r. jeanloz and d.l. heinz- experiments at high temperature and pressure: laser heating through the...

8/3/2019 R. Jeanloz and D.L. Heinz- Experiments at High Temperature and Pressure: Laser Heating Through the Diamond Cell

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J OURNAL DE PHYSIQUEColloque C8, suppliment au n O l l ,Tome 45, novembre 1984 page C8-83

EXPERIM ENTS A T H IG H TEMPERATURE AND PRESSURE :LASER H EA TIN G THROUGH THE DIAMOND C EL L

R . Jeanloz and D .L . HeinzDepartment of Geology and Geophysics, Unive rsi ty of Cali fornia,Berkeley, CaZifornia 94720, U.S.A.R6sum6 - I1 e s t p o s s i b l e d e r 6 a l i s e r d e s e x pg ri en c es q u a n t i t a t i v e s 5 hau-t e s p r es s i on s e t t e m p gr at u re s s t a t i q u e s , e n c h au f fa n t l ' i n t g r i e u r d 'u nec e l l u l e 5 enclumes de diarrant A l ' a i d e d ' u n l a s e r c o n t i n u . Des ~ e m p 6 r a -tu re s de 1500 2 5000K o n t 6 t 6 a t t e i n t e s d a n s t o u t e l a gamme d e p r e s s i o n s :10 a 100 GPa. Le c ha u ff a ge e s t o b t en u en f o c a l i s a n t l e f a i s c e a u d ' un l a -scr Nd: YAG e t l e s t e m p g r a tu r e s s o n t d k t e rm i n ge s p a r r a d i o m6 t r i e a v c cune p r gc i s i on d ' e nv i r on 200K. Le p r o f i l de t e m pgr a tu r e de l a z one c ha uf -f 6 c p a r l e l a s e r e s t d gt erm in g p a r un f i l t r a g e s p a t i a l , o bt en u p a rb a la y a ge d ' un e f e n t e . Dans c e s c o n d i t i o n s l e s t e m p 6 ra t u r e s d e f u s i o ns o n t d g t e rm i nc e s s o i t p a r l a m es ure d e l a t e m p6 r at u re o b s er v ge 2 l ' i n t e r -f a c e l i q u i d e - s ol i d e , s o i t 2 p a r t i r d e l a t e mp g ra t ur e c o r re s po n d an t 2l ' a p p a r i t i o n d e l a ph as e v i t r e u s e l o r s q u e l ' o n au gme nte pr o gr e ss i ve m en tl a p u i s s a n c e d e c h a u ff a g e .Abs t r ac t -Quanti ta t ive experiments are possible a t susta ined high pre ssu resand tem pe rat ure s by means of CW-laser heating th roug h t he diamond-anvil cell.Tem pera ture s of 1500 to 5000 K have been reached through out th e 10 to 100GPa pr es su re ran ge. H eating is achieved by a focused Nd: YAG laser beam, andtem pe rat ur es ar e determined radiometrically with a n accu racy of a bou t 200 K.The variation of t em pe rat ure acro ss th e laser-heated spo t is derived by meansof spat ial Altering with a slit tha t can be scann ed. In this way, th e melting tem -pera ture can be determined ei ther f rom the tem perature observed at th eliquid-solid interf ace or from the pea k temp eratu re a t which glass is first pro-du ced with increasing lase r power.

IN TRODUCTIONTwo of t h e most impo rtant a dva ntag es of t h e diamond-anvil cell for high-p ressu re res ear chderive from th e streng th of diamond an d its tran spar ency across a broad range of t h e elec-tromagnetic spect rum. Thus, no t only is it possible to achieve ultrahigh s tatic pressu res of100 GPa or more, bu t t he ability to observe th e sample in situ and probe it with electromag-netic radiation while a t pres sure is an especially imp orta nt featu re of t he diamond cell. Inth e present case . we use this tra nsparency to c arry o ut quanti ta tive experiments a t s imul-taneously high pressures and temp eratures. CW laser radiation which is absorbed by thesample. but n ot by th e diamond anvils. is used to achieve temp eratu res of several tho usan dKelvin. By th e same token, one can observe th e therm al radiation emitted fro m th e hot sam-ple located between th e diamonds; th e blackbody-like radiation makes it possible to deter-mine th e sample temperature a t high p ressures.High temper atur e experim ents a t elevated pres sure s ar e of inter est for studies of phas eequilibria, as well a s for the syn thesis of h igh-pressure phases th at can often be quenchedand examined a t roo m tem pera ture an d pressure . Melting is among th e most importanttransitions th at ca n be examined, but high-p ressure react ions t ha t would be kineticallyimpeded a t low temperatures are a lso of inte rest . As temp eratu res of 5000 K or more can beachieved at pres sure s of 10-100 GPa, th is experimental tec hnique is also of dire ct geophysi-cal intere st for studying materials a t th e conditions of t h e Earth's interior (Fig. 1). Indeed,beginning with th e pioneering work of Ming and Bass ett /I/ , mos t of th e res ear ch withlaser-heated diamond cells has been carri ed out by t he geophysical community. To date.however, this work has been more exploratory th an quantitative: temper atur e in th e diamondcell ha s been roughly es timated by means of optical pyrometry. and little h as been done tocontrol th e temp eratu re o r to derive quantitat ive phase equilibria from su ch experiments.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984817
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JOURNAL DE PHYSIQUE

P,T Range ofDiamond-Cell Experiments

L

PRESSURE (GPa)Fig. 1 - Pressu re- temp erature range th at is accessible in diamond-cell experiments a s com-pare d with the rang e of estimated tem per atur es (g eotherm s) in th e lower mantle and oute rcore of t he Earth. Cur rent es tim ates of t he melting cu rves of iron and mantle silicate s ar ealso shown. The highes t pressures so far achieved with th e diamond cell ar e a t room tem-pera ture , bu t exter nal heating an d cooling (for cryogen ic experiments ) allow tem pera ture sbelow about 1000 K to be reached. With laser heating, t em pe ra tu re s exceeding 2000 to 4000K have been reached a t pressures up to 100 GPa.

We descr ibe here a spectrorad iometric technique t h at is curr ent ly being developed in ord erto quantify the high-pressure la ser heating experim ents. Although many of th e results a re ofa preliminary nature, our major conclusion is th at it is possible to ca rry o ut phase equili-br ium studies throughout t he pressure - tempe rature regime i l lustrated in Figure 1. Thedetails of o ur cu rre nt techniques, an d some of t he problems t ha t remain unsolved, ar e sum-marized in the following sections.

E X P E R I M E N T A L T E C H N I Q U E SThe nat ur e of laser heating a t the sample in th e diam ond cell is qualitatively summarized inFigure 2. Typical dimensions ar e a sample thickn ess and di ame ter of 5 to 20 p m and 50 to250 p m , espectively, with the sample being contained by a metal gasket between the dia-mond anvils. The highest te mp era tu res a re achieved in th e focal spo t, which is of dimension1 0 t o 5 0 p m dependin g on th e focusing optics used ( se e below). Because of th eir high th er-mal conductivity and large size, relative to t he sample, th e diamonds ac t essentially asinfinite he at sinks. Thus. large te mp er atu re variations ap pe ar due to conduction both in thevertical (2 ) and radial ( T ) dimensions; and the gau ssian inte nsity distribution acro ss th e focalspot a lso contr ibutes to th e radial tem perature depend ence (Fig. 2).The tempe ratu re variations with position pre sen t a major tech nical difficulty in making q uan-titative measurements. In par t icular , the spatia l temperature Aeld must be determined inorder to carry out phase equilibrium experiments. There is an important chemical advan-tage, however, in th e fac t tha t t he ho t, reactive p ar t of th e sample is only in co nta ct withcold mate rial of th e same composition (cold sample materia l). Problems of contamina tionar e minimized, and t he diamond anvils ar e pro tect ed fro m damage caused by reactions withth e sample. This fact i l lustra tes the importance of properly focusing the laser a t th e middleof th e sample thickne ss in orde r to achieve high temp erat ure s.Our complete la se r heat ing s ystem is schematicall y illust rated in Figure 3. The 1064 nm radi-ation from a Nd:YAG l aser with 100 W (multimode) CW power is used to he at t he samplebetween the diamond anvils. A mode-selecting ap er tu re is used to isolate th e focusableTEMOOmode. which c arr ies a maximum power of 2 5 to 30 W a t th e sample.


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' \OWERDIAMONDFig. 2 - Cross section illustrating a sample being lase r hea te d within th e diamond cell. TheNd:YAG las er beam is focused within the sample, which is con tai ne d by a m etal gask etbetween t h e diamond anvils. Typical dimensions ar e given. an d t h e rad ial ( T ) and ver tical (2)dep end enc e of tem pera tur e in the sample is schematically illustrated.

t TEMPERATURE

Dlchro~cMirror

Wavelength

Fig. 3 - Summ ary of the laser heating system used in th e pres en t experiments. The beamfrom a CW Nd:YAG las er (TEMOO mode) is Focused i nt o t h e hi gh -p re ss ur e dia mo nd cell byme an s of a microscope system containing a dichroic mirror. The dichroic mirror reflects thelaser beam b ut transmits th e thermal radiat ion emitted from t he sample a t visiblewaveleng ths (wavy lines). The sp ec tru m of t he th erm al radiation is det ermi ned by a mono-chrom ator located above th e dichroic mirror ; f rom this s pec tru m the emissivity and tem-pe rat ur e of th e sample ar e determined. Also, the fluc tuations in lase r power and sampletem pe ra tu re ar e monitored as a function of time.

The focusing is achieved with a beam exp an de r (optional) an d a long working-distance objec-tive: typically we use a Leitz UM 20 objective (0.33 numeri cal ap ertu re), which yields a beamwaist of about 10 prn (or 50 p n ) a t th e sample when using (or no t using) a 6X beamexpander .The laser power ca n be varied by changing th e power at t he pum p lamps. For a given experi-ment, however , we use a polarizer- a t te nua tor system located r ight af t er th e beam expanderin or der to change t he laser power tha t is received a t the sample. By changing the poweroutsid e th e las er cavity in this way, ther e is no change in m ode -str uct ure o r focal level of t helas er beam. The efficiency of th is atte nu at or is suc h th at between 5 W (minimum) and 23 W(maximum) rea ch th e sample a t peak laser power.


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C8-86 JOURNAL DE PHYSIQUE

The microscope system which focuses the laser beam is also used to observe t he sample inth e diamo nd cell. Visual observation (dir ect or with a closed circui t television) is import antfor reproducibly focusing the laser in the sample and for monitoring changes in the samplea t high pressure s an d temperatures. The thermal radiat ion from th e sample passes throughth e "hot mirror" (high-pass dichroic Alter) th at reflects the laser bea m thro ugh the objec-tive. This ther mal radiation is focused onto the e ntra nce slit of a holographic-grating mono-chro mat or (12 nm/rnm dispersion and 833 p m slit widths) with a silicon de te cto r. Ext radichroic filters are placed in fron t of th e entr ance slit in orde r to assu re t h at none of th eprim ary (1064 nm) lase r radiation en ter s the monochrom ator. Also, a low-pass (IR t ransmit-ting) filter located a t the polarizer-a ttenuato r prevents any of th e pump-lamp radiatio n fromcontam inating t he sample spec tra. As a result, even with maximum laser power almost noradia t ion is observed through the spec t romete r when a ref lect ive ( n ~ n a b s o r b i n ~ )urface isplaced in th e sample area .The s pe ct ral response of thi s system, which is summarized in Figure 4, illustrates th e exclu-sion of radiatio n a t 1064 nm. The str uc tu re in th e response curve is caused mainly by th emultilayer dichroic filters th at ar e used as well as by th e det ecto r respons e. As is evident,however, thi s sy ste m which is maximized for visible wavelengths is ideal for mea suri ng t h ethe rma l radiation of samples a t about 1500 to 5000 K.At lower tem pera ture s, t h e light inten-sity is ofte n t oo low to ob tain a reliable sp ec tru m (especially for samples of low emissivity).At higher tem perature s, t he l imita tion der ives more f rom our d at a reduction technique, asdescribed below, than from fundamental optical constrai nts.The de te ct or ou tp ut is amplified. and sm oothed as a fun ctio n of time (100 to 220 pF capaci-to rs give smoothing time con st an ts of ord er 1 to 2 sec.), before being read by a Hewlett-Packard 9826 minicomputer which is used to process the sp ect ra l data . In ord er to scanover a ful l spect rum 4 to 8 minutes ar e usually required, but it is often possible to determi neth e tem pe rat ur e more rapidly. Speciflcally, only a fraction of th e spe ct ru m need be col-lect ed if th e s amp le emissivity varies little with wavelength (ie., th e greybody model applies),as is often t he cas e. Thus, tem pera tur e ca n be reliably mea sur ed with th is system in lesstha n one minute. Nevertheless, sufficient time is available for characterizing th e sp ec tru m ofthe thermal radiat ion, as continuous heating experiments last ing well over one hour havebeen successfully accomplished in t he diamond cell.

WAVELENGTH (nm)Fig. 4 - Spectra l respon se of th e present radiometer system is compared with the laserwavelength (arrow at 1064 nm) and the thermal emission from blackbodies (Planck func-t ions) a t 2000 K, 3000 K and 4000 K. The system response (intensity in arb it ra ry units)includes th e effects of Alters. dichroic mirrors, lenses and the det ector; c han ges in ord er-sorting liters ar e shown by shor t vertical lines. l 'here is additionally a scale fac tor rangingover 10 for t he s yst em response. The Wien approximation to the Planck function is shownfor illustrative purpos es (s ee text).


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DATA REDUCTION A ND SYSTEM CALIBRATIONThe thermal emission from th e Laser-heated sample is analyzed in term s of the greybodymodel which r ela tes sp ect ral int ensity . I(A), to emissivity. e, and tempera ture . 7?

with A , h, k and c being wavelength , Planck 's const ant , Bol tzmann 's con stant an d t he velocityof l ight, respectively. In or de r to test t he greybody assumption th at & is independent ofwavelength it is convenient to use Wien's l inearized approximation to the Planck function (1).Thus. deflning a normal ized int ens ity

and a normalized frequency

which ar e both observable, Wien's relation ca n be expr essed a s a l inear equation in 'I'-I:J = lne - W T - ' (3)The problem with using (3 ) as an approximation t o (1) is th at it is only valid a t relatively lowtem per atu res . As is eviden t fro m Figure 4, however, th e Wien equation a ccura tely rep ro-

duce s th e Planck relat ion for the p res ent experim ental condi tions up to tempera tures ofabou t 5000 K. The adva ntage , on th e oth er han d, of reducing th e dat a in term s of (3) is tha tit is easy to tes t for th e validity of th e greybody model (i.e.. c heck th at r is indep enden t of Aby checking the linearity of J versus ? ), and uncer ta in t ies in the spec t ra l measurements c anbe direct ly propagated to th e corresponding uncertain t ies in T and c . Starting with thisanalysis , h igh-temperature da ta ca n be i terat ively reduced to c orre ct for the d ifferencebetween t h e Wien and Pla nck fun cti ons . if nec ess ary .The spectroradiometer syste m is cal ibrated with re spec t to a s tand ard tungsten-fllamentlamp of sp ec tr al em itt an ce which is known absolutely. To da te , however, we have no t cali-brat ed t he emissivi ties absolutely in t he laser-heat ing expe riments at pressure. The reasonis tha t the su r face a re a o f emi t tance in th e d iamond ce ll mus t be accura te ly measured inevery experiment; this is possible (using optical techniqu es / 2 / ) but dimcult. Still, emissivi-t ies a re typically determined to a f actor of 2 to 5, and what is most important is tha t th erelative wavelength dependence of E is accur ately measured. Also, th e main conc ern ha sbeen in measuring te mp er at ur e and, a s is evident from (3). this is independ ent of how wellt.he absolute emissivity is known.As a check on the acc urac y of th e spectra l temp eratu re determinat ions during laser heat ing,th e zer o-pre ssure melting of sever al metals has bee n exam ined between 1500 an d 5000 K.High-purity metal wires were heated in an argon atmosphere to prevent oxidation. The tem-pe rat ure was measu red as in exper imen ts with the diamond cell . The main difference withth e high-pressure exper imen ts is t h a t a muc h larg er region is uniformly melted ( - 10 0 pmchar acter istic dimension) beca use of t he abs ence of th e diamond hea t sinks.The melting temp eratu re was brackete d in a cr ude but unambiguous way by th e d irec tobserv ation (o r lac k ther eo f) of flowage an d form ation of a bead of me lt. As is evident fromFigure 5, the pr esen t re sul ts a r e consis tent with the known melting tem pera ture s in all casesbut that of tungsten, which exhibi ts a 5 pe rc en t discrepancy. Although Larger than t h eest imated err or on the t em per atu re determinat ion (derived from the spectral f i t to the grey-body funct ion), an acc ura cy of 5 perce nt is plausible for th e pres ent experiments. It shouldbe noted, however, th at su rf ac e tension might obscure t he oc cur ren ce of l iquid flow andres ult in an overestimate of t he m elting tem pe rat ure , as observed. Another difficulty worthmentioning is th at beca use of th e nonlinearity of t he laser-sample coupling it is not alwayspossible to achieve temperatures near the melting point in both the solid and liquid phases(cf. iron and zirconium).FLUCTUATfONS IN LASER POWEROne important cause of uncertain t ies in th e pre sen t experiments is tha t the laser outputfluctuates by a few per ce nt a s a function of t ime. These fluctuations ar e monitored bymea ns or a sil icon det ec tor on the back (high reflecta nce) m irror of the laser: ther e issufficient light leakage to con tinuousl y mo nito r th e power (Fig. 3). As a resu lt of t he las er-


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JOURNAL DE PHYSIQUE

Fig. 5 - Melting of four metals under th e laser beam a t zero pressu re. The difference betweenth e radiometrically mea sured tem per atu re and the known melting tem per atu re (T,. given a tth e botto m) is plotted fo r Fe, Zr, Mo and W. Open symbols refer to the direct observation offlow whereas closed symbols indicate th at t he sample ap pe are d solid. In all cases t he m etalwas in a s tr e a m of Ar.

in tensi ty f luctuations , sample tempe ratur es vary by a bout 200 K. This val ue is of s imil armagn itude a s the disc repan cies observed in the zero- pressu re melting experiments. Wepart ly surm ount th e problems caused by temp eratu re f luctua t ions by means of the temporalsmoothing th at is appl ied to the s pect rome ter outpu t . It is clear, however, that furtherimprovements ar e possible. Among oth er solutions, th e lase r ou tp ut could be smoothed witha fe edback system or the s pectr um could be col lected much more rapidly by mca ns of ade tec to r a r ray .From a different perspective, the laser-intensity fluctua tions can actually b e advantageous.For a sample th at is passively absorbing the l ase r radiation , one would expect. th e temp era-tu re f luctuat ions to correlate with the changes in la ser in tensi ty (due t o he at flow a loss ofhigh-frequency comp onen ts and a slight phase shift would be ex pecte d, as is schematicallyshown in Fig. 3). In con tra st, the occu rrenc e of react ions (in particula r. melting) in the sam-ple would be exp ected to resu lt in a loss of co rrela tion between th e tem pe ra tu re and thelaser in tensi ty . That is , k inetics and the la tent h ea t (which ac ts as an ext ra he at s ink)preve nt th e sample tem per atu re from closely following th e cha nge s in energ y deposition byt h e l a s e r b eam.These expecta tions a r e born out by laser-heating exper imen ts in the diamond cell (Fig. 6). Are f rac to ry compound such as AIZOg is so lid at e levated tem pera tures , a nd i t exhibits a closecorrelat ion between laser in tensky and sample tem pe rat ure (monitored here by the in ten-sity of the rm al radiation a t 600 n m wavelength). The dom ina nt oscillations a r e of a few Hz infrequ ency , and th e effects of t herm al conduction a re clearly evident (phase shift and loss ofhigh fre quenc y compon ents in th e temp era tur e fluctuatiorls).The r esul t for a sample t ha t is part ial ly molten unde r th e laser be am is markedly different(Fig. 6). In this case. the correlation between laser intensity and sample temperature ispoor. Qualitatively, this e ffect is analogous to what is obser ved in Differential ScanningCalorimetry experiments. Thus, although more work is required to develop this technique,th e corr elation of la ser an d tem pe rat ure fluctu ations could provide a powerful tool for th e insitu deter mina tion of high -tem perat ure reactions, an d specifically melting, in th e diamondcell.


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Fig. 6 - Oscil loscope traces i l lustrat ing the fluctuations in sample temperature associatedwith fluctuations in laser ou tpu t (see Fig. 3). In e ac h of t he four ru ns t he upp er an d Lowertrace s cor respond respectively to the l aser power and sample tempe ratur e as functions oftime. Durations for each run a re 1 and 2 seconds. as indicated; las er intensity and sampletemp eratu re ar e in arbitrar y units. The tra ce s a re well correlate d in th e two runs with A1 O3(upper half) and poorly correlated in the two runs with (Mg Fee, ) Si04 (oli,ne s ta rgn gmaterial; lower half). These experime nts were car rie d out a?'%5 to$& GPa pres su re in thediamond cell, and a few per ce nt P t were mixed in with th e AL2O3 in ord er to absorb the las erbeam.

Fig. 7 - Axial v i ew in tran sm itte d white light of four glass blobs crea .ed by las er he ating in th ediamond cell a t 30 GPa pres sure . The starti ng mater ial was Mgo Feo lSiOQ which was con -verte d to th e high-pressur e perovskite phas e by laser heating &? ppr$ssures above 25 CPa.The large st glass blob (top) is 18 p m in diameter.


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MELTING EXPERIM ENT S AND THE SPATIAL VARIATION OF TEMPERATUREAn alt ernativ e an d mo re definitive proof th at samples ar e melted in th e diamond cell is th eformation of glass on quenc hing (turn ing off t he laser) . In this ca se, i t is advanta geous towork with silicates or oxides t h a t a r e good glass formers. An example is i l lustrat ed in Figure7 , in which t h e star tin g mat eria l is the hi gh-pres sure perovskite pha se of pyroxene; t his wassynthesized in situ a t a bo ut 30 GPa from sta rtin g m aterial of composition Mg.o,88 Fe 12Si03.Aside from its glass-form ing ability this perovskite is of in te re st beca use it 1s co ns it er ed t obe the do mina nt miner al of the earth's lower mantle/3/. Indeed silicate perovskite, which isstable only above abo ut 20 GPa, is probably th e single most abun dan t mineral within o urplanet .Four glass blobs a re evident in th e microphotograph, th e la rgest be ing formed a t the highestlaser power. With decrea sing power t he quenc hed blobs become smaller , an d a t low power noglass is observed. Thus, a t low power the pea k te mp era ture in th e diamond cell is below th emelting point of t h e samp le (Fig. 8). With increasing la ser power, bot h th e avera ge an d pe aktempera tures a re obse rved to inc rease , and once the peak tem pera ture in te r sec ts th e me l t-ing point a small am ou nt of glas s is formed on quench ing. With higher la se r power, a la rge rregion is melted a nd th ere fore a lar ger am ount of glass is observed.Based on Figure 8 , two approa ches ar e suggested for determining th e melting tem pe ra t ure ofsamples in th e diamond cell . On th e one hand , if the pea k tem per atu re is determ ined fo r th er u n in which glass is A r s t observed with increasing lase r power, this brac ket s t h e meltingtemp era tu re f rom below. On th e oth er hand, the melting point can a lso be dete rmine d a thigher laser power by me asur ing th e tempe ra ture a t th e locat ion of the glass-crysta l in ter -face. This is more diff icult, but more satisfactory, than the preceding technique because itinvolves a direc t measur eme nt of t he coexis tence temp era tur e be tween melt a nd sol id; th a tis , th e exper iment is inherent ly reversed.In e i the r case , i t is evident tha t th e tem pera tu re var ia t ion ac ross th e sample (Figs. 2 a n d 8)mu st be direct ly meas ure d for quantitative phase-equilibrium studies to be possible. As af lrs t cut to this problem wc tre at t he la ser heated spot as a disc source of light, with tem-pe ra t ure varying as a funct ion 07 radia l dis tance f rom the ce nte r of th e spot . Hence thetem per atu re f leld can be directl y mea sured by mea ns of a movable slit which is place ddirec t ly on, an d or ient ed perpendicular to , the en tra nce s l i t of the monochromator (Fig. 9).

t t i t glass

Distance-INCREASING LASER POWER-Fig. 8 - Schematic i l lustra t ion of the temp era tu re a s a funct ion of radial dis tance a cross th elaser-h eated spot. At low las er power t he peak tem per atu re is below the melting point of t he

z m p l e . Tm (left panel) . With increa sing laser power the average te mp era tur e, TaV, inc rea sesand th e peak tem pe ra t ure inte rsec t s th e melt ing temp era tu re (middle panel). At this pointglass is formed on que nchin g if th e sam ple material is a glass former. At hig her las er power,a large r region is melted an d hen ce a larger am ount of glass is found afte r the l ase r isturn ed ofl (r ight panel) .


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SAMPLING25pm SLIT (moveable)MONOCHROMATORSLIT 8 3 3 ~

(stotionory)

Fig. 9 - A vertical sampling slit can be t ran sla ted ac ros s the image of t he sample which isfocused at the statio nary, horizontal en tr an ce sli t of t he monochromator. By scanning theslit horizontally acr oss th e image of t he las er- hea ted sp ot it is possible to determine th e spa-tial variation of t em pe ra tu re in th e sample . The sampl ing slit is 10 to 25 p m wide, th e mono-chrom ator s li t is 833 p m wide and the laser -heate d region is typically 250 p m in diameter.

This sampling slit blocks o ut all but a thin st ri p of th e sample image which is focused a t theen tr an ce slit. We use sampling slits of 10 and 25 p m dimension which ca n be moved with aprecision of 1 pm. s th e magnification of th e samp le image is approximately 5x a t the slit. aspa tial averaging (proje cted slit width) and positioning of 2 to 5 p m and 200 nm respectively,is achieved acr oss the tem per atu re field in th e sample.Because of th e vertical (on-axis) variation of tem per atu re (Fig. 2), th e disc approximationused her e must always res ult in an und ere sti ma te of th e peak tempe rature . Nevertheless,th is gives a relatively good est ima te of t h e spati al variation of te mp er at ur e. The slit-sampling acr oss a disc is an exarnple of a to mo gra phi c problem, with t he invc rsior~ iven byan Abel transf orm /4/. Therefore, th e spe ctr al intensity as a function of radial dis tar ~c e, , iscalcu lated from th e spe ctr al intensity tha t is mea sured a s a function of sli t position, z:

Combining ( 4) with (1 ) or (3 ) provides an esti ma te of th e position-dependerit t em pe rat urean d emissivity: T(T) an d E (7).The result of one such sc an for a lase r heatin g exper iment on magnesium silicate perovskitein th e diamond cell is shown in Figure 10. This figure i llu st ra te s sorrie of th e difficulties th a tmu st stil l be resolved, as well providing c onc ret e re sul ts from the pre sen t techniq ue. The7(r)curve was constrain ed by the da ta between 5 and 25 p m radial dist .ance and by the aver-age temperature (measured over the whole field of view) of 4100 (i 00) K . Near the cen te rof t he hot s pot the resu lts were extreme ly unstable becau se of th e fluctuations in lightintensity a s a function of t ime . That is , th e size of t h e molten region flu ctuate s in responseto th e fluctua tions in las er intensity. Beca use of its small dimension (6 p m diameter, basedon t he size of th e q uench ed glass). fluctu ations of only I to 2 p m in the size of th e moltenregion drastically alIect t,he amount. of the rm al radiatio n th at is emitted from the c ent rala r e a .Beyond th e 50 p m beam-waist diamet er t he te mp er at ur e drops of! and. for th e opticalconfiguration used in this run , the decrea sing light intensity produ ces large uncertain ties inthe tempera tu re de te rmina t ions a t T > 20 p m . We believe th at t he minimum in f l ~ )ear 10prn radial distance may be du e to th e lase r being slightly defocused. We have found tha t i t iscritical to reprod ucibly focus t he las er in th e middle of t h e sample thickn ess (F'ig. 2) in orde rto get reproducib le profiles. Beca use of variat ion in th e thick nes ses of diamond anvils and inth e index of ref ractio n of diamond as a fun cti on of pre ssu re. th e laser focusing must becarefully checked in eac h expe rime nt. One way tha t this c an be accomplished is by fir ~di r~g


10/10

J OURNAL DE PHYSIQUE

Radial Distance (,urn)Fig. 10 - Profile of tem pe ra tu re a s a function of radial dista nce from the c en te r of th e laser-hea ted s po t for a sample of silicate perovskite a t 35 GPa in th e diamond cell. The averageternperaturc acros s th e hot regions is 4100 K and the beam-waist diam eter is 50 p m in thisexperiment. The t em per atu re distribution. T(T), satisfies the average temp eratu re as well astemperature points derived by inversion (see text). Error bars show the est imated uncer-taint ies and open symbols have un cer tai nti es well over 100 percen t. Upon turning off thelaser, a glass region 5 p m in diameter was quenched, and the corresponding temp erat ure a tthe solid- liquid interface is found to be near 3000 K.

the focal position th at produces the narrowest temperature distr ibution, corresponding t othe sm allest beam waist in th c sample.According to Figure 10, th e liquid-crystal i nterfa ce is a t abou t 3000 K for the si l icateperovskite a t 35 GPa. This value is in acco rd with th e peak te mpe rat ure a t which glass firstapp ear s with increasin g las er power. Interestingly, the melting te mp er at ur e of t.hisperovski te appears to be cons tant a t about 3000 K between 30 and 60 GPa press urc, an d t hismay have important geophysical ramifications /5/.The main conclusion fro m thi s work, however, is t h at co ntin uous lase r heating car1 be used t oachieve temp era tu res of several thous and Kelvin a t pres sures of 10 to 100 GPa in th e di a-mond cell. Average t empe ratur e, and bot h th e spatial and temporal variation of t emp era tu rein the sample can be monito red by spectroradiometry. Thus, phase-equilibrium exper imentsare possible a t high p ressu res a nd tem pe rat ure over time scales of m inutes to hours.

ACKNOWLEDGEMENTSWork su pp or te d by th e U.S. National S cien ce Foundat ion , NASA and th e A.P. Sloan Founda -tion.

REFERENCES/ I / . MING. L.C.. and BASSETT, W.A.. Rev. Sci:Instrum., 45, (1974) 1115-1118./ 2 / . SCOTT, C. an d JEANLOZ, R., Rev. Sci. Ins tru m, 55, (1984) 558-562./3/. JEANLOZ, R., and THOMPSON, A.B., Rev. Geophys. Space Phys., 21, (1983) 51-74./4/. DEANS. S.R.. The Rado n T ra ns fo rm a n d Som e of It s Applications,J. Wiley and So ns , New York (1983)./5/. HEINZ. D.L.. an d JEANLOZ. R.. US-Japan Confe rence on Parti al Melting.Proceedings. Eugene. Oregon. USA. (September 1984).

r. jeanloz and d.l. heinz- experiments at high temperature and pressure: laser heating through the...

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