DT/T beyond linear theory

The major contribution to the anisotropy of the temperature of the Cosmic Microwave Background (CMB) radiation is believed to come from the interaction of linear density perturbations with the radiation previous to the decoupling time. Assuming a standard thermal history for the gas after recombination, only the gravitational field produced by the linear density perturbations present on a $\Omega\neq 1$ universe can generate anisotropies at low z (these anisotropies would manifest on large angular scales). However, secondary anisotropies are inevitably produced during the nonlinear evolution of matter at late times even in a universe with a standard thermal history. Two effects associated to this nonlinear phase can give rise to new anisotropies: the time-varying gravitational potential of nonlinear structures (Rees-Sciama RS effect) and the inverse Compton scattering of the microwave photons with hot electrons in clusters of galaxies (Sunyaev-Zeldovich SZ effect). These two effects can produce distinct imprints on the CMB temperature anisotropy. We discuss the amplitude of the anisotropies expected and the relevant angular scales in different cosmological scenarios. Future sensitive experiments will be able to probe the CMB anisotropies beyong the first order primary contribution.Comment: plain tex, 16 pages, 3 figures. Proceedings of the Laredo Advance School on Astrophysics "The universe at high-z, large-scale structure and the cosmic microwave background". To be publised by Springer-Verla

Silicon carbide based thermal spray powder, method of preparation and use

A silicon carbide-based thermal spray powder contains at least one boride chosen from zirconium boride, titanium boride and hafnium boride. The powder is prepared by mixing and aggregation of powders containing the compounds in question. Said thermal spray powder is used to deposit, via the plasma spraying technique, a silicon carbide-based coating on a metallic or non-metallic substrate. The figure shows the X-ray crystallogram obtained, for a silicon carbide-based powder, according to the invention, after thermal spraying. The substantial identity of this crystallogram with the one obtained prior to thermal spraying demonstrates that the silicon carbide has been deposited on the substrate without decomposing

Silicon carbide-based thermal spray powder, method of preparation and use

A silicon carbide-based thermal spray powder contains at least one boride chosen from zirconium boride, titanium boride and hafnium boride. The powder is prepared by mixing and aggregation of powders containing the compounds in question. Said thermal spray powder is used to deposit, via the plasma spraying technique, a silicon carbide-based coating on a metallic or non-metallic substrate. The figure shows the X-ray crystallogram obtained, for a silicon carbide-based powder, according to the invention, after thermal spraying. The substantial identity of this crystallogram with the one obtained prior to thermal spraying demonstrates that the silicon carbide has been deposited on the substrate without decomposing

Mechanical properties of ceramic matrix composite for high temperature applications obtained by plasma spraying

Nose and wing leading edges for future generations space vehicles will withstand very high temperature in an oxidizing environment. UHTC (Ultra High Temperature Ceramics) materials are very promising candidate materials for such applications. An innovative, proprietary methodology was developed to produce, by plasma spraying deposition, a ceramic composite containing SiC particles (25 wt%) dispersed in a ZrB2 matrix. With such a technique both coatings and self standing parts were fabricated. In the present paper, the results of mechanical characterisations, carried out on self standing samples, are presented. Tensile and bending properties were determined by mechanical tests on as sprayed samples and on samples exposed at high temperature (2173 K) in oxidising conditions. Experimental results clearly evidenced the possibility to use the plasma spraying technology and suggest that the so fabricated ZrB2-SiC material is suitable to be adopted as protective coating

High temperature behaviour of plasma sprayed ZrB2-SiC composite coatings

ZrB2-SiC composites are considered a class of promising materials for aerospace applications such as nose and leading edges of re-entry vehicles. Results on such materials obtained by hot isostatic pressing have confirmed their high resistance to the oxidation at temperature up to 2000degreesC. Ongoing work has shown that such materials can be obtained in the form of coatings by means of Plasma Spraying techniques. On this regard, the most critical aspect was correlated to the decomposition of the SiC phase at a temperature quite lower than the melting point of ZrB2. Experimental evidence indicated that such decomposition can be avoided when a proper methodology of preparation of the starting powders is adopted, and if suitable thermal spraying parameters are selected. In any case, high temperature oxidation testing (up to 1800degreesC) confirmed the outstanding behaviour of this materials obtained by plasma spraying. This paper is focussed on preliminary studies of oxidation behaviour for plasma sprayed ZrB2-SiC Composites suitable for thermal protection shields