Hierarchically Structured Materials

Coatings of ZrB2 and TiB2 for photothermal solar absorber applications were prepared using chemical vapor deposition (CVD) techniques. Oxidation tests suggest a maximum temperature limit for air exposure of 600 K for TiB2 and 800 K for ZrB2. Both materials exhibit innate spectral selectivity with an emittance at 375 K ranging from 0.06 to 0.09, a solar absorptance for ZrB2 ranging from 0.67 to 0.77 and a solar absorptance for TiB2 ranging from 0.46 to 0.59 ZrB2 has better solar selectivity and more desirable oxidation behavior than TiB2. A 0.071 μm antireflection coating of Si3N4 deposited onto the ZrB2 coating leads to an increase in absorptance from 0.77 to 0.93, while the emittance remains unchanged.

Chemical vapor deposited (CVD) amorphous silicon alloyed with carbon or nitrogen (¿-Si:X, X=C or N) to retard high temperature crystallization is a promising absorber material for photothermal solar energy conversion. Films are prepared by decomposing silane containing gas mixures, a technique which is known to incorporate hydrogen into ¿-Si in some cases. Using the 16.45 MeV resonance of the 1H(19F,¿¿)16O reaction we made the first measurements of the hydrogen incorporation in CVD a-Si:X films (X=C,N). We have made three observations. First, the incorporation efficiency of hydrogen into CVD a-Si increases by a factor of twenty as the carbon content increases from 0 to 35 atomic percent which indicates that previous studies of multicomponent systems may need to be reevaluated since this enhancement in incorporation efficiency involves hydrogen--a key alloyant in a-Si. Second, the quantity of hydrogen incorporated increases at a greater than linear rate as a function of carbon content which implies that the presence of hydrogen in the films is not accidental but is a necessary part of film growth. Third, the hydrogen content of a-Si decreases to almost zero after high temperature anneal which may help explain reported shift in optical constants.

Amorphous silicon holds considerable promise as a photothermal absorber, but high-temperature-induced crystallization limits its usefulness. To attempt to retard the crystallization, we produced CVD a-Si films alloyed with C, N, B, or Ge. These films crystallized differently than did the non-intentionally doped amorphous material. The crystallization temperature was increased from 680 C to 950 C for 18 at.% C-alloyed a-Si, and even then more than 10 hours were required for crystallization. This retardation of crystallization gives alloyed a-Si absorbers sufficient life expectancy for converters operating at temperatures up to 700 C.

By pyrolytic decomposition of silane in the presence of dopant gases, a set of amorphous silicon films was prepared that contains various concentrations of carbon, nitrogen, boron or germanium. The effect of these dopants on the crystallization process and the optical properties is investigated. Films containing aböut 18 at % carbon show the properties most favorable for solar absorbers. The crystallization is retarded to temperatures near 1000°C, and the solar absorptance is greater than that of non-intentionally doped CVD amorphous silicon. From the experimentally determined activation energy of crystallization, the structural lifetime for such absorber films is extrapolated to be in excess of several decades for continuous operation at 700°C. For identical thicknesses of absorber layers, spectrally selective stacks of stabilized amorphous silicon deposited on top of a molybdenum reflector have higher solar absorptance than stacks composed of polycrystalline silicon on a silver reflector, amorphous silicon on molybdenum having been tested at temperatures in excess of 500°C.