High IR Reflectance Research Articles

Composition and microstructure of black molybdenum photothermal converter layers deposited by the pyrolytic hydrogen reduction of MoO 2Cl 2

Black molybdenum films combining a high IR reflectance with a significant solar absorptance were produced by the pyrolytic hydrogen reduction of MoO 2Cl 2 in the temperature range 550–710°C on both quartz and metal substrates. Characterization by X-ray, transmission electron microscopy, scanning electron microscopy and Auger analyses showed that the films are composite materials consisting of small molybdenum particles suspended in a matrix of MoO 2 grains. The composition and structure of the films can be adjusted by variations in the substrate temperature and hydrogen concentration, resulting in corresponding changes in the optical properties. Although the films described here are comparable in structure with black chrome, they are more durable at elevated temperatures and were tested at 500°C in a roughing pump vacuum.

Deposition of transparent heat-reflecting coatings of metal oxides using reactive planar magnetron sputtering of a metal and/or alloy

Highly conducting visually transmitting filters were made on glass and these also showed a high IR reflectance. The filters were made by planar magnetron sputtering of InSn and Cd 2Sn in an ArO 2 atmosphere which was activated by an r.f. discharge to give very high rates of deposition (with excellent properties) onto substrates which were at room temperature. The key to the achievement of these properties lay in the control of the activating field and the rate of deposition compared with the rate of admission of oxygen gas. The properties of InSn oxide and Cd 2Sn oxide films in their visual transparency, electrical conductivity, electrical mobility and IR reflectance match those made by conventional vacuum techniques and chemical vapour deposition and they do not require any heating of the substrate. The high rates obtained with this method, with no requirement for substrate heating, offer an economical technique of coating glass for the suppression of radiation loss in glazing elements.

Highly reflecting molybdenum thin films having significant solar absorptance

A high solar absorptance coupled with a high IR reflectance is necessary for efficient photothermal solar conversion. The most common selective blacks operate on the principle of the absorber-reflector tandem, in which a highly reflecting film is overcoated with a layer which absorbs over the solar spectral range. Molybdenum films fabricated by chemical vapor deposition (CVD) under oxidizing conditions and subjected to post-deposition annealing and passivation provide both optical functions—high IR reflectance together with sufficient solar absorptance—in a single layer. Film deposition proceeds under atmospheric pressure and at 300°C through pyrolytic decomposition of molybdenum carbonyl (Mo(CO) 6) in the presence of an oxygen bleed. Films thus deposited contain a high concentration of oxygen and exhibit typically a solar absorptance of a = 0.77 and a thermal emittance for 500°C black-body radiation of e = 0.31. In subsequent annealing in a reducing atmosphere at 770°C this high solar absorptance is nearly retained whereas the thermal emittance is lowered to values of the order of e = 0.08. The passivation of these black molybdenum films against deterioration through operation in open air requires an overcoating. In this case Si 3N 4 deposited by CVD was used. This passivation layer can be used as an antireflection coating if it is of the proper thickness, raising the solar absorptance to values greater than a = 0.91 while only slightly increasing the thermal emittance to e = 0.11. Changes in the optical properties of black molybdenum during annealing are related to alternations in structure and composition. Since annealing proceeds at temperatures in excess of 700°C no thermal deterioration processes are expected in the absence of oxygen at the lower operating temperature of 500°C. This was demonstrated through testing at 500°C in a roughing pump vacuum; no changes in the films' optical characteristics were observed after 1000 h. All depositions and anneals proceed at atmospheric pressure, facilitating a continous flow-through procedure important for large-scale economic fabrication.

Abstract: Transparent conducting films

Transparent conductors are becoming of increasing importance because of increased interest in the interactions of light with electrically or electronically active materials, and for their high ir reflectivity which is useful for solar energy applications. The information in this presentation is largely organized by film deposition process rather than the film material because, ostensibly, the same material deposited by two different deposition techniques ordinarily yields two widely varying sets of physical properties. This is not surprising because of the complex physics and chemistry of the materials used for this purpose. Broadly speaking, there are two classes of materials: semiconducting oxides and very thin metals. The properties of the oxides depend critically upon their oxidation state and the nature and quantity of impurities trapped in the film. The properties of the thin metals depend upon nucleation and early coalescence phenomena and impurities. Both the electrical and optical properties of both classes depend strongly on the nature of the substrate used (impurities that can diffuse from the substrate into the film and the refractive index of the substrate). Following a brief review of the physics of conductivity and transparency in these materials, detailed analysis of the following film deposition processes are given: hydrolysis of chlorides, pyrolysis, evaporation and sputtering of thin metals (with and without nucleation modifying layers), reactive evaporation and sputtering, and sputtering of oxide targets. The important process-control parameters that affect the electrical and optical properties of transparent conducting films are discussed.