American Mineralogist, Volume 75, pages 1020-1028, 1990

Some simplifications to multianvil devices for high pressure experiments

D. War.xrnr* M. A. Canpnurnn" C. M. HrrcnDepartment of Earth Sciences, Dowmng Street, Cambridge CB2 3EQ, United Kingdom

Ansrru,cr

We describe a simplified multianvil system for high pressure and high temperatureexperiments based on the well-established octahedron-within-cubes geometry in use inmany other laboratories. Departures in desigrr from previous devices using this geometryinclude (l) use of a loose supporting ring that accommodates appreciable elastic strainunder loading from within by an unanchored cylindrical cluster of removable tool steelwedges, (2) choice of height to diameter of the cylindrical cluster of wedges so that thenegative cube geometry ofthe interior cavity that they surround does not have its angularrelationships distorted under appreciable strain, (3) casting of composite gasket materialsdirectly upon low porosity octahedral pressure media. The first two of these departuresfrom established practices allow a drastic reduction in hardware cost, bulk, and productiondifficulty relative to current installations. The assembled multianvil module is cost effec-tive, as well as being small and light enough to be inserted by hand in many of the standardhydraulic presses used for piston-cylinder work. The third departure reduces the risk ofblowouts, of tedium, and of irreproducibility (during learning stages) of the normal pro-cedure of cutting and attaching pyrophyllite gaskets. Using gaskets of Al'O, in epoxy ona range of standard sizes of chrome magnesia octahedra, we are able to reproduce standardcalibration point transitions with the same forces (and in some cases somewhat less force)required by other laboratories to achieve the same transitions with pyrophyllite gaskets.The overall performance of pressure, volume, and temperatwe (PVI) for this design iscomparable to or better than extant octahedron-within-cubes devices. These developmentsshould make multianvil experiments accessible to many existing piston-cylinder labora-tories at modest incremental cost.

InrnouucrroxPressure is an important variable in mineralogical and

petrological materials research. In condensed phases thehigh bulk modulus of the materials under investigationusually requires that pressures in the range oftens to hun-dreds ofkilobars be applied in order for interesting prop-erty variations or phase transitions to occur. Practicaldevices producing static pressures at the upper end ofthisrange are largely limited to diamond-anvil cells and avariety of multianvil devices. For large volume yields,development work has largely centered on the latter.

Multianvil devices were used in the 1950s in diamondsynthesis work; for example, see von Platen (1962) andthe summary in Hall (1960). Kawai and Endo (1970) usedthe octahedron-within-cubes geometry nested within aspherical seat pressurized by oil outside a rubber con-tainment membrane. Split spherical or cylindrical seats,sometimes without the oil bag and sometimes with trun-cations to sectors of the seats, have been the basis ofmany subsequent versions of the nest for the octahedron-

* On leave from I-amont-Doherty Geological Observatory andDepartment of Geological Sciences of Columbia University, Pal-isades, New York 10964, U.S.A.

within-cubes devices (e.9., Kumazawa et al., 1972; lto etaI., 197 4; Akaogi and Akimoto , 19771, Yoneda et al., 1986;Takahashi, 1986; Ohtani et al., 1987). Ohtani (1987) hasmodified the nest for the anvils from a sphere to a longcylindrical cluster of wedges contained by a pair of sep-arable, massive rings. The simplifications we describe arerelated to the previous desigrr of Ohtani (1987), in thatthe wedges we use form a cylindrical cluster. Howeverthe wedges of the present design are loose and free to

Fig. l. Plan and section drawings of the multianvil module.Uniaxial hydraulic press into which this module is positionedfor axial loading is not shown. The plan view has been drawnwith the top pressure distribution plate (G,), the top three wedges(A-u), and the cubes plus the pressure medium assembly (C, D)removed. Critical dimensions listed as follows. (Gaps, their di-mensions, and their filling materials are discussed in the text.)WEDGES. (A) AISI M2 tool steel hardened to R. 62. Squareface at 35o16'to axis ofmodule; square face side 2.015 in. [5.] 18cml; lowest corner of square face 0.25 in. [6.35 mm] from basalsurface; when 3 wedges assembled with gap of I mm betweenthem, the cylindrical diameter is 7.990 in. 120.295 cml; surfacefinish 8 micro inches or better. This high a hardness is unnec-essary, and a subsequent version at R" 58 has also performedwell. Nevertheless, the continued use ofM2 steel is recommended

0003-004x190/09 l 0-l 020s02.00 1020

WALKER ET AL.: SIMPLIFIED MULTIANVIL DEVICES

PLAN VIEW

, STRAIN GAUGE

r021

r LINE OF SECTION

+#o r_L!-LJ!9-L-!-l lo

C M

sEcTroN vrEw

for its superior corrosion resistance. CONTAINMENT RING. (B)Stress ring of AISI H13 low-alloy tool steel hardened to R" 49and double tempered as supplied by Edward B. Williams Man-ufacturing Company, Foundry Lane, Smethwick, Warley, WestMidlands 866 2LQ, United Kingdom. Nominal0.2olo proof stressis 14.5 kbar. O.D. 10.01 in. (25.425 cm), I.D. 8.000 in. (20320cm), length 5.0 in. (12.7 cm). Surface finish on bore better than8 micro inches; 0.020 in. (0.51 mm) radius on all corners. AN-VLS. (C) Tungsten-carbide cubes with corner truncations. Edgelength 1.000 in. (25.40 mm); surface finish 8 micro inches orbetter. Hertel grade KMY (60/o Co) used in testing. PRESSUREMEDIUM. (D) MgO with 50/o CrrO. compressed to about 200/oporosity; developed and supplied by Alan Hardstaff of G.R.-

Stein Refractories, Central Research Lab., Sandy I-ane, Worksop,Nottinghamshire S80 3EU, United Kingdom. SAFETYRING. (E) Mild steel. 10.007 in. (25.4178 cm) I.D.; wall thick-ness 5/8 in. ( I 6 mm); press fi t onto ring B. SCATTER SHIELD. (F)

Polycarbonate (Makrolon); 3.25 turns of a strip 3 mm thick and5 irt. (12.7 cm) wide rolled onto exterior of E and pinned to Ewith 2 dozen bolts. PRESSURE DISTRIBUTION PLATES' (G)

Tool steel annealed AISI Ol hardened to R. l8; 1.5 in. (3'8 cm)thick; 12 in. (30.5 cm) in diameter; raised "pillbox" of 7.80-in.(19.81-cm) diameterand 0.5-in. (1.27-cm) altitude forms an O-ring

closure on bore ofB. Tunnels and shafts bring coolant and wiring

between wedges. (Al alloy used successfully in a subsequent ver-

sion.)

lo22 WALKER ET AL.: SI},iPLIFIED MULTIANVIL DEVICES

Fig. 2. Photographs of the wedges and partially assembled module. Holes in the bases of the wedges are for dowel pins and boltattachments used to promote uniformity in the fabrication of the wedges. Partially assembled view corresponds to the partiallyassembled plan view ofFigure l, except for the oblique view point.

float within the ring and the wedge cluster's ratio of lengthto diameter is less than unity, rather than being a fewtimes greater. Massive support of the cluster is not pres-ent, and substantial elastic strain of the supporting ringis induced during loading. Coolant is easily introducedbetween the components within the ring. Essentially wedescribe a soft-shell cylindrical nest for wedges which drivethe standard octahedron-within-cubes payload. Thismodular device can be retrofitted into the hydraulicpresses of many existing piston-cylinder laboratories.

EqurrvrnNr DEscRrprroN

Plan and section drawings of the module are given inFigure l. This module is inserted into a uniaxial hydrau-lic press to perform experiments. Uniaxial loading drivesthe lower set of wedges (A,_r) toward the upper set ofwedges (A-u) in the bore of the containment ring (B),forcing the tungsten carbide cubic anvils (C) to convergeon the octahedral sample cavity (D). The containmentring (B) is jacketed by the safety ring (E) with a light pressfit. Strain gauges are pennanently mounted to the exterior

ofthe safety ring, which is surrounded by a polycarbonatescatter shield (F) with attached handles for manipulationof the ring and assembled module. Loading of the moduleis accomplished through pressure-distribution plates (G).Materials specifications and dimensions of these com-ponents are given in caption for Figure l.

Several gaps exist between components. Between thecarbide anvils (C) are gaskets, to be discussed in detailbelow. Between the anvils (C) and the wedges (A) arepads [0.020 in. (0.5 mm) thick] of glass-filled epoxideresin sheet (Tufnol grade l0G/40), which relieve anystress-causing irregularities on the interfaces between an-vil and wedge, as well as accommodate the slip betweenanvil and wedge necessary for convergence during load-ing. Similar pads are in use in most multianvil labora-tories. A gap I mm in plan view exists between the wedg-es (A), which is filled with Tufnol sheet 0.030 in. (0.76mm) thick to position the wedges during module assem-bly. Between the wedges (A) and the pressure distributionplates (G) are disks 0.040 in. (l mm) thick of Tufnol.These disks electrically isolate the wedges, absorb surface

WALKER ET AL.: SIMPLIFIED MULTIANVILDEVICES 1023

irregularities, and accommodate the lateral motion of thewedges during loading [up to 0.010 in. (0.25 mm)]. Be-tween the wedges (A) and the bore of the containmentring (B) is a gap of 0.005 in. (0.13 mm) filled with twosheets of polyester film. The outer sheet [0.003 in. (0.076mm) thick] is coated on its interior glossy surface withpolytetrafluorethylene (PTFE) spray (equivalent to Tef-lon). The inner sheet [0.002 in. (0.051 mm) thick] of poly-ester has its glossy side out to the PTFE-coated glossyside of the outer sheet. This configuration of lubricatedinterfaces between polyester sheets provides a slip surfaceto accommodate motion of the wedges (A) on the boreof the containment ring (B). The polyester sheets alsoprovide electrical isolation of the ring from the wedgesand absorb stress concentrations from surface flaws anddebris on the bore of ring (B). The plastic sheets alsoisolate the bore of ring (B) from the mixture of solubleoil and water that circulates between the wedges (A) andbetween the cubes (C) as a coolant. Acetate sheets of theappropriate thickness have been found to work equallywell, and the choice depends largely on ease ofprocure-ment.

Photos of the partially assembled module are shown inFigure 2. The weight of the module is low enough that asingle small person can slide it into a hydraulic presswithout the aid of an air pad, lift, or rail system. In massand ease of manipulation it is directly comparable to astandard Boyd and England (1960) assembled piston-cyl-inder stack. We have tested this module in a uniaxialpress normally used for piston-cylinder work that coulddeliver about 500 tons of thrust from an 8-in. (20.32-cm)ram.

I-o.corNc AND srRArN TEsrs

As uniaxial loading proceeds, the wedges converge onthe cubic cavity and are consequently forced outwardagainst the bore of the ring (B). The ring assembly swellsas a result, and the wedges move radially as well as lon-gitudinally along the module axis. The swelling of the ringunder load was monitored directly with a movable dialindicator gauge and with permanently mounted straingauges on the ring (E). No circumferential anisotropy ofswelling can be detected at the outer surface of the ringonce the assembly of interior wedges and cubes has beenproperly fabricated in terms of symmetry and uniformityof dimensions. In fact this lack of strain anisotropy of thering makes it possible to test the correctness of the fab-rication of the wedges and uniformity of the cubes. Di-mensional imbalances ofjust a few 0.001 in. (0.025 mm)are easily detected by exterior strain anisotropies of thering. A pair of typical plots of ring swelling on the outerdiameter vs. hydraulic load is given in Figure 3. The plotin small open circles shows the swelling of the ring underload when only a single sheet offrosted polyester is usedto lubricate the gap between the wedges (A) and the bore(B). The ring swelling is linear in applied load but showssubstantial hysteresis during unloading. The ring bindsthe wedges, and they do not immediately slip back during

2 0 0 4 0 0 5 0 0 8 0 0

Bars Oil Pressure8"Ram

Fig. 3. Strain vs. load on press. Linear loading curves showmarkedly nonlinear unloading. Bore friction on the wedges isreduced through the expedient ofproviding a lubricated slip sur-face of plastic on plastic.

unloading. The extent of this hysteresis suggests that asmuch as 300/o of the thrust may be dissipated into borefriction against the wedges. This is contrasted with theplot of large solid symbols for the doubleJubricated filmconfiguration generally used. Again the ring strain is lin-ear in applied stress, but a substantially higher strain isrealized for the same load. Furthermore, during unload-ing much less hysteresis is encountered. The extent ofreduction of the width of the stress-strain loop suggeststhat bore friction has been reduced to the l0o/o of thrustlevel by the expedient ofproviding a lubricated slip sur-face of glossy plastic on glossy plastic. Many other lubri-cation combinations were tested, with none showing muchbetter performance that the one described here, althoughseveral were considerably messier in practice. It is worthnoting that PTFE is not the key ingredient here becausea single 0.005-in. (0.127-mm) film of PTFE performedonly marginally better that a single sheet of frosted poly-ester. Two dry sheets of glossy-to-glossy polyester per-formed better that either single sheet polyester or PTFE,so it appears that the plastic-to-plastic slip surface is thekey ingredient, upon which further lubrication providesincremental improvement.

uJof

azaTEFg@

Io

za(EFa

: : J6 <d F -

zutoz

t024

That frictional loss to the bore was the cause of theloading/unloading strain hysteresis was obliquely con-firmed by measurement of ring strain along the axis ofthe module when internal loading proceeded only throughthe top pressure distribution plate. This could be accom-plished by resting the lower end of the ring directly onthe flange of the lower pressure-distribution plate andtaking measurements with the dial indicator gauge up anddown the outer ring surface. Strain measurements corre-sponding to the case of large hysteresis and poor lubri-cation in Figure 3 were about 200lo higher in the top thirdofthe ring than in the bottom third. In this configuration,only the top wedges were free to travel along the bore,and they sustain their frictional losses before stress istransmitted through the cubic anvils to the bottom set ofthree wedges. Thus the bottom three wedges push andswell the bottom of the ring to a lesser extent, determinedby the bore friction. The hysteresis measurements ofloading and unloading and the longitudinal anisotropy ofstrain measurements both suggest that the extent of thrustloss to bore friction is about 20-30o/o in the worst casessuch as for the light symbols in Figure 3.

One might hesitate to think of the l0o/o frictional lossof the heavy symbols in Figure 3 as a best case. Howeverwhen one considers how much of the total applied thrustis lost to gasket support rather than sample pressurization(roughly 50-900/0, depending on the lab and gasketsystem), a 100/o loss to friction is easier to tolerate. Thereis evidently much more potential scope for improving theefficiency of sample pressurization through attention togasketing, where the major losses occur, than in removingthe last small residual frictional losses. In light of thecalibrations reported below, bore friction cannot be con-sidered a major drawback to this design.

Nevertheless it should be possible to reduce this fric-tional loss still further in subsequent designs by splittingthe single ring we have used into a pair of rings stackedon top of one another with compressible separators be-tween. Rings capable of independent movement towardone another, each holding three wedges in place, will onlyencounter frictional losses at the tips of the wedges, whichoverlap the opposing ring. Splitting the ring into two hasthe added attraction that each ring is lighter and moreeasily manipulated. It also makes the inner cubic cavitymore accessible for loading because the upper ring neednot be fitted until the lower ring, lower wedges, and cubesare all firlly assembled. A split ring also provides accessfor penetrating beam experiments (although there is noreason why suitable ports could not be introduced at thestrategic places in a single ring). It is likely that futurerefinements in this design may proceed along these lines.A cautionary note about splitting the ring into two is thateach ring must be proportionately larger that simply halfof the single ring. This is because of the weaknesses as-sociated with end stresses on the ring maryins, where thetwo rings approach one another and support the wedgetips. The steel we have used in the device reported here

WALKER ET AL.: SIMPLIFIED MULTIANVIL DEVICES

might need to have its wall thickened by as much as afactor of approximately 1.5 before it could be split intotwo rings and be expected to provide adequate supportto the wedges.

A feature of some interest in Figure 3 is the extent towhich the ring swells. At a load of 500 tons the ring di-ameter grows by almost 0.020 in. (0.51 mm). Clearly thewedges have a nontrivial radial displacement under load.The roughly comparable dimensions of wedge radius andheight are a key feature in keeping the central cavity fromdistorting cubic angular relationships. An additional re-sult of the extent of swelling of the ring under load is thatthe O-ring seals between the pressure distribution plates(G) and the ring bore (B) will no longer provide a seal.Thus it has proved handy to have a ready supply ofrubberbands stretched onto the outside of the scatter shield (D.As the loading proceeds, these rubber bands can bedropped into the gap between the face of the pressuredistribution plates (G) and the end of the ring (B) to forman end seal. Without this precaution, the circulating cool-ant will bathe the outside as well as the inside of themodule, with untidy consequences. If safety concernsmandate reducing the exposure of the investigators to theapparatus under load, it will not be possible to decide inreal time the number of rubber bands appropriate for thespecific loading of the experiment. Alternate sealing pro-cedures would need to be adopted. The use of O-ring sealson the end of the ring (B) is unlikely to be effective be-cause ofthe close tolerances required for closure and thedynamic nature of the approach of the pressure distri-bution plate's (G) flange to the end of the ring (B) duringan experiment. A more suitably compressible gasket ma-terial must be used to fill this gap before loading.

Ar-rcNprnNr

The separated, unattached character of the wedges mightbe thought a design defect in terms of the difficulty inmaintaining wedge alignment. In fact the reverse is truebecause the wedges are sufficiently lubricated and the ge-ometric relations of the components are such that anyunbalanced forces arising from misalignment are directedtoward, and are capable of, nudging the wedges and cubesinto proper alignment. The ability to self-align is criticallydependent on the wedges being exactly symmetrical andthe cubes being uniform in size. If this initial conditionis satisfied, then the apparatus shows a remarkable ca-pacity for self-organization. Even deliberate initial errorsof up to 2 mm from center in the placement of Cu oc-tahedra within the cubes produced highly symmetricaltest pieces after closure of the jaws. The thickness of Cufins extruded between the tungsten-carbide cube facesduring jaw closure were uniform to within the +0.05 mm(0.002 in.) measurable on a vernier caliper. The distanceof extrusion of the fins was symmetrical enough that com-pleted test pieces were in some demand as trinkets. Spe-cial initial or periodic alignment procedures to the wedgesare unnecessary with this design.

CoNr.c.rNMnNT RrNG DURABTLTTy AND sAFETyThe durability of the containment ring assembly under

load is of some interest from the point of view of safety.Rough calculations using the nominal proof stress (14.5kbar) suggest the ring would reach 0.20lo strain at a uni-axial load approaching 700 tons. In point of fact thisamount of strain was encountered at a uniaxial load of500 tons, which also was the practical limit of the hy-draulic system of the press used. The discrepancy be-tween the designed and the observed strain may reflectsome degradation of the nominal proof stress during thedouble tempering used to increase ring toughness, and itmay reflect the crudeness ofthe calculations used. In anyevent, as long as lhe 0.2Vo strain limit is observed, anextended service life for the ring is anticipated. To ensurethis limit is not exceeded, three strain gauges are attachedto the exterior of the safety ring (E). Gauge voltages aremonitored on a chart recorder along with other vital pa-rameters of the experiment such as temperature, sampleconductivity, and hydraulic system pressure, as indicatedby a strain gauge attached to the outside of the ram.

Safety precautions taken against the unlikely event ofring failure include the safety ring (E) and the scatter shield(D. The mild steel of the same batch of tube stock usedfor the safety ring was certified to have passed a full flat-tening test without rupture. One measure of the dangerassociated with the failure of a pressure vessel is thequantity of elastic energy stored before rupture. Com-pound vessels ofthe Boyd and England (1960) type haveat least as much energy stored as the energy of assemblyof the two tapered rings and the insertion of the core.This is conservatively estimated at l0 kJ; any energystored in samplo pressurization increases the hazard. Bycontrast, an upp€r bound upon the elastic energy storedin this loaded multianvil rnodule is provided by the prod-uct of the maximum force (500 tons) and the cube closuredistance (2 mm). The elastic energy stored is undoubtedlyvery much less than this product because much of thework of closure is partitioned into anelastic deformationand extrusion of the gaskets, and an average force mightbe more appropriate to use than the maximum. Even so,this product is less than l0 kJ. Past experience with sev-eral catastrophic failures of compound pressure vessels ofthe Boyd and England (1960) design has shown mild steelto be effective as a safety ring. Because the safety ring (E)of the present design is at least three times more massivethan the safety rings proved to have withstood cata-strophic failures of more highly stressed compound ves-sels, we have felt reasonably confident in approachingthis vessel under full load. Nevertheless it should be care-fully noted that the use of inexpensive mild steel in thiscapacity is contrary to the best recommended practice,and that 3O0-series stainless steels are more ductile athigh strain rates and may afford an extra margin of safety(Getting, 1979). Our extra margin of safety (not countingthe rubber bands rnentioned above) is the polycarbonatescatter shield (D.

I025

Fig. 4. CIop) Sketch of pressure rnedium with integrally castgasketing fins. @ottom) Photographs of actual examples of thesecombinations of pressure medium and gasket, as well as an ex-ample of a recovered experiment (ZnS transition) which wasloaded to 200 kbar on 3-mm TEL tungsten-carbide anvils. Theanvils are seen to have sustained appreciable plastic strain. Theextrusion of the gaskets is satisfuingly uniform, and the gasketsclearly maintain their integrity even during unloading.

G.c,sKErs AND PRESsURE MEDrA

Two innovations in gasketing and pressure media wereemployed. The first was to reduce to about 20olo the po-rosity of the semisintered chrome magnesia (950/o MgOand 5Vo CrrOr) from which octahedra were sawed. Re-duction of initial porosity is beneficial for pressure-gen-eration efficiency because less compressive stroke is wast-ed in closing the initial porosity presont. Chrome magnesiawith only 20olo porosity is quite easily machinable. Puremagnesia in this low initial porosity is slightly less easilymachined, is a poorer insulator at very hrgh temperature,and is a poorer pressure medium, as indicated by thestudy of force needed to recover fixed transition points,which is discussed below. Development work toward fur-ther densification of chrome magnesia is appropriate.

Our gasket system is illustrated in Figure 4. Fins thatserve as gaskets during cube convergence are cast directly

WALKER ET AL.: SIMPLIFIED MULTIANVIL DEVICES

Al rO3 /EpoxyFin Gaskets

MgO-5%Cr2O3

Octahedron

{{k--+-t ' --Bi l tr- V

l So /oCr2Og

OCTAHEDRAAl203/Epoxy

GASKETS

1026 WALKER ET AL.: SIMPLIFIED MULTIANVIL DEVICES

atooJY 'ooulE

aaIIJt(L

FORCE2

1 5 0

o soo loooBars Oil Pressure

8"RAMFig. 5. Fixed point calibrations for three sizes ofanvil TEL

for the low porosity (20%) chrome-magnesia pressure mediumand integral gasket fins in standard use in this lab.

on the edges of the octahedra. We have experimentedwith several materials, the most successful to date beinga composite of AlrO, (a gntty powder supplied by BDHunder the name calcined alumina) bonded by epoxy resin(Buehler Epoxide) in a ratio of 2:l by weight. The com-posite is prepared by mixing powder with premixed resinand hardener. The material for pressing into the moldsshould be very sparingly wetted with the epoxy. A goodmeasure of the richness of mixture desired is that the mixshould wad and cling to a wooden spatula without look-ing at all wet. PTFE cubes with truncations to simulatetungsten-carbide anvils are spaced apart with PTFE sheetof the desired fin thickness (2 mm in all cases). Thechrome-mrgnesia octahedron is placed in the segmentedmold, and the composite is squashed into the spaces tobecome fins after curing. Figure 4 contains a photographofthe finished product, which can then be drilled or cutfor heater insertion, wiring connections, and so forth. Thisprocedure ensures a minimum of initial space due to themismatch of gasket and MgO, reduces the chances of gas-ket misplacement, and replaces the tedium of cutting andpasting pyrophyllite with the tedium of cleaning up afterepoxy dribbles.

This gasket composite is much less brittle than pyro-

Bars on S Ram5 0 0 7 5 0

FORCE

Fig. 6. Calibrations for 8-mm TEL on Bi transitions. (A)Cambridge lab results for three ceramics. Chrome magnesia isslightly more efrcient at translating thrust into sample pressurethan chrome-freg negnesia. Nonporous ceramics such as Ar-emco 5l I are more efficient still, but suffer other problems dis-cussed in the text. (B) Cambridge results for dense chrome mag-nesia with gaskets of epoxy and AlrO, are compared to those forsome other labs: Misasa (Takahashi, 1986), ANU (Ohtani et al.,1987), and Ehime (Ohtani, pers. comm.). Even with about l0Yoofthrust lost to bore friction, the use ofdense, integrally gasketedpressure media in this lab gives an overall performance in theefrciency of converting thrust to sample pressure that is com-parable to or better than currently acceptable standards. Thusbore friction is not a serious drawback to this multianvil nestdesign. Adoption ofdenser pressure media and these gasketing

il:ff:*"r could improve the performance of friction-free de-

phyllite and forms an extremely robust stockwork aroundthe pressure medium. We suspect that the toughness ofthese integral gaskets, in addition to the self-alignmentcapacity ofthe wedges, plays a role in the low incidenceof blowouts experienced in this lab-one in six months,which occurred at only 25 kbar through the ill-consideredinclusion of polyester in the pressure medium. The per-

formance of this composite material as a high pressuregasket may be judged objectively from the calibrationsreported in the next section.

C,l,r,rnru.noNs

Three standard electrical resistance-drop transitions areshown in Figure 5 as a function ofram oil pressure andequivalent tons offorce ofuniaxial loading: Bi I-II at 25.5kbar, Bi III-V at 77 kbar, and ZnS at 155 kbar (Lloyd,l97l; Piermarini and Block, 1975). Three sizes of anvil

oi c E

ooJY

G P a u '$ roooo

1 U l' E

o-

WALKER ET AL.: SIMPLIFIED MULTIANVIL DEVICES t027

truncation were tested: truncated edge lengths (TEL) of8, 5, and 3 mm, which corespond to octahedral pressuremedia of initial edge dimensions 14, ll, and 8.5 mm. Inall cases gaskets were 2 mm thick and extended 3 mmfrom their line ofexterior intersection with the surface ofthe octahedron (which is always chrome magnesia in Fig.5). A regular behavior is obvious. Furthermore, this plothighlights the fact that pressures in the normal range ofother octahedron-within-cubes devices can be achievedby this design. We have successfully loaded 3-mm trun-cations to about 200 kbar without blowout or otherequipment failure. Figure 4 shows a sample assembly andits gaskets recovered from one such experiment.

Comparison of our 8-mm TEL results for chrome mag-nesia is made to results for chrome-free magnesia in Fig-ure 6,4'. It is clear that chrome magnesia is a slightly betterpressure medium. An even higher efficiency of conver-sion of thrust into sample pressure was experienced usinga nonporous, castable ceramic made of MgO and zirconia(Aremco 5 I l). Only about 7 5o/o of the forced needed toachieve the Bi I-II transition with the other materials wasrequired in this case. Unfortunately this material is notfully suitable for other reasons. Its mechanical nonuni-formity promoted fin disintegration shortly after passingthe transition from Bi II to Bi III. Nevertheless this resultdoes indicate the benefits to be realized by reducing pres-sure medium porosity.

Figure 6B contains comparative results from other labsreporting Bi calibrations at 8-mm TEL. Our results arequite comparable to, or in some cases somewhat betterthan, those of the other labs in terms of the efficiency inconverting thrust into sample pressure. Clearly this min-imalist design does not sacrifice performance for simplic-ity ofapparatus construction or ease ofgasket preparation.In this context, anxiety over bore friction in the presentdesign would seem to be misplaced. An obvious corollaryis that other labs enjoying equipment free of bore frictionmight be able to improve their performance by adoptingthese gasketing procedures with lower porosity pressuremedia.

In other respects, such as heating, the performance ofthis design is also comparable to extant equipment. Wehave been able to pass the Mo melting point at 100 kbaron 8-mm TEL assemblies. Exactly the same strengths andweaknesses of all multianvil heating systems are sharedby this one. However the present design offers one ad-vantage in that it is easy to circulate coolant within thesealed ring to keep the pads between the anvils and wedg-es from softening when hot and to keep the tungsten-carbide cubes cool. When coolant circulates, we have de-termined the temperature at the 8-mm TEL anvil tips tobe about 250 T' when the hot spot of a tube furnace witha diameter of 3.4 mm is in excess of 2000 "C at 100 kbarwithin l4-mm chrome magnesia octahedra. Without thecoolant, the outer surfaces of the anvils approach thistemperature in the steady state even when the interiorsample temperature only approaches 1000 "C. Coolingthe carbide should improve its performance.

CoNcr,usroNs

A simple design is presented that duplicates the PWperformance of larger, standard octahedron-within-cubesmultianvil devices. The device could be retrofitted intomany existing piston-cylinder laboratories to enable re-search to 200+ kbar to be performed more widely.

Acxxowr,nncMENTs

The first author's participation in this project was made possible by afellowship fiom the Guggenheim Foundation and a sabbatical leave fromColumbia University's Iamont-Doherty Geological Observatory as a Vis-iting Fellow of Clare Hall. Development costs were supported by a grantfrom the Paul Instrument Fund ofthe Royal Society and by the Depart-ment of Earth Sciences, University of Cambridge. We have benefited fromdiscussions with and technical assistance from a large number ofpeopleincluding, but not limited to, C.B. Agee, P. Beattie, P. Buchan, F.R. Boyd,B. Cullum, I.C. Getting, N. Johnson, T.J.B. Holland, R. Hudson, T. Gas-parik, R.C. Lieberman, E. Ohtani, W.F. Sherman, P. Smitl, D.C. Pres-nall, M. Welch, and L. Wojnar. We thank F.R. Boyd for bringing the vonPlaten referenc€ to our attention. We wish to specially thank Alan Hard-staff of the GR-Stein Refractories' Central Research Iaboratory for hisefforts in developing and supplying us with low porosity chrome-magnesiaceramic. We are grateful to all rhese supporting organizations and col-leagues. Usefirl reviews of this manuscript were provided by I.C. Gettingand D.C. Presnall. Department of Earth Sciences Contribution #1671.Lamont-Doherty Contribution #4659.

Rnrnnntcns crrEDAkaogi, M., and Akimoto, S. (1977) Pyroxene-garnet solid-solution equi-

libria in the systems M&SLOr,-Mg4lrSip,, and FenSLO'r-Fe'AlrSirO',at high pressures and temperatures. Physics of the Earth Planetary In-reriors, 15,90-106.

Boyd, F.R., and England, J.L. (1960) Apparatus for phase equilibriummeasurements up to 50 kilobars and temperatures to 1750pC. Journalof Geophysical Research, 65, 7 4l-7 48.

Getting, I.C. (1979) Lubrication and safety in compound pressure vessels.In ICD. Timmerhaus and M.S. Barber, Eds., High pressure science andtechnology, vol. l, p. 835-845. Plenum, New York.

Hall, H.T. (1960) High pressrue methods. In High temperature technol-ogy, p. 145-156, 335-3t6. McGraw-Hill, New York.

Ito, E., Matsumoto, T., and Kawai, N. (1974) High-pressure decompo-sitions in manganese silicates and their geophysical implications. Phys-ics of the Earth Planelary Interiors, 8, 241-245.

Kawai, N., and Endo, S. (1970) The generation ofultrahigh hydrostaticpressure by a split sphere apparatus. Reviews ofScientific Instruments,4r,425-428.

Kumazawa, M., Masaki, K., Sawamoto, H., and Kato, M. (1972) Guideblocks and compressible pads for the practical operation ofmulti-anvilsliding system for the production ofhigh pressure. High Temperature-High Pressure, 4, 293-t10.

Lloyd, E.C. (1971) Editorialintroduction and summaryto: Accurate char-acterization of the high-pressure environment. U.S. National BureauStandards Special Publicarion, 326, l-3.

Ohtani, E. (1987) Abstract from the Kobe High Pressure Conference.Ohtani, 8., Irifune, T., Hibberson, W.O., and Ringwood, A.E. (1987)

Modified split-sphere guide block for practical operation of a multi-anvil apparatus. Hig[ Temperature-High Pressure, 19, 523-529.

Piermarini, G.J., and Block, S. (1975) Ultra high prcssure diamond-anvilcell and several semiconductor phase transition pressures in reliation tofixed point prcssure scale. Reviews of Scientific Instruments, 46, 973-980.

Takahashi, E. (1986) Melting ofa dry peridotite KLB-I up to 14 GPa:Implications on the origin ofperidotitic upper mantle. Journal ofGeo-physical Research, 9 1, 9367 -9382.

IO28 WALKER ET AL.: SIMPLIFIED MULTIANVIL DEVICES

von Platen, B. (1962) A multiple piston, high pressure, high temperature in an MA8 type apparatus to generat€ 15 GPa in a 1.8 cm3 sarirple

apparatus. In R.H. Wento{ Ed-, Modem very high pressure tech- volume.HighTemperature-HighPressure, 18,301-310.niques, p. 118-136. Butterworths, Washington, DC.

Yoneda, A., Kato, M., Kozuki, Y., Sawamoto, H., Kurnazawa" M., and MlNuscrn'r RECETVED Nover,rser 27, l9E9Makino, R. (1986) The use of sandwich-typ€ composite metal gaskets Mnruscmrr AccEprED Jur-v 18, 1990