Solar Energy Materials and Solar Cells54(1998)19—26
Characterisation of CVD-tungsten—alumina cermets for high-temperature selective absorbers
A.Berghaus*,A.Djahanbakhsh,L.K.Thomas
Institut fu(r Metallische Werkstoffe-Metallphysik—Technische Uni v ersita(t Berlin,Hardenbergstr.36,
10623Berlin,Germany
Received1February1997
Abstract
Solar selective tungsten—alumina cermets were deposited with a low-pressure cold-wall CVD-system utilising the simultaneous pyrolytical decomposition of tungstenhexacarbonyl and aluminiumtriisopropylate(ATI).The deposition results in amorphous W—WO
V—Al O films.After a post-deposition annealing in pure hydrogen at800°C for a maximum of1h the films consist offine grained -alumina and metallic tungsten.The hemispheric reflectivity was measured f
rom0.2—25 m and compared to the emissivity,measured with a thermopile set-up. An absorptivity of0.85and an emissivity of0.04could be obtained for a single layer on copper substrates.Improvement of the absorption by a gradient of the tungsten content in the layer was investigated. 1998Published by Elsevier Science B.V.All rights reserved.
Keywords:Tungsten—alumina cermets;Absorption
1.Introduction
The CVD-process is well suited to deposit systems for a complete selective absorber consisting of adherence-layer,diffusion barrier,ir—reflecting layer and absorbing cermet layer with a composition gradient in one uninterrupted process.In previous work the predominantly investigated systems were cermets consisting of tungsten
*Correspondence address:Surface/Interface260Santa Ana Court,Sunnyvale,CA94086USA.E-mail: aberghaus@surfaceinterface
0927-0248/98/$19.00 1998Published by Elsevier Science B.V.All rights reserved.
PII S0927-0248(97)00219-5
20  A.Berghaus et al./Solar Energy Materials and Solar Cells54(1998)19—26 embedded in tungstenoxides with various oxygen content[1—5].Tungstenoxide as matrix bears some difficulties for long-term use because of its relatively high vapour pressure at higher temperatures.Gradient layers cannot be produced for principal reasons[6].
In extension of the previous work,here we investigated the potential of the CVD-process to simultaneously deposit a refractory metal in a stable ceramic matrix. As an example we utilised tungsten in alumina.Alumina is known as a high temper-ature stable ceramic.Tungsten is well suited because of its intrinsic selectivity and its high melting point.Several selective absorber systems are based on refractory metals and alumina[7].Up to now only sputtered or evaporated layers were investigated. The possibilities of the CVD-process were not tested.
2.Experimental
2.1.Precursors
Tungstenhexacarbonyl(W(CO) )is an easy substance to handle.It is stable in air, solid at room temperature and sublimes readily with a sufficient vapour pressure at temperatures between50°C and90°C.Depending on the atmosphere it decomposes at temperatures above170C,leading to a depo
sit of tungsten with various amounts of oxygen and carbon incorporated and volatile carbon monoxide.
The deposition of alumina from the pyrolytic decomposition of aluminium-tri-isopropylate(Al(C H O) )(ATI)was intensively investigated.It is a moisture sensitive,air stable,solid substance.It sublimes in vacuum without decomposition. It can be melted and kept as supercooled liquid under argon for several days [8—10].
ATI has the highest vapour pressure of all aluminum alkoxides.The decomposition leads to alumina and aluminumhydroxide at approx.¹'200°C.At¹'350°C only alumina is present.
2.2.CVD equipment
The CVD-system consists of a gas supply for argon,nitrogen and oxygen,regulated by massflow controllers,a source-system for W(CO) and for ATI,a horizontal cold-wall reactor and a pumping system.The gasstream behind the reaction zone can be analysed by mass-spectroscopy.
Within the reactor susceptors of carbon allowed several geometries for the deposition process.The susceptors were coated with a layer of metallic tungsten to prevent reactions of the carbon.Here we report about samples perpendicular to the gasstream.The susceptor can be heated to a maximum tem
perature of approx. 1000K within a few minutes by3lamps with a total power of3kW.The temperature is measured by a thermocouple within the susceptor a few millimeters below the sample.
A.Berghaus et al./Solar Energy Materials and Solar Cells54(1998)19—2621
2.3.Partial pressure control of solid precursors
Special care was taken to ensure a stable concentration of the precursors in the gasstream.
For the tungstencarbonyl the optical absorption at254nm was monitored and the concentration of the precursor calculated.For the ATI a micropump was used to deliver the melted and supercooled liquid into an overheated atmospheric pressure chamber whereflash evapoation takes place.The concentration of the precursor was controlled by adjustment of the transport rate of the micropump.
2.4.Substrate preparation
Substrate materials were silicon,quartzglass,copper and stainless steel of type 1.4301.Mostly metallic substrates were used because of their usage as infrared reflectors and because of a good thermal contact to the susceptor.They were disks of a diameter of40and4mm thickness.They were subsequently ground and polished with siliconcarbide paper and a diamond suspension(particle size2
m),then cleaned with detergent,destilled water,ethanol and methanol and dried in a stream of hot air.
2.5.Deposition process
The substrates are mounted on the susceptor.Before deposition,the reactor is cleaned by subsequent evacuation to a base pressure below0.1Pa and a backfill with dry nitrogen to the desired total pressure.After stabilization of the gasstreams and heating the susceptor to673K the deposition is started by switching the gasstream from a bypass to the reactor.After deposition the stream is switched back.Cooling down of the samples takes15—25min.Typical deposition conditions are given in Table1.
2.6.Annealing
After deposition the samples contain more oxygen than can be attributed to the System W-Al O alone.Some of the samples were subjected to a stepwise post deposition heat treatment in hydrogen at temperatures between650°C and900°C for 1—18h.The contents of tungsten,aluminum and oxygen were monitored by EDX and the reflectivity was measured.
2.7.Analysisreactor pressure中文
The deposition rate and density of the layers were calculated from the substrate weight before and after the deposition and the thickness of the layers measured by
a talystep instrument(Hommel).The hemispheric reflectance was measured from
0.2—25 m with a Fourier transform spectrometer(MIDAC)in the far infrared and
a Perkin-Elmer 9instrument for the visible and near ir range and compared to the
22  A.Berghaus et al./Solar Energy Materials and Solar Cells54(1998)19—26
Table1
Typical conditions for the deposition of the CVD-layers
Carriergas Argon,nitrogen or forming gas
(8%H in N )
Temperature of susceptor673K
Total pressure266P
Total gasflow F  600sccm
Temperature of W(CO) -source323—343K
Flow through source100sccm
ATI-source pumprate64.3 mol/min
Flow through ATI-source400sccm
Deposition time1—120min
emissivity,measured with a thermopile set-up.Optical constants were determined from ellipsometric measurements.
3.Results and discussion
3.1.Composition
The deposition produces amorphous W—WO V—Al O cermetfilms.Thefilms adhered well to quartz-,copper-and stainless-steel substrates.Metallic tungstenfilms on stainless steel for high infrared
reflectance withstand thermal cycling when depos-ited on an intermediate cermet layer.After a post deposition annealing in pure hydrogen at800°C for a maximum of1h thefilms consist offine grained -alumina and metallic tungsten.Fig.1shows the composition of cermetfilms as investigated by EDX vs.the composition of the gasstream.Changing the ratio of the concentration of the precursors W(CO) and ATI results in a change of the composition of the deposited layers.The oxygen content of all samples analysed was almost independent of the relative concentration of precursor.The tungsten content varies between9and 32at%and the aluminum content,respectively,depending on the relative concentra-tion of the tungsten precursor in the gasstream.The W-and Al-contents seem to saturate at30and15at%.A depositon without ATI results in afilm consisting of 41%W and59%O.
Annealing of thefilms in hydrogen leads to a change in composition.Above550°C hydrogen reduces various tungsten oxides to metallic tungsten.Alumina cannot be reduced by hydrogen at the temperatures applied.The surplus oxygen can be at-tributed to a formal tungsten oxide WO V.After approx.4h at650°C the annealing is finished(x P0)and thefilms consist of pure tungsten in the alumina matrix(see Fig.2).
Fig.1.tration of precursors in the gasstream during deposition.
3.2.Stucture
The surface of the films deposited on copper substrates appears to be almost structureless in SEM.Only at high magnification they reveal nodular structures with a typical diameter of less then aprox.100nm.It was shown by Thomas that for WO V -films [5]the total pressure during deposition influences the size of these nodules.The lower the pressure,the smaller the diameter.
Diffraction experiments were done to investigate the crystal structure of the as-deposited films and the annealing process.Pure tungsten-,alumina-and cermet films were deposited on silicon wafers.These films were investigated with monochromatic X-Rays (Cu k  )after deposition and then subjected to a stepwise annealing at various temperatures.Fig.3shows the diffraction diagrams of a 2.7 m thick cermet film with an initial W-content of 11at%during this annealing procedure.The theoretical positions of the diffraction lines of FCC  -W,BCC  -W and  -Al  O  are indicated.After deposition the diffraction pattern of a metallic film shows a mixture of  -tungsten and  -tungsten.While the CVD-tungsten films appear to be crystalline after deposition the tungsten in the cermet layer is amorphous for X-rays.The embedding in the ceramic matrix seems to impede the formation of crystalline metallic particles.Only th
e broad background indicates a mixture of very fine grained  -tungsten and  -tungsten.During the annealing mainly three processes appear to take place.The oxygen content is reduced,therefore tungstenoxide is reduced to A.Berghaus et al./Solar Energy Materials and Solar Cells 54(1998)19—2623

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