Qi-Chu Zhang

Residual stress, microstructure, and structure of tungsten thin films deposited by magnetron sputtering

The residual stress and structural properties of tungsten thin films prepared by magnetron sputtering as a function of sputtering-gas pressure are reported. The films were analyzed in situ by a cantilever beam technique, and ex situ by x-ray diffraction, cross-sectional transmission electron microscopy (TEM), x-ray photoelectron spectroscopy, electron energy-loss spectrometry, and energy-filtered electron diffraction. It is found that the residual stress, microstructure, and surface morphology are clearly correlated. The film stresses, determined in real time during the film formation, depend strongly on the argon pressure and change from highly compressive to highly tensile in a relatively narrow pressure range of 12–26 mTorr. For pressures exceeding ∼60 mTorr, the stress in the film is nearly zero. It is also found that the nonequilibrium A15 W structure is responsible for the observed tensile stress, whereas the stable bcc W or a mixture of bcc W and A15 W are in compression. Cross-sectional TEM eviden...

Recent progress in high-temperature solar selective coatings

This paper reviews two recent significant innovations relating to solar selective absorbing coatings. Through fundamental analysis and computer modelling, we have developed a double cermet layer film structure for solar selective surfaces with better solar performance than surfaces using a homogeneous cermet layer or conventional graded film structure. A second innovation reduces the cost of depositing high-temperature solar coatings. This innovation has two main features: (1) the ceramic and metallic components in the cermet are simultaneously deposited by direct current (DC) sputtering, and (2) the ceramic component is deposited by DC reactive sputtering and the metallic component by DC non-reactive sputtering. Metal-aluminium nitride (M-AlN) cermet solar coatings have been deposited by two-target DC magnetron sputtering technology. An Al metal target is used to deposit the AlN ceramic component in the cermet by DC reactive sputtering in a gas mixture of argon and nitrogen. Tungsten, molybdenum and stainless steel (SS), which have good nitriding resistance, are used to deposit the metallic component by DC non-reactive sputtering in the same gas mixture. M-AlN cermet solar coatings with the double cermet layer film structure were successfully deposited, achieving a solar absorptance of 0.92–0.96 and normal emittance of 0.04–0.05 at room temperature. A commercial-scale cylindrical DC magnetron sputter coater for depositing the SS-AlN cermet selective surfaces on batches of tubes has been constructed and successfully operated. SS-AlN cermet solar collector tubes have been produced. Minor modifications to this commercial-scale coater, such as exchanging the SS target for a tungsten or molybdenum target, would enable the production of W-AlN or Mo-AlN cermet solar collector tubes. Good thermal stability of SS-AlN, W-AlN and Mo-AlN cermet solar collector tubes at a high temperature of 350–500°C in vacuum is expected. The cost of these high-temperature solar collector tubes should be much lower than solar collector tubes produced using conventional sputtering technology, DC sputtered Mo metal component and RF-sputtered Al2O3 ceramic component, for solar thermal electricity applications.

Very low‐emittance solar selective surfaces using new film structures

New cermet film structures suitable for selective solar absorbers, composed of two cermet sublayers with identical metal volume fractions in each sublayer, on metal reflectors with an antireflection dielectric layer coating are described. The absorbing cermet sublayers have thickness and volume fractions such that solar radiation is absorbed internally and by phase cancellation interference. They are substantially transparent in the thermal infrared region. Modeling of the new cermet layer structures has revealed a number of different selective surfaces, involving a variety of practical cermet materials, with better performance than recent published results. For example, the best predicted ratio of absorptance to normal emittance α/e at room temperature is 46 for a Cu‐SiO2 cermet, 49 for a Cu‐Al2O3 cermet and a Au‐Al2O3 cermet. The best laboratory results of α/e at room temperature thus far are 46 for a Cu‐SiO2 cermet, and 45 for a Au‐Al2O3 cermet. From computer modeling, using published experimental diel...

High efficiency MoAl2O3 cermet selective surfaces for high-temperature application

Abstract Highly efficient MoAl2O3 cermet solar absorbers have been designed with a numerical model and deposited experimentally. The typical film structure is an Al2O3 anti-reflection layer on a double MoAl2O3 cermet layer on a Mo or Cu metal thermal reflector. In numerical calculations of the thermal emittance at high temperature for these selective surfaces, the temperature dependencies of the complex refractive indices of the metal reflector and cermet in the infrared region have been considered, and the dielectric functions of the cermet materials are evaluated using Shengs approximation. An optimization calculation yields a photothermal conversion efficiency as high as 0.914 at 350°C for a concentration factor of 26 for the film structure consisting of a double cermet layer on a Mo metal thermal reflector with an Al2O3 anti-reflection coating. The corresponding normal absorptance and hemispherical emittance at 350°C are 0.96 and 0.11, respectively. MoAl2O3 cermet selective surfaces using the double cermet layer structure were deposited by vacuum co-evaporation, and an absorptance of 0.955 and near normal emittance of 0.032 at room temperature have been achieved. An emittance of 0.08 at 350°C is estimated based upon room temperature experimental data for the film structure of a double cermet layer on a Cu metal thermal reflector with an Al2O3 anti-reflection coating.

New cermet film structures with much improved selectivity for solar thermal applications

A new cermet film structure of solar thermal absorber is presented. The typical film is similar to that of the single cermet layer case, except the single cermet layer is replaced by two isotropic cermet sublayers. The calculated results have been shown that low‐ and high‐temperature performances using this new selective surface structure are excellent. The value of the ratio of absorptance to normal emittance α/e, 46, for deposited film has been achieved.

Composition, residual stress, and structural properties of thin tungsten nitride films deposited by reactive magnetron sputtering

Thin tungsten nitride ( WN x ) films were produced by reactive dc magnetron sputtering of tungsten in an Ar–N 2 gas mixture. The effects of the variation of nitrogen partial pressure on the composition, residual stress, and structural properties of these films as well as the influence of postdeposition annealing have been studied. The films were analyzed in situ by a cantilever beam technique, and ex situ by x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, x-ray diffraction, and transmission electron microscopy(TEM). It was found that at N concentrations below 8 at. %, the films (typical 150 nm in thickness) were essentially bcc α-W. An amorphous phase was observed in the range of about 12–28 at. % N. When N concentrations reached ∼32 at. % or above, a single-phase structure of W 2 N was formed. Annealing of the as-deposited films resulted in crystallization of the amorphous or an improved crystallinity of the W 2 N structure, which was related to the N concentration. Stresses of all W and WN x films were compressive. As the N concentration was increased, the stress decreased and reached its lowest value for amorphous samples near 20 at. % N. Past this point, the compression of films rose again. These results can be ascribed to structural changes induced by the pressure-dependent variation in the average energy of particles bombarding the film during deposition. Cross-sectional TEM studies showed that all crystalline WN x films had columnar microstructures. The average column width near stoichiometry of W 2 N was ∼20±5 nm near the film surface.

High solar performance selective surface using bi-sublayer cermet film structures

Numerical modelling calculations have been used to design solar selective absorbers with new cermet film structures, composed of two cermet sublayers, each having a different metal volume fraction, located between a conventional metal reflector and a dielectric anti-reflection layer. These selective surfaces may use a variety of practical cermet materials and all achieve better solar thermal performance than any published results. For example, our best predicted ratio of absorptance to normal emittance α/e at room temperature is 46 for a CuSiO2 cermet. The best experimental result of α/e at room temperature is also 46 for the same cermet. From a computer optimazation using published experimental dielectric functions of CoAl2O3 cermets, we clearly show improved performance is obtained by using two cermet layers rather than one. For example, an absorptance of 0.90 and normal emittance of 0.024 at 50°C (α/e = 37) could be obtained for a film composed a two cermet sublayers on a Mo reflector with an Al2O3 antireflection layer. For an optimized low emittance double cermet coating, hemispherical emittance at 350–400°C is very close to hemispherical emittance at room temperature.

Structural properties and nitrogen-loss characteristics in sputtered tungsten nitride films

Abstract A combination of X-ray photoelectron spectroscopy (XPS), parallel electron energy-loss spectroscopy (PEELS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and transmission electron diffraction (TED) were used to investigate structural properties and nitrogen-loss characteristics of thin WNx films prepared by reactive magnetron sputtering of tungsten in an Ar–N2 gas mixture. XRD θ−2θ scans combined with plan-view and cross-sectional TEM showed that the as-deposited WNx films were amorphous in structure. Annealing of the as-deposited films at 600°C or above resulted in crystallization of the amorphous phases, forming either a two-phase structure consisting of W2N and b.c.c. W or a single-phase structure of W2N, which was related to the initial nitrogen concentration in the films. The 150-nm thick crystalline films near a stoichiometry of W2N had a columnar microstructure with an average column width of 15–20 nm near the film surface, whereas the column grains were larger for substoichiometric films. Thermal stability and nitrogen-loss characteristics of nitride films were also studied by in situ annealing in the TEM and PEELS system. The results indicate that between 600 and 800°C the W2N phase was stable. Nitrogen in the film started to evaporate to vacuum at approximately 820°C and was fully released after 900°C annealing.

OXYGEN-INDUCED AMORPHOUS STRUCTURE OF TUNGSTEN THIN FILMS

A combination of energy-filtered electron diffraction, electron energy-loss spectroscopy, transmission electron microscopy, and x-ray diffraction are used to establish that oxygen impurities incorporated in tungsten films prepared by magnetron sputtering in the early stage of the deposition play a dominant role in the formation of an amorphous phase. Energy-filtered electron diffraction data collected from a range of amorphous films were Fourier transformed to a reduced density function (RDF) and matched with an amorphous model. The results show that better agreement with the experimental RDF is achieved if the amorphous model consists of a random continuous matrix of clusters with W3O-like symmetry.