The future X-ray astrophysics missions such as the ESA's XEUS will require innovative technologies and approaches resulting in lighter mirror shells in order to achieve high sensitivity and high angular resolutions at a still reasonable weight of the mirror assembly. Initiated by the Czech participation to the ESA XEUS mission, we have started a wide collaboration among the Czech and Slovak scientific institutes in order to exploit the possibilities of the suitable alternative technology to recently widely used electroforming replication.
Glass has 4 times less volume density if compared with nickel in common use. The recently developed advanced Lobster Eye X-ray optics modules (Hudec et al., this conference) are based on such gold-coated foils, only 100 microns thick, spaced at 300 microns. The recently developed mirror test assembly module (Fig. 1) is based on 50 30 x 30 cm gold-coated glass foils 0.75 mm thick, spaced at 12 mm, with possible thickness decrease up to 0.1 mm in future. The glass foils may be used either as flats, or alternatively may be shaped or thermally slumped to achieve the required geometry.
We have developed double-sided X-ray reflecting foils and flats of various thickness within the US - Czech Science and Technology Program. They are based either on a combination of composite and electroforming technologies, or on gold-coated composites, and exhibits a low weight and a very smooth surface. Analogous flats and foils may find applications in future X-ray optics experiments.
The recently available technologies such as the thermal spray deposition allows the ceramics materials to be considered as carriers or shell material of the X-ray mirrors. Their volume density may be four times less than electroformed nickel, or even less if special materials such as lithium (volume density 0.5) will be used.
From the materials already sprayed with the WSP, the fused basalt, garnet almandin, mullite, cordierite, steatite, wollastonite, Si, and AlSi (10) seem to be promising (due to low volume densities of 2 ... 3) for the production of X-ray optics carriers/shells.
Tests with replication of light ceramics layers (to produce free standing shells) have been carried out, with deposit thickness between 0.2 and 1.0 mm. Various materials including SiC, Al2O3, ZrSiO4, TiB2, MoSi2, B4C, Ti-TiN-TiO2, Al2O3-13TiO2, and mullite have been investigated and tested.
The mechanical strength was tested in compression on standard samples (0.785 cm2 in area, 1.0 mm in thickness), while the pressure needed for destruction was registered. The mechanical resistance measured amounts to 60 … > 800 kPa depending on the sample material.
The roughness measurements has indicated that all the surfaces of replicated layers adjacent to the masters have the same roughness as the corresponding substrate. The residual internal stresses in replicated layers have been also investigated. The excellent planar test replicated flats were produced by the deposition on Mo substrates and following separation at 970oC, all the residual stresses are then minimized and the resulting flat replica is undeformed.
We propose completely new innovative materials such as glossy carbon as possible alternative substrate/material for future large X-ray telescopes like the ESA project XEUS where the severe weight constraints exclude the classical recently widely used technologies and approaches. As far as we know, this material has been never discussed and studied before for this type of application.
While the preparation of thick, glass-like carbon products is still difficult, the thin layers and films are much easier to be produced. In the thin film glass carbon techniques, the resin is coated on to a silica-glass plate, cured, peeled, and carbonised, and, if necessary, it can be graphitized. It is obvious that this may be considered as one of promising completely new techniques to be exploited as a possible alternative for future large segmented X-ray mirrors/foil telescopes.
The glass-like carbons have bulk densities around 1.5 g cm-3 (although they can be as small as 1.4 g cm-3 and even 0.6 g cm-3 if an extended porosity may be accepted) which are almost equal to those of the conventional synthetic graphite and lower than any previous material considered for future large area X-ray mirrors. The glossy carbons with high porosity can even reach bulk densities of 0.6 g cm-3. The bending strength of glass like carbons amounts to 50-200 MPa, the Young's modulus to 20-32 GPa, and the C.T.E. amounts to about 1 x 10-6 C-1.
The applications of glass-like carbons have been rather limited for the past few dozens of years. It is just recently that they have attracted much more interest in terms of industrial applications. Among the parameters, the glass like carbons seems to be favourable because of their low density and low thermal expansion. The large-size composite glass-like carbon thin plates have been already successfully produced for fuel cell separator s (Marsch et al. 1997).
Although mostly discussed and applied because of their unique magnetic properties, the amorphous (glossy) metals and alloys also exhibit excellent mechanical properties if compared with classical crystalline materials. The results of studies obtained so far indicate that the mechanical stiffness may be nearly 4 times better and hence nearly 4 times improvement may be expected in the weight reduction of the mirror assembly.
On the other hand, the application of this technique in the development of innovative X-ray optics is completely new and needs to be carefully tested and further exploited. From the three available technologies to produce amorphous metal alloys, only one (the electrodeposition) seems to be suitable for X-ray mirror shells. We have started study and technology developments in this direction with expected preliminary results within one year.
The replication allows in general to transfer the multilayer structures from mandrels to (mostly inner, i.e. hollow) surfaces where the direct multilayer deposition is either difficult to be carried out with the required quality or even impossible. We have successfully replicated multilayer mirror flat samples using the epoxy replication technology. The tests have confirmed the still good performance of the multilayers after the replication process. In the first stage, the flat samples have been replicated and tested. The study continues with replication of layers deposited on rotational symmetric surfaces (mandrels).
There are several promising alternative methods to produce large precise and lightweight X-ray mirror shells for future X-ray astronomy satellite missions. The first prototypes and tests have indicated that the glass foils, double-sided composite flats, light ceramics as well as amorphous metal alloys are among the suitable techniques to be further exploited.
