One of the most important characteristics of a mosaic crystal is its mosaic spread or, as it is sometimes called, mosaicity.
In an idealized model, a mosaic crystal consists of spatially disoriented ideal crystals – crystallites and mosaicity is understood as a measure of the spread of crystallite orientations in a certain volume. In graphite mosaic crystals, the mosaic spread represents deviation of C-axis of single crystallites from the average direction of the C-axis of the analyzed volume. In the basal plane crystallites are randomly orientated.
for Flat graphite monochromators and graphite optics
For quality control of flat and slightly
bent crystals and optics, mosaic spread is
determined on a specially adapted Bruker D8
ADVANCE ECO diffractometer using a rocking curve
measured in a Bregg-Brentano geometry
Measurements are carried out at several points, depending
on the size of the sample, twice in each position. A standard
spot size is ca. 1 cm² at the sample location.
Instrument: D8 ADVANCE ECO (Bruker),
Anode material:
Cu, using Kα1 with wavelength 1.5406 Å
Tube parameters:
40 kV, 25 µA
Medium:
Air;
Detector: SSD160
The envelope of the scattered radiation intensity at different incidence angles is a rocking curve that more often is well approximated by a normal Gaussian distribution .
Mosaic Spread of a crystal is defined as the FWHM (Full Width of Half Maximum) of this rocking curve. When evaluating mosaicity from the rocking curve, the contribution from internal broadening and deviation from the planarity of the crystal surface is not taken into account.
for Curved graphite optics
Cylindrical graphite optics is measured at CuKα₁ line in von Hamos geometry at the first diffraction maximum.
The mosaic spread is defined as the FWHM of the signal intensity distribution of the signal in the sagittal direction and the spectral resolution is calculated in the meridional direction.
During the measurement, almost the entire surface of the sample is illuminated. Only very large graphite optics are measured at multiple points.
The measurements fulfilled with a two-coordinate detector (CCD camera) allow to evaluate resolution, reflectivity and mosaic spread of graphite optics/crystal. The approach, theoretical basis and the design of the instruments were developed by the group of prof. B. Kanngießer from TU Berlin. [Anklamm L., 2014], [Malzer W., 2018].
Mosaic spread calculated as follows: m = arctan(FWHM / 2Rsag), Where: m – mosaic spread [°], FWHM – full width at half maximum of integrated intensity curve in sagittal direction, Rsag – radius of crystal optics A spectral resolving power of E/ΔE is calculated as FWHM of the Cu Kα1 line in meridional direction
