Preparation,quality characterization,rvice performance evaluation and its modification of sapphire crystal for optical window and dome application
Guigen Wang a,*,Hongbo Zuo b ,Huayu Zhang a ,Qibao Wu c ,Mingfu Zhang b ,Xiaodong He b ,Zhaohui Hu a ,Lin Zhu d
a
Shenzhen Graduate School,Harbin Institute of Technology,Shenzhen 518055,PR China b
Center for Composite Materials,Harbin Institute of Technology,Harbin 150080,PR China c背上的痣
China Fangda Group Co.Ltd.,Shenzhen 518055,PR China d
Shenzhen Taoyuan Middle School,Shenzhen 518055,PR China
a r t i c l e i n f o Article history:
Received 1July 2009Accepted 7August 2009
Available online 13August 2009Keywords:
教学八大原则
Sapphire crystal Window and dome
Mechanical-optical properties Precipitation hardening
a b s t r a c t
Sapphire (i.e.,Al 2O 3single crystal)window and dome are important nsor components.Large-sized sap-phire crystal was prepared by an improved Kyropoulos method,and its machining technique of ultrasonic vibration was also reported.The quality and structure were evaluated by illumination of a He–Ne lar transmission electron microscopy (TEM),double crystal X-ray diffraction (DCXRD)and Raman spectros-copy.For practical window and dome application,its high-temperature mechanical strength and optical transmission were both measured,and a modification measure of sputtering Mg film on sapphire surface followed by annealing and subquent quenching was demonstrated.Large-sized sapphire crystals with high quality and different profiles were successfully grown by adjustment of pulling velocity and crucible size,design of suitable temperature field.Moreover,a ries of sapphire windows and domes were fabri-cated for different optical application.The biaxial flexure strength of sapphire crystal decreas with increasing temperature,and the infrared transmission is also slightly degraded at high temperature.In addition,it should be noted that there is sharp strength
drop in the temperature range of 400°to 600°C which is related with twin formation in sapphire crystal.Submicron MgAl 2O 4spinel precipitation can effectively harden sapphire crystal and do not induce large optical-scattering loss,which is a potential modification method for window and dome application of sapphire crystal.
Ó2009Elvier Ltd.All rights rerved.
1.Introduction
The windows and domes of nsors often become the single point of failure for optoelectronic devices and systems becau of their interaction with harsh environment.The windows and do-mes must overcome thermal shock failure due to rapid accelera-tion and deceleration,withstand abrasion from raindrops and sand particles,and meanwhile yet provide maximum signal trans-mission and precision,etc.[1,2].
An ideal window or dome,which satisfies all the requirements,does not exist up to now.Selection of a window or dome is often bad on compromis instead of choices.There are only a small number of window and dome materials available and each comes with its own limitations [1].Owing to its excellent overall proper-
ties,sapphire (Al 2O 3)single crystal continues to prevail in durable window/dome materials field but is also always limited by its size and high cost becau there are increasing numbers of airborne electro-optic systems that require large aperture optical windows and domes with high quality [3,4].In addition,newer and higher technical requirements are made for window/dome materials of supersonic aerocraft with high performance,such as higher resis-tance to thermal shock [5].
In this paper,large-sized and high-quality sapphire crystals were grown by an improved Kyropoulos method,which were both char-acterized by its optical quality and crystalline structure.Their machining technique of ultrasonic vibration was reported for fabri-cation of optical window and dome,and the mechanical and optical properties were studied systematically in high temperature envi-ronment.Finally,a measure of sputtering Mg film on sapphire sur-face followed by annealing and quenching was discusd for avoiding rvice failure of sapphire optical window and dome dur-ing hypersonic flight.
0261-3069/$-e front matter Ó2009Elvier Ltd.All rights rerved.doi:10.1016/j.matdes.2009.08.015
*Corresponding author.Tel./fax:+8675526032709.E-mail address: (G.Wang).
Materials and Design 31(2010)
706–711
Contents lists available at ScienceDirect做好准备的成语
Materials and Design
j o u r n a l h o m e p a g e :w w w.e l s e vier.c om/loc ate/mat
狼心狗肺des
2.Experimental
2.1.Growth of sapphire crystal
Sapphire crystal was grown by an improved Kyropoulos method [6].In our well-designed crystal growth system,molybdenum cru-cible was supported by one large-diameter tungsten rod with blind hole on the bottom.The special cage-shaped heater was consisted of a ries of clo-packed molybdenum bars.In addition,the ther-mal shield elements included top multi-layers molybdenum sheets on the crucible,side-wall molybdenum sleeve surrounded by insu-lation layer of alumina foam and bottom thick alumina blank.
Before crystal growth,the crucible was charged with high-purity alumina biscuit which was pre-sintered at high temperature,sap-phire ed crystal with a definite direction(the most common crystal orientation of sapphire are h11 20i;h1 100i;h1 102i;
<,a-direction,m-direction and r-direction)wasfixed to the bottom of a top water-cooling heat exchanger,and the crystal growth chamber was then pumped down to a high vacuum of 1.0Â10À4Pa.
When the geometrical center of ed crystal was aimed at the cooled center of ,the interction site of convection lines of melt surface)by its continual drop,lift and rotation,the crystal eding process was started.The crystal-eding and subquent shoulder-expanding process were both adjusted by micro-pulling the ed crystal with a velocity range of0.1to0.2mm/h and changing cooling-waterflow in heat exchanger.At the growth stages of iso-diameter and tailing,it were well achieved by pulling the ed crystal with a velocity range of0.3to0.5mm/h and decreasing furnace temperature,also the cooling process was con-trolled automatically to achieve in-situ annealing with pre-estab-lished program.
2.2.Crystal characterization
For as-grown sapphire crystal,it was obrved by illumination of a He–Ne lar(wavelength:k=543nm,output power: P=4mW.source:Lar&Photonics Division,Spectra-Physics Inc.).Its crystalline structure was investigated by TEM(Instrument type:Philips CM-12,acceleration voltage:U=120kV)and Raman (Instrument type:Jobin–Yvon HR800,exciting wavelength: k=458nm,lar power:P=20mW)experiments.Moreover,the crystalline perfection was evaluated by double-crystal X-ray dif-fraction(DCXRD)study using synchrotron radiation source.
2.3.Fabrication of optical window and dome
Due to its hexagonal crystal system,sapphire crystal with zero birefringence[0001]orientation is required for optical elements with high precision[7].For sapphire boule with preferred growth directions of aÀ½11 20 ;mÀ½1 100 and rÀ½1 102 ,the (0001)facet plane often appears on crystal growth profile becau of its lowest growth velocity[4].Firstly the sample near and parallel to facet plane was cut by cylindrical cutting and oriented for veral times in order tofind out the exact sapphire(0001)reference plane.The standard cylindrical sapphire crystal was then drawn out from large as-grown boule by drilling equipped with ultrasonic vibration grinding composite tool[8].Finally a ries of machining procedures were performed,such as wire cutting or scooping,fol-lowing rough die grinding,fine grinding and polishing.
2.4.Measurement of mechanical and optical properties of sapphire crystal
Three-points or four-points bending test is common for strength measurement.But as far as hard window and dome mate-rial is concerned,the ring-on-ring equibiaxialflexure test is pre-ferred in consideration of measurement accuracy and sample preparation[9,10].The hot-presd SiC ceramic with high strength was adopted for mould material,the average diameter of upper pressure ring and lower support ring was10and28mm,respec-tively.The size of sample was U36mmÂ2.0mm,and there were graphite lubricating papers between sample and mould.The strength measurement was perfor
med with Instron-1186 Electronic Universal Testing Machine.The loading velocity was 0.5mm/min,and the testing temperature was in the range of 20°to800°C.
In addition,its high-temperature infrared transmission spec-trum was also measured using FT-IR spectrometer(Nicolet Nexus 870)with heating cell.A ries of(0001)plane disc-shaped sam-ples were polished with size of U13mmÂ3mm,the measuring wavelength and temperature was in the range of2000to 7000nm and20°C to800°C.
2.5.Modification treatment of sapphire crystal for window and dome application
For precipitation-strengthening,Mgfilm was deposited on sap-phire substrate at300°C using a DC magnetron sputtering system with Mg metal target(99.995%purity).During the sputtering pro-cess,the distance between the sample and target was about 85mm,the argon gas(99.9995%purity)pressure and sputtering current was adjusted to0.4Pa and400mA.The deposition time was30min.The Mg-sputtered sapphire substrate was treated by following successive procedures:annealing for30h at1600°C in air,quick quenching from1600to1200°C in about20min and aging at1200°C for50h before slow cooling to room temperature finally.
3.Results and discussion
电脑防火墙在哪里
3.1.Large-sized and high-quality sapphire crystal&sapphire window and dome
As shown in Fig.1,a ries of sapphire crystals were grown suc-cessfully by above-mentioned improved Kyropoulos method.In or-der to meet the specific demand of different window and dome applications,there are mainly three kinds of sapphire , U240mmÂ200mm,U240mmÂ310mm,U325mmÂ260mm, in which different profiles depend on pulling velocity,crucible size and temperaturefield.The pulling velocity influences boule high-ness[11].In addition,the larger the as-grown crystal is,the larger the crucible is needed for achieving in-situ annealing and the lower the temperature gradient is required[6].
All as-grown sapphire boules have complete necks,smooth shoulders and good iso-diameter regions.The surfaces of boules are clean and smooth,on which nofind crepes.Moreover,the pho-tography of sapphire boule with size of U240mmÂ200mm,illu-minated by a He–Ne lar,is illustrated in Fig.2.As to the large-sized sapphire boule,there are just a few scattering centers in the shoulder and tail regions,the visible cloud and pores are not found in its most region.The indicate that no apparent mechan-ical vibration and temperaturefluctuation happened during the growth process of large-sized sapphire,which verifies elementary the reliability of our designed growth system.
For practical application,the sapphire samples often originate from iso-diameter growth region of boule.Further analysis results of crystal structure and quality by TEM,Raman and DCXRD meth-ods are shown in Figs.3–5.The calibration of periodic parallelo-gram patterns in TEM image(Fig.3)show that as-grown sapphire belongs to Al2O3hexagonal single crystal system.There
G.Wang et al./Materials and Design31(2010)706–711707
are complete ven apparent Raman peaks located at 379.365to 751.852cm À1in Raman spectrum (Fig.4),which is in good agree-ment with natural sapphire [12].The double-crystal diffraction curve,as shown in Fig.5,has only a sharp main peak and no big satellite ones.The FWHM experimental value is about 11cond,which approaches its theoretical value (8.6cond)calculated by Darwin X-ray dynamic diffraction equation [13,14].All the
indicate that as-grown sapphire boule prepared by our improved Kyropoulos method usually has good crystal quality,and is well-suited for fabrication of optical window and dome.
Fig.6gives a ries of photographies of finished sapphire win-dows and domes.They have different shapes (e.g.,bar and circular plate windows,sphere domes)and their certain sizes (length or diameter)all exceed 140mm,which can meet the surging demand to some extent for sapphire windo
ws and domes [15–17]
.
Fig.1.Large-sized sapphire crystal grown by improved Kyropoulos method (a)U 240mm Â200mm,(b)U 240mm Â310mm,(c)U 325mm Â260
mm.
Fig.2.The obrvation of large-sized (U 240mm Â200mm)sapphire crystal using He–Ne lar
source.
Fig.4.Raman scattering spectrum of sapphire
crystal.
Fig.3.The electron diffraction pattern of sapphire
crystal.
Fig.5.The double-crystal X-ray diffraction rocking curve in (0006)reflection.
708G.Wang et al./Materials and Design 31(2010)706–711
3.2.Service performance evaluation of sapphire crystal
Sapphire windows and domes are typical structure and function integrating devices.With the development of high-speed and ul-tra-high-speed aircraft,optical window and dome will face com-presd air flow during fighting,which leads to vere aerodynamic heating effects such as thermal crack,thermal radia-tion and thermal distortion [18],so it is necessary to have a thor-ough knowledge of their rvice performance.
As shown in Fig.7,the biaxial flexure strength of sapphire crys-tal decreas with increasing temperature,and especially it hap-pens sharp drop in 400°to 600°C.From the obrvation for fracture morphologies of sapphire crystal (Fig.8)during its strength testing,there are mainly irregular fragments at room tem-
perature (25°C)(Fig.8(a)),however,there are six similar and sym-metrical ones at elevated temperature (600°C)(Fig.8(b)).It indicates the fracture at room temperature was influenced by ran-dom microflaw or other microdefect on crystal surface,and its inhomogeneous distribution induced larger difference of measured strength for a ries of specimens [19].As to the fracture at high tempe
rature,it was mainly caud by slip interction of
f 1 1
02g h 1 100i twins,which is consistent with most specula-tion [20].There were many fragments after loading at high temper-ature,which also shows that there was little stress difference in various crystal regions after thermal polishing during high-tem-perature testing process.
Fig.9also illustrates the IR transmission spectra of sapphire crystals measured at different temperatures.It can be en that its transmittance exceeds 85%from 2.0to 4.0l m at room
temper-
Fig.6.A ries of finished sapphire windows and
domes.
Fig.7.The biaxial flexure strength of sapphire crystal at various
temperatures.Fig.8.Fracture morphologies of sapphire crystals during strength testing at room and high
temperature.
国外旅游
Fig.9.The transmission spectra of sapphire crystal at various temperatures.
G.Wang et al./Materials and Design 31(2010)706–711709
ature.However,it begins to decrea with increasing temperature.The behavior was caud by the in
cread lattice absorption and anharmonically induced broadening in the phonon density of states,which was also discusd in elwhere [21].3.3.Precipitation hardening of sapphire crystal
From the above analysis,it is especially necessary to increa high-temperature strength of sapphire crystal,in which precipita-tion hardening is regarded as a potential method recently [22,23].As shown in Fig.10,‘‘column-like”Mg film with [002]preferred-orientation was deposited on sapphire substrate by DC magnetron sputtering.After annealing,quenching and aging treatment at high temperature,its structure and surface morphology are illustrated in Fig.11.The highly [111]-oriented spinel film was grown on
sapphire substrate by following reactions:2Mg +O 2?2MgO,MgO +Al 2O 3?MgAl 2O 4,and its crystalline orientation was related to the lowest lattice mismatch between MgAl 2O 4(111)film and Al 2O 3(0001)matrix [24,25].The grown spinel film has a
triangle-like surface morphology with ½1 1
学习的谚语
0 ;½10 1 and ½01 1 edges [26],and the spinel film thickness (18.68l m)is four times as much as Mg film (4.52l m),which show Mg film was converted fully into spinel film after high-temperature annealing [27].
As en from the cross-ction morphology (Fig.12)of sapphire crystal after above-mentioned treatment measures,submicron/nano spinel reinforcing cond pha was precipitated in sapphire matrix which region is not far from but not near the film/substrate interface.Investigating above possible formation mechanisms dee-ply,Mg and Al ions can diffu each other,and MgAl 2O 4spinel formation reaction occurred during annealing at high
temperature
Fig.10.The structure and cross-ctional morphology of sapphire crystal after sputtering Mg film:XRD pattern (a)and SEM micrograph
(b).
Fig.11.The structure and surface morphology of sapphire crystal after modification treatment:XRD pattern (a)and SEM micrograph
(b).
Fig.12.The cross-ctional morphology of sapphire crystal after modification:The figure on the right (b)is a magnification of the left part of (a).
710G.Wang et al./Materials and Design 31(2010)706–711
[28].Moreover,after quick quenching,the solubility of Mg ions de-cread with temperature,some Mg ions in sapphire matrix cannot diffu fast enough to go back to Mgfilm,which led to MgAl2O4 spinel precipitation.The submicron/nano MgAl2O4spinel was pre-cipitated directionally becau there was also the lowest lattice mismatch between MgAl2O4(111)and Al2O3(0001)[24,25].
The modification results of sapphire crystal by spinel precipita-tion are described in Table1.The shows that MgAl2O4spinel pre-cipitation can hardened sapphire matrix effectively to large extent (15.2%).However,it decread infrared transmission at3l m slightly(3.7%)after polishing the MgAl2O4spinel reactionfilm. The small and dispersive spinel precipitates can prevent twin slip and thus improve microcrack propagation resistance at high tem-perature,and meanwhile the do not lead to large light-scattering in sapphire crystal[22,29].It is anticipated the sapphire crystal with directionally aligned spinel precipitates has desirable strength and adequate infrared transmission,an
d can be applied widely in its rvice environment of aerodynamic heating.The influence of the size and distribution of precipitates on the com-prehensive properties sapphire crystal will be discusd in our fu-ture work.
4.Conclusion
Large-sized and high-quality sapphire with different profiles can be grown by an improved Kyropoulos method,in which it is necessary for adjustment of pulling velocity and crucible size, and design of suitable temperaturefield.A ries of sapphire win-dows and domes can be fabricated by ultrasonic-vibration assisted machining for different optical application.
The biaxialflexure strength and infrared transmission of sap-phire crystal both decrea with increasing temperature,and espe-cially there is sharp drop in biaxialflexure strength at elevated temperature of600°C,which is related to twin formation in sapphire crystal.Submicron MgAl2O4spinel precipitation can effectively harden sapphire matrix and does not induce large opti-cal-scattering loss.It is suggested that it is a potential modification method for sapphire window and dome by growing Mgfilm on sapphire surface followed by annealing,quenching and aging.
Acknowledgements
This project was supported by International Science and Tech-nology Cooperation Program of Sino-Russian(No.2009DFR50160 and No.2009DFR50350),China Postdoctoral Science Foundation, Weapon&Equipment Pre-rearch Foundation of General Arma-ment Department of China,Natural Science Foundation of Guang-dong Province(No.9451805707003351),The Basic Rearch Plan Program of Shenzhen City in2009(No.34).The authors are also highly thankful for discussion with Dr.Zheming Liu and Prof.Zhi-chen Chen from National Central University(NCU)in the modifica-tion experiment.
References
[1]Bayya SS,Chin GD,et al.VIS–IR transmitting windows.Proc SPIE
2005;5786:262–71.
[2]Tropf WJ,Thomas ME,Frazer RK.Windows and domes:past,prent,and
食府
future.Proc SPIE2003;5078:80–9.
[3]Gentilman R,McGuire P,et al.Large-area sapphire windows.Proc SPIE
2003;5078:54–60.
[4]Tatartchenko VA.Sapphire crystal growth and applications.In:Capper P,
editor.Bulk crystal growth in electronic,optical and optoelectronic materials.New York:John Wiley&Sons,Ltd.;2005.p.299–338.
[5]Harris DC.Materials for Infrared windows and domes:properties and
performance.2nd ed.Washington:SPIE;1999.
[6]Xu CH,Zhang MF,Meng SH,Han JC,Wang GG,Zuo HB.Temperaturefield
design,process analysis and control of SAPMAC method for the growth of large size sapphire crystals.Cryst Res Technol2007;42:751–7.
[7]Tokunari M,Hayakawa H,et al.Development of an automatic birefringence
measuring device of mirror substrates for gravitational wave detectors.J Phys Conf Ser2006;32:432–8.
[8]Yang XH,Han JC,Zhang YM,Zuo HB,Zhang XJ.Rearch on ultrasonic vibration
grinding of the hard and brittle materials.Chin J Aeronaut2006;19(z1):9–13.
[9]Fessler H,Fricker DC.A theoretical analysis of the ring-on-ring loading disk
test.J Am Ceram Soc1984;67:582–8.
[10]Cheng M,Chen WN,Sridhar KR.Biaxialflexural strength distribution of thin
ceramic substrates with surface defects.Int J Solids Struct2003;40:2244–66.
[11]Perner B.Growth of sapphire by the Kyropoulus method.Cryst Res Technol
1979;14:661–4.
[12]Karr CJ.Infrared and Raman spectroscopy of lunar and terrestrial mineral.New
York:Academic Press;1975.
[13]Pinsker ZG.Dynamical scattering of X-rays in crystal.Berlin
Heidelberg:Springer-Verlag;1978.
[14]Lee WE,Lagerlof KPD.Structural and electron diffraction data for sapphire(a-
Al2O3).J Electron Microsc Tech1985;2:247–58.
[15]Hall JM.Army applications for multi-spectral windows.SPIE
1997;3060:330–4.
[16]Barish BC,Camp J,Kells WP,et al.Development of large size sapphire crystals
for lar interferometer gravitational wave obrvatory.IEEE Trans Nucl Sci 2002;49:1233–7.
[17]Gentilman RL,McGuire PT,et al.Low-cost sapphire windows.Proc SPIE
2003;5078:12–7.
[18]Klein CA,Gentilman RL.Thermal shock resistance of convectively heated
infrared windows and domes.Proc SPIE1997;3060:115–29.
[19]Black D.Strength-limiting surface damage in single-crystal sapphire imaged
by X-ray topography.J Am Ceram Soc2001;84:2351–5.
[20]Harris DC,Frederick S,Black DR.Factors that influence mechanical failure of
sapphire at high temperature.Proc SPIE1997;3060:226–35.
[21]Wang GG,Zhang MF,Han JC,et al.High-temperature infrared and dielectric
properties of large sapphire crystal for eker dome application.Cryst Res Technol2007;43:531–6.
[22]Chen WW,Harris NH.Method of making thermal shock resistant sapphire for
IR windows and domes.US Patent1997;5,702,654.
[23]Harris DC,Kirkpatrick A,Johnson LF.Effect of ion implantation on the strength
of sapphire at300–600°C.J Mater Sci2001;36:2195–201.
[24]Kumar P,Dregia SA,Sandhage KH.Epitaxial growth of magnesia and spinel on
sapphire during incongruent reduction in molten magnesium.J Mater Res 1999;14:3312–8.
[25]Li DX,Pirouz P,Heuer AH,Yadavalla S,Flynn CP.A high-resolution electron
microscopy study MgO/Al2O3interfaces and MgAl2O4spinel formation.Philos Mag A1992;65:403–25.
[26]Yanina SV,Carter CB.Terraces and ledges on(001)spinel surfaces.Surf Sci
2002;513:L402–12.
[27]Sieber H,Hes D,et al.TEM investigations of spinel-forming solid state
reactions:reaction mechanism,film orientation,and interface structure durning MgAl2O4formation on MgO(001)and Al2O3(1–102)single crystal substrates.Z anorg allg Chem1996;622:1658–66.
[28]Whitney WP,Stijbican VS.Interdiffusion studies in the system MgO–Al2O3.J
Phys Chem Solids1971;32:305–12.
[29]Sparks M,Duthler CJ.Theory of infrared absorption and material failure in
crystals containing inclusions.J Appl Phys1973;44:3038–45.
Table1
The modification results of sapphire crystal by spinel precipitation.
Sapphire sample Infrared transmission
(3l m)(%)Vicker hardness(H v) (GPa;1kg Load)
Pristine86.716.4±0.52
After precipitation treatment83.518.9±0.37
G.Wang et al./Materials and Design31(2010)706–711711
