American Mineralogist, Volume 79, pages 145-147, 1994
A high-temperature assembly for 1.91-cm (t/n-in.) piston-cylinder apparatus
Prnnnn flunoN, DoN R. Blrnn, Plur. B. Torr
Department of Earth and Planetary Sciences,McGill University, 3450 University Street,Montr6al, Qu6bec}{3A2A7, Canada
AssrRAcr
A l.9l-cm (%-in.) piston-cylinderassemblysuitablefor high-temperatureexperimentation has been designed.It can easily hold four capsules3 mm in diameter encircling a
thermocouple at the center of the graphite furnace. The overall design is simple in order
to provide the samples and thermocouple a uniform, anhydrous environment poor in
contaminants. The major components are made of NaCl, fused silica glass,and crushable
alumina. No pressurecorrection is necessaryat temperaturesabove 1400 "C, and experiments can be performed to 2100 "C. The vertical thermal gradient over the central 4.5
mm of the furnacein this assemblyis <4'Clmm below 1700'C and reachesa maximum
of l0 'Clmm at 2 100lC with either hot piston-out or hot piston-in pressurizationtechniques.
Trrnl.ssnNnr-v
The piston-cylinder apparatus (Boyd and England,
1960)is widely usedto perform experimentsat pressures
and temperatures occurring in the lower crust and uppermost mantle. Phase equilibria studies at high pressures and at temperatures exceeding 1600 "C require a
piston-cylinder assemblythat is low in contaminants,anhydrous,frictionless,and capableof maintainingalarge
volume for multiple capsulesand that generatesa smooth
thermal gradient.A l.9l-cm (3/+-in.)
assemblycontaining
no hydrous parts such as talc or pyrophyllite has been
designedin order to fulfill thesecriteria.
The furnace assembly is illustrated in Figure l. The
pressuremedium is made of three stackedNaCl cylinders
whose total length is about I mm longer than the other
parts of the assemblybecauseof the high compressibility
of salt. NaCl has the advantagesof being inexpensive,
being free of impurities, requiring no machining, and
needinglesselectricalpower than talc assembliesto reach
the sametemperature(Boettcheret al., l98l). The NaCl
is separatedfrom the graphite furnace by a fused silica
sleeve;a Pyrex sleeveis used in experimentsbelow 1400
"C. The inner part of the assembly consists of Norton
AN900 crushablealumina (99.7 wto/oAlrO3) with an axial
hole for the thermocouple. Sample capsulesare inserted
in the assemblyby cutting the Norton AN900 crushable
alumina in two pieces and drilling cavities around the
axial hole at the top ofthe bottom piece.The part ofthe
axial hole adjacent to the sample capsulesis slightly enlarged compared with that in the top piece of crushable
alumina to avoid thermocouple breakagedue to slight
misalignmentbetweenthe two crushablealumina pieces.
To locate the thermocouple at the furnace center, the axial hole in the bottom crushablealumina piece is packed
tightly with reagent-gradealumina powder or filled with
a rod machinedfrom AN900 crushablealumina topped
with about 1 mm of reagent-gradealumina powder. Ther0003404x/94l0l 02-ol 45$02.00
mocouple wires are insulated with McDanel 998 alumina
(99.8 wto/oAlroj). Both alumina pieces,Norton AN900
and McDanel 998, provide an anhydrousenvironment
low in contaminants and have virtually the same chemical composition and thermal conductivity and similar
physical properties.This assemblydesignfavors isotropic
pressuredistribution and a smooth thermal gradient.Also,
thermocouple drift is minimized by use of high-purity
alumina (Mao and Bell, 197l;Mao et al., l97l; Presnall
et al., 1973). A rod of fused silica glass 5 mm long is
necessaryat the base of the furnace to prevent thermocouple breakage because of mechanical distortion; at
temperaturesbelow 1400.C, a Pyrex rod l0 mm in length
is used.The steelbaseplug at the top of the assemblyis
insulated from the pressurevessel with a Pyrex sleeve.
Pyrex is preferred to conventional pyrophyllite sleeves
becauseit is anhydrous, can be cut to size easily, and is
cheaper.To lubricate the assembly,a Pb foil 0.05 mm
thick (not shown in Fig. l) is wrapped around the salt
cell. No MoS, lubricant is used.This assemblydesignis
free of most sourcesof Hr, HrO, and S gases.Four capsules3 mm in diameter that are 4 mm long may be easily
accommodated. Alternatively, a single capsule of larger
dimensionscan be placedin the centerof the assembly,
and the thermocouple located slightly above the hot spot.
Pnnssunn cALTBRATToNAND MEASUREMENToF THE
THERMAL
GRADIENT
Temperatureswere measured with WerRe3-WrrRe(type D) thermocouples made from wires calibrated by
EngelhardIndustries. This calibration is traceableto National BureauofStandardstest no. 243484A and has an
accuracynot exceedin1+0.7o/o.No pressurecorrection
was applied to the emf of the thermocouples.Pressure
calibration of this assemblyat I GPa was performed using the melting temperatureof diopside (1513 + 3 'C)
reportedby Boettcheret al. (l 982),which agreesto within
145
146
HUDON ET AL.: HIGH-TEMPERATURE ASSEMBLY
1 , 9 1 - c nA S S E M B L Y
n
LZ-Z
srEELBAsEPLUG
:
pyoFfi
Distonce from
cenier of furnoce (mm)
6
2095
f'fl
Nocr
2085
fl
tl
FusEDsrlrcA GLAss
2075
cRAPHTTE
19 0 0
t
O 18 9 0
l':I
L.l
ALUMINA
CRUSHAELE
trI..=]
hd
THERMOCOUPLECERAHIC
q)
H.DANEL
es@ALUHINA
N
"o"ro"
o""oF
cAPSULE
Fig. I . Crosssectionof the I .9I -cmhigh-temperature
assembly for the piston-cylinder
apparatus.
l0 "C with earlier data (Boyd and England, 1963; Williams and Kennedy, 1969).The pressurizationand heating procedure for both hot piston-out and hot piston-in
techniques was the same up to 1300 .C. A pressureof
0. 15 GPa was first applied to ensure electrical contact
between the piston, the graphite furnace, and the base
plug. The assemblywas then heated to 1300 .C at a rate
of 300'C/min; pressurewas increasedsimultaneouslysuch
that, at 500, 1000, and 1300 "C, the pressurewas approximately0.4,0.7, and 0.85 GPa. The temperaturewas
then raised at 100 "C/min up to 1400 "C, and then at 50
'Clmin to the experimental
temperature. For the hot piston-out technique,pressurewas increasedto 1.2 GPa at
1400'C; after reachingthe experimental temperature,the
pressurewas decreasedslowly to 1.0 GPa. For the hot
piston-in technique, pressurewas increasedslowly while
the assemblywas heatedfrom 1300.C to the temperature
of the experiment,in order to reach 1.0 GPa at the temperature desired. Between 1300 and 1400 "C, careful
pumping must be performed for both techniquesbecause
the fused silica glass sleeve softens in this temperature
range, resulting in a concomitant drop in pressure.Pressurewas controlled within +0.008 GPa, and temperature
within + I "C. Results of the calibration indicate that no
friction correction,within a precisionof +0.04 Gpa, is
necessaryat temperaturesof 1400"C and above for either
hot piston-out or hot piston-in pressurizationprocedures.
The vertical thermal gradients in this assembly were
measuredat I GPa and temperaturesbetween 1000 and
2100 "C. During the measurementof the thermal gradient, the temperature was increasedat 300 "C/min to 950
"C and then raised at 50 'Clmin for the duration of the
experiment. Pressurizationand heating of the assembly
followed similar P-Z paths,as describedabove,for pressurecalibration. Thermal gradientswere measuredat 100
oCintervals betweenwelded thermocouples
located at the
center of the furnace and 2.25 mm either above or below
f,
o
18 8 0
1700
16 9 0
Q)
o_
(- 16 8 0
c
./
o7
,/\
z
4-dA
| /uu
\
\
(l)
F 15 0 0
14 9 0
14 8 0
10 0 0
990
980
1 8 2 5 1 9 . 2 5 2 0 . 2 5 2 1. 2 5 2 2 . 2 5 2 3 . 2 5 2 4 . 2 5
D i s t o n c ef r o m b o i t o m o f f u r n o c e ( m m )
Fig. 2. Thermal gradients measured at 2100, 1900, 1700,
I 500, and I 000 'C at 1.0 GPa over the central part ofthe furnace
assembly.Squaresrepresentdata collectedwith four Pt capsules,
and circles represent data collected with four Mo capsules.At
1000, 1500, and 1700 "C a circle is superimposedon a square
at the distanceof 21.25 mm from the bottom of the furnace. A
second-degreepolynomial has been used to fit the data points.
The short-dashedline shows the thermal profiles reported by
Cohen et al. (1967) whereasthe long-dashedlines depict profiles
measuredby Boyd and England(1963),both in I .27-cmassemblies. The thermal profiles of Boyd and England (1963) have
been slightly shifted to bring the locations of maximum temperature into alignment for all three profiles.
the center. After the experiment, the location and the
distance between the thermocouples were checked to make
sure they did not move as a result of pressurization. A
time interval of 90-240 s was required at each temperature for stabilization of the temperature readings from
both thermocouples. All measurements were performed
with loaded capsules to simulate actual experimental conditions in which Pt capsules are used at temperatures below 1700 'C and Mo capsules are used at higher temperatures. At 2100 'C the temperatures measured by both
thermocouples fluctuated by +5 "C; however, the tem-
HUDON ET AL.: HIGH-TEMPERATUREASSEMBLY
perature gradient between the thermocouples remained
constant to within + I 'C.
Figure 2 showsthe vertical thermal gradientsmeasured
over the central 4.5 mm of the graphite furnace in this
assembly. At temperatures of 1700 oC and below, the
gradient is <4 "C/mm, and at 2100 "C, the maximum
gradientis l0 "C/mm. This assemblyhas a zone 5.4 mm
in length in which the temperatureis no more than l0'C
below that of the hot spot, up to 1400 'C; at higher temperatures the length of this zone diminishes to 3.5 mm
at 2100 "C. Vertical thermal gradientsin our l.9l-cm
assemblyare similar to thosemeasuredin other l.9l-cm
assemblieswith straight-wall graphite furnaces(kistner,
I 979; Kushiro, 1976). Vertical thermal gradientsin 1.27cm (Yz-in.)assembliesare equal to or greater than those
we have measured(Fig. 2). The gradientin the 1.27-cm
assemblyof Cohen et al. (1967)is similar to the oneswe
measured,whereasgradients reported by Boyd and England (1963) at 1500 and 1700 "C are steeperthan ours
(Fig. 2). Dunn's measurements(Dunn, 1993)at 1200 "C
in his 1.27-cm assembly(similar in designto the standard
Depths of the Earth | .27 -cm assembly)demonstratethat
only the central 3.75 mm of his assemblyare within l0
"C of the hot-spot temperature.
Radial temperature gradients appear similarly small,
based on textural observations ofcross sectionsthrough
sample capsules.This is supported by radial temperature
gradient determinations in other types of assembliescarasried out by other investigators.In a l.9l-cm (3/a-in.)
sembly made of NaCl, pyrophyllite, and graphite at 2001000 "C and l-3 GPa, there was a temperaturedifference
of l0lobetween the center and inside wall of the graphite
furnace, the maximum temperature being closest to the
furnacewall (Leistner,1979).ln a 2.54-cm(l-in.) assembly made of NaCI and graphite the radial temperature
differencebetweenthe center and a spot I mm away from
the inside furnacewall was no largerthan 1.5 'C at 1200
.C and 1.5 GPa (Bohlenand Boettcher,1982).Additionally, alkali halide melting experiments carried out in a
2.54-cm (l-in.) assemblymade of NaCl and graphiteat
0.5 GPa showedthat the thermal gradient acrossthe sample was <5 "C (Bohlen, 1984).
Drscussrox
Assembliesof our design routinely operate to temperatures of 1900 .C for experiments of 60 min long and
have beenusedin 20-min experimentsat 2000'C and in
5-min experimentsat 2100 "C at I GPa. High-temperature experimentsquench to I 00 "C ar a rate of about I 500
'Clmin. At temperaturesabove 2100'C at 1.0 GPa, the
assemblyundergoesgradual deterioration becauseofthe
melting of the alumina near 2200'C, leading to failure of
the experiment. Hot piston-in conditions are preferred at
high temperaturesbecausewe observed a systematicin-
147
creaseof the thermal gradient during the decompression
step of the hot piston-out technique. This increase is
probably the result of a changein the wall thickness of
the graphite furnace in the vicinity of the four sample
capsulesthat appearsto occur when the assemblyis decompressed,or to the stresseffects on the two thermocouples(Mao and Bell, l97l; Mao et al., l97l).
AcxNowr,rocMENTS
This paper representspart of the first author's Ph.D. thesis at Mccill
University. Thanks go to J.N. Ludden for the loan ofthe welding apparatus, to H. Bossenyi for help in the laboratory, and to C. Dalpe for helpful
discussions. This work was funded by a Leroy fellowship and a Geological
SocietyofAmerica researchgrant (no. 4957-92) to P.H., and by a Natural
Sciences and Engineering Research Council of Canada operating grant
(no. OGP0089662)to D.R.B.
Rrpnnrnrcns CITED
Boettcher,A.L., Windom, K.E., Bohlen, S.R., and Luth, R.W. (1981)
lrw-friction, anhydrous, low- to high-temperature furnace sample assembly for piston-cylinderapparatus.Review of ScientificInstruments,
52.1903-1904.
Boettcher,A.L., Burnham, C.W., Windom, K.E., and Bohlen, S.R. (19E2)
Liquids, glasses,and the melting ofsilicates to high pressures.Joumal
of Geology, 90, 127-138.
Bohlen, S.R. (1984) Equilibria for precisepressurecalibration and a frictionless fumace assembly for the piston-cylinder apparatus. Neues
Jahrbuch liir Mineralogie Monatshefte,9, 404-412.
Bohlen, S.R., and Boettcher, A.L. (1982) The quartz=coesite transformation: A precisedetermination and the effectsofother components.
Joumal of GeophysicalResearch,87, 7073-7 078.
Boyd, F.R., and England, J.L. (1960) Apparatus for phase-equilibrium
measurementsat pressuresup to 50 kiloban and temperaturesup to
I 750 qC.Journal of GeophysicalResearch,65, 74l-748.
-(1963)
Effect of pressureon the melting of diopside, CaMgSirOu,
and albite, NaAlSirO8, in the range up to 50 kilobars. Journal ofGeophysicalResearch,68, 3l l-323.
Cohen, L.H., Ito, K., and Kennedy, G.C. (1967) Melting and phaserelations in an anhydrous basalt to 40 kilobars. American Journal of
Science.265. 475-5 I 8.
Dunn, T. (1993) The piston-cylinder apparatus.In R.w. Luth, Ed., Experimentsat high pressureand applicationsto the Earth's mantle. MineralogicalAssociation ofCanada Short Course Handbook, 21,39-94.
Kushiro, l. (1976) A new furnace assembly with a small temperature
gradient in solid-media, high-pressureapparatus.CarnegieInstitution
of Washington Year Book, 75, 832-833.
in piston-zylinder
kistner, H. ( I 979) Temperaturgradienten-Messungen
Pressen.Fortschritte der Mineralog1e,57,Beiheft 1, 8l-82.
Mao, H.K., and Bell, P.M. (1971) Behavior of thermocouplesin the single-stagepiston-cylinder apparatus.CarnegieInstitution of Washin4on
Year Book. 69.207-216.
Mao, H.K., Bell, P.M., and England,J.L. (1971) Tensionalerrors and
drift of thermocouple electromotive force in the single-stage,pistoncylinder apparatus.CamegieInstitution of WashingtonYear Book, 70,
2Er-287.
Presnall, D.C., Brenner, N.L., and O'Donnell, T.H. (1973) Drift of Ptl
Rl0Rh and W3Re/W25Re thermocouplesin single stagepiston-cylinder apparatus.American Mineralogist, 58, 77 l-777 .
Williams, D.W., and Kennedy, G.C. (1969) Melting curve of diopside to
50 kilobars. Journal of GeophysicalResearch,74, 4359-4366.
Mnvuscnrrr REcETvED
Jvr-v 12, 1993
MeNuscmsr AccEprEDSerrErtsrn 9, 1993