Microstructure- Property Relationships in Macro-Defect ... · Microstructure-Property Relationships...

6
Microstructure- Property Relationships in Macro-Defect-Free Cement Jennifer A. Lewis and Waltraud M. Kriven Introduction The term "macro-defect-free" refers to the absence of relatively large voids (or de- fects) that are normally present in conven- tional cement pastes due to entrapped air or inadequate mixing. A decade ago, Birchall and co-workers developed a novel processing method that avoids the formation of these strength-limiting de- fects. 1 " This method, outlined schematically in Figure 1, consists of mixing hydraulic cement powder, a water-soluble polymer, and a minimal amount of water under high shear to produce a macro-defect-free (MDF) cement composite. Several cement/ polymer systems can be processed by this flexible technique, although the calcium aluminate cement/polyvinyl alcohol- acetate (PVA) copolymer system is most common: 1 * MDF cements display unique properties relative to conventional cement pastes. For example, the flexural strength of MDF cement is more than 200 MPa as compared to values on the order of 10 MPa for conventional pastes. 2 One can view MDF cements as a type of "inorganic plas- tic." As is the case with plastic processing, fillers such as alumina, silicon carbide, or metal powders can be added to MDF ce- ment to modify its performance properties (e.g., abrasion resistance, thermal or elec- trical conductivity, and hardness). 7 The combined attractiveness of inexpensive raw materials and flexible, low-temperature processing has generated great interest in this new class of advanced cement-based materials. Unlike conventional cements, MDF ce- ments incorporate a polymeric constituent and a rather small amount of water. The polymeric phase serves many functions: (1) it acts as a rheological aid facilitating particle packing, (2) it fills the pore space between cement particles, and (3) it reacts chemically with cement hydration prod- ucts to form an integral microstructural element. 4 ' 710 During the high-shear mixing step (refer to Figure 1), the water plasticizes the polymer, giving the paste a workable MI>K Cement Processing Methodology I'olVITKT/PlilStk-izi Premixing (low sheiirl High Shear Mixing (twin roll mill) Calendaring (to form sheets) Drying/Curing (sirci MDF Cement Figure 1. Process flow diagram for MDF cement fabrication. viscosity. The temperature must be con- trolled during mixing to prevent the paste from stiffening due to premature setting (or hydration) reactions. On complete mix- ing, the volume fraction of cement parti- cles in the paste approaches 0.70. This high-volume loading is obtainable due to the broad particle size distribution (e.g., 0.3-70 /xm) of the cement powder. In sub- sequent pressure curing and dry curing steps, water reacts with the cement parti- cles to form hydration products. 31 " The chemical composition and morphology of these products will be discussed in detail in the next section. MDF cement-based composites have many potential applications (e.g., substrates for electronic packaging, structural, or acoustic-damping materials), since they may be filled with secondary phases, metallized, coated, or even stacked and laminated to incorporate fiber-reinforcing layers or other materials between MDF sheets. 7 However, their use is currently limited to low-humidity environments. The hydrophilic nature of the polymeric constituent in MDF cement causes these materials to be highly moisture sensitive. Moisture absorption has a negative impact on most of their performance properties (e.g., flexural strength, dielectric constant, dimensional stability)."" 15 However, with a thorough understanding of microstructure- property relationships in MDF cement, one can envision both an emergence of commercial applications as well as the development of a generic class of MDF materials based on alternative inorganic/ polymer systems. In this article, we present an over- view of our current understanding of microstructure-property relationships in the calcium aluminate/PVA system. We be- gin by highlighting recent microstructural studies that include both the microstruc- tural analysis and microhydration behav- ior of MDF cement composites. We then discuss the organic phase composition and its percolative nature within the micro- structure. Finally, we relate these micro- structural observations to their influence on the bulk properties of MDF cement, and discuss their implications for the further development of MDF materials systems. Microstructural Analysis and Microhydration Behavior The significant improvement in strength and toughness of MDF cements relative to conventional cement pastes is attributed not only to their low porosity, but also to the chemical reactions that occur between the PVA polymer and the inorganic ions produced when calcium aluminates dissolve in water. A detailed study of the micro- 72 MRS BULLETIN/MARCH 1993

Transcript of Microstructure- Property Relationships in Macro-Defect ... · Microstructure-Property Relationships...

Page 1: Microstructure- Property Relationships in Macro-Defect ... · Microstructure-Property Relationships in Macro-Defect-Free Cement ference between the thic PVk A coating on Cu (representative

Microstructure-PropertyRelationships inMacro-Defect-FreeCement

Jennifer A. Lewis and Waltraud M. Kriven

IntroductionThe term "macro-defect-free" refers to

the absence of relatively large voids (or de-fects) that are normally present in conven-tional cement pastes due to entrappedair or inadequate mixing. A decade ago,Birchall and co-workers developed anovel processing method that avoids theformation of these strength-limiting de-fects.1" This method, outlined schematicallyin Figure 1, consists of mixing hydrauliccement powder, a water-soluble polymer,and a minimal amount of water underhigh shear to produce a macro-defect-free(MDF) cement composite. Several cement/polymer systems can be processed by thisflexible technique, although the calciumaluminate cement/polyvinyl alcohol-acetate (PVA) copolymer system is mostcommon:1* MDF cements display uniqueproperties relative to conventional cementpastes. For example, the flexural strengthof MDF cement is more than 200 MPa ascompared to values on the order of 10 MPafor conventional pastes.2 One can viewMDF cements as a type of "inorganic plas-tic." As is the case with plastic processing,fillers such as alumina, silicon carbide, ormetal powders can be added to MDF ce-ment to modify its performance properties(e.g., abrasion resistance, thermal or elec-trical conductivity, and hardness).7 Thecombined attractiveness of inexpensiveraw materials and flexible, low-temperatureprocessing has generated great interest inthis new class of advanced cement-basedmaterials.

Unlike conventional cements, MDF ce-ments incorporate a polymeric constituentand a rather small amount of water. The

polymeric phase serves many functions:(1) it acts as a rheological aid facilitatingparticle packing, (2) it fills the pore spacebetween cement particles, and (3) it reactschemically with cement hydration prod-ucts to form an integral microstructuralelement.4'710 During the high-shear mixingstep (refer to Figure 1), the water plasticizesthe polymer, giving the paste a workable

MI>K Cement Processing Methodology

I'olVITKT/PlilStk-izi

Premixing (low sheiirl

High Shear Mixing(twin roll mill)

Calendaring(to form sheets)

Drying/Curing(sirci

MDF Cement

Figure 1. Process flow diagram for MDFcement fabrication.

viscosity. The temperature must be con-trolled during mixing to prevent the pastefrom stiffening due to premature setting(or hydration) reactions. On complete mix-ing, the volume fraction of cement parti-cles in the paste approaches 0.70. Thishigh-volume loading is obtainable due tothe broad particle size distribution (e.g.,0.3-70 /xm) of the cement powder. In sub-sequent pressure curing and dry curingsteps, water reacts with the cement parti-cles to form hydration products.31" Thechemical composition and morphology ofthese products will be discussed in detailin the next section.

MDF cement-based composites havemany potential applications (e.g., substratesfor electronic packaging, structural, oracoustic-damping materials), since theymay be filled with secondary phases,metallized, coated, or even stacked andlaminated to incorporate fiber-reinforcinglayers or other materials between MDFsheets.7 However, their use is currentlylimited to low-humidity environments.The hydrophilic nature of the polymericconstituent in MDF cement causes thesematerials to be highly moisture sensitive.Moisture absorption has a negative impacton most of their performance properties(e.g., flexural strength, dielectric constant,dimensional stability).""15 However, with athorough understanding of microstructure-property relationships in MDF cement,one can envision both an emergence ofcommercial applications as well as thedevelopment of a generic class of MDFmaterials based on alternative inorganic/polymer systems.

In this article, we present an over-view of our current understanding ofmicrostructure-property relationships inthe calcium aluminate/PVA system. We be-gin by highlighting recent microstructuralstudies that include both the microstruc-tural analysis and microhydration behav-ior of MDF cement composites. We thendiscuss the organic phase composition andits percolative nature within the micro-structure. Finally, we relate these micro-structural observations to their influenceon the bulk properties of MDF cement, anddiscuss their implications for the furtherdevelopment of MDF materials systems.

Microstructural Analysis andMicrohydration Behavior

The significant improvement in strengthand toughness of MDF cements relative toconventional cement pastes is attributednot only to their low porosity, but also tothe chemical reactions that occur betweenthe PVA polymer and the inorganic ionsproduced when calcium aluminates dissolvein water. A detailed study of the micro-

72 MRS BULLETIN/MARCH 1993

Page 2: Microstructure- Property Relationships in Macro-Defect ... · Microstructure-Property Relationships in Macro-Defect-Free Cement ference between the thic PVk A coating on Cu (representative

Microstructure-Property Relationships in Macro-Defect-Free Cement

structure, microchemistry, and microhy-dration behavior of MDF and of a layeredCaAl2O4 and PVA model system has beencarried out by Kriven and co-workers89'""2"using a variety of advanced techniques:scanning electron microscopy (SEM), trans-mission electron microscopy (TEM), high-resolution electron microscopy (HREM),energy dispersive spectroscopy (EDS),parallel electron energy loss spectroscopy(PEELS),8*"18 and x-ray photoelectron spec-troscopy (XPS).W1" The TEM sampleswere prepared by ultramicrotomy to pre-

g 2. TEM micrograph (bright field)of the general microstructure of MDFcement. Calcium mono- and di-aluminatcgrains (3-7 fun in size) are dispersed ina PVA matrix (containing dissolved Caand Al). An interfacial interphase(=200 nm thick and containing 5-8 nmcrystallites) is shared by the organicpolymer and inorganic aluminate grains.

Figure 3. HREM micrograph of theinterfacial interphase showing the latticeimage of dispersed mctastablc CaAI2Or, •8H2O hydration products in the form offine crystallites, 5-8 nm in size, whichwere sometimes twinned.

cisely determine the microchemistry ofdifferent regions within the MDF cementmicrostructure.

The general microstructure of MDF ce-ment consists of crystalline agglomeratesof calcium mono-aluminate (CaAl2O4) andcalcium di-aluminate (CaAl4O7) grainsranging from 3 ^m to 7 /am in size, ran-domly dispersed in the PV\ polymer matrix(Figure 2). Between the inorganic grainsand the organic polymer, an interfacial in-terphase region of varied thickness (e.g.,100-500 nm) was observed. HREM81618

showed that the interphase regions con-sisted of an amorphous polymer matrixcontaining a fine dispersion of crystallites(5-8 nm in size) that were often twinned(refer to Figure 3). Laser optical diffrac-tometry identified the crystallites as themetastable, pseudo-hexagonal, hydrationproduct Ca2Al2O5 • 8H2O. The expectedhydration product (if any) after process-

ing at 80°C would have been Ca3Al2O,, •6H2O.17 A typical TEM/EDS816 w spectrumtaken from the interphase region in an Al-coated specimen revealed the presenceof Ca, Al, and O. A corresponding PEELSspectrum from the same region confirmed,unambiguously, the presence of carbon,since C-K and Ca-L lines (which overlapin EDS) were differentiate in PEELS. Soit was inferred that in the interfacial region,water reacted with the cement (CaAl2O4)in the presence of polymer, formingthe metastable calcium mono-aluminateoctahydrate.

The chemical nature of the bulk poly-mer matrix and of the polymer within theinterphase region was probed by XPS analy-sis of the model PVA/CaAl2O4 samples,which were prepared by spin-coating a¥VA solution onto a dense, sintered CaAl2O4substrate. The results of the analysis arepresented in Figure 4. Essentially, the dif-

LLJ

LU

CLT

292 291 290 289 288 287Binding Energy, eV

286 285 284

292 291 290 289 288 287Binding Energy, eV

286 285

Figure 4. High-resolution experimental (EX) and synthesized (SYN) XPS C(1s) spectra from(a) 1-fnm-thick polyvinyl alcohol film on Cu foil or on CaAl2Ot pellet and (b) 10-nnt-thickpolyvinyl alcohol film on a dense CaAl2O4 pellet heat treated for 24 h at 80°C.

MRS BULLETIN/MARCH 1993 73

Page 3: Microstructure- Property Relationships in Macro-Defect ... · Microstructure-Property Relationships in Macro-Defect-Free Cement ference between the thic PVk A coating on Cu (representative

Microstructure-Property Relationships in Macro-Defect-Free Cement

ference between the thick PVA coating onCu (representative of the unreacted ma-trix polymer) and the thin coating on theCaAl2O4 pellet (representative of thechemically reacted, interfacial polymer)was the presence of an A1OC complex.This implied that Al ions could be chemi-cally incorporated into the polymeric chains,likely serving as a crosslinking agent. Apossible reaction sequence would be first,dissolution of the CaAl2O4 into H2O ac-cording to the equation:

2CaAl2O4 + 8H2O * 2Ca2+(aq)

+ 4Al(OH)-4(aq), (1)

followed by the crosslinking of the poly-mer by Al in a polycondensation reactionof the type:

4(CH2-CHOH),, + n[Al(OH)4]-M -»

«{(CH2-CHO)4A1J- + 4MH2O. (2)

The water released in this type of reactionwould then be available to form the meta-stable Ca2Al2O5 • 8H2O hydration product:

CaAl2O4 + 5.5 H2O ->

0.5 Ca2Al2O5 • 8H2O + A1(OH)3. (3)

In order to understand the mechanismof water/MDF interactions at the micro-structural level, an in-situ environmentalcell TEM study was undertaken.17 Micro-structural changes were monitored con-tinuously for 10 h, as a moist hydrogenatmosphere was passed into the cell con-taining a 3-nm-thin TEM foil of MDF ce-ment. As the hydration proceeded, thebulk polymer matrix swelled noticeably.After 8 h, the microstructure consisted ofswollen globules of polymer into whichthe interfacial interphase had progressivelybeen dissolved. These observations sug-gest that moisture attacks both of these keymicrostructural features, which can becollectively described as the binder phase.

Binder Phase Composition andPercolative Nature

The binder phase acts to "bind" unre-acted cement grains together within theMDF cement microstructure, and thusshould greatly influence the performanceproperties of MDF cements, as demon-strated by several investigators.5'911"15 Tounderstand the influence of the binderphase on bulk properties, we must firstdiscuss the binder composition and its dis-tribution (or percolative nature) withinthe MDF cement microstructure. As dis-cussed above, the binder phase consists ofregions that contain the bulk PVA matrix,and the interphase regions that contain

both PVA and hydration products in theform of a nanocomposite.

Representative initial and as-cured com-positions of MDF cement materials areshown in Table I. The as-cured composi-tion was determined by assuming that hy-dration occurred according to Equation 3.

The interphase regions comprise about63 vol% of the total binder phase; only20% of the total PVA content, however, is

contained within these regions.21 The dis-tribution of binder within the MDF cementmicrostructure can be directly observedby microscopy or investigated by com-puter modeling. While an understandingof the two-dimensional MDF cement micro-structure provided by microscopy studiesis important, it is the three-dimensionalmicrostructure that must be linked to theproperties of MDF cements. Computer

Table I: Representative Initial and As-Cured Compositions of MDF Cement.

Component

HAC Cement(Secar71)

PVA(Gohsenol KH-17s)

GlycerolDeionized Water

Component

Unreacted cement(CaAI2O4, CaAI4O?

Ca2AI2O5 • 8H2OAI(OH)3

Plasticized-PVA

NonvolatilesVolatiles

Plasticized-PVAH2O

Initial CompositionWeight %

84.25

5.90

0.589.27

As-Cured CompositionWeight %

78.7

•)10.14.46.8

87.7

6.85.5

Volume %

65.21

12.30

1.3521.14

Volume %

66.6

13.24.6

15.6

70.4

15.614.0

Table II: Simulation Results on Interphase Region Properties in MDF Cement.

Interphase RegionThickness (nm)

100200300400500

Volume Fractionof Interphase Region

0.06750.13900.20670.26150.2987

Connected Fractionof Interphase Region

0.8990.9900.99950.9999961.0

% of Binder inInterphase

Region

20.241.661.978.289.4

Table III: Simulation Results on Connectivity of PVA Matrix in MDF Cement.

Grid Size

150200300

Resolution (nm)

1000750500

Volume Fraction ofPVA Matrix inMDF Cement

0.12740.12730.1274

Fraction of PVAMatrix Connected

to Top

0.02990.71300.9568

Fraction ofPVA MatrixPercolated

00.69540.9561

74 MRS BULLETIN/MARCH 1993

Page 4: Microstructure- Property Relationships in Macro-Defect ... · Microstructure-Property Relationships in Macro-Defect-Free Cement ference between the thic PVk A coating on Cu (representative

Microstructure-Property Relationships in Macro-Defect-Free Cement

modeling provides a route for bridging thegap between observable two-dimensionalmicrostructure and performance properties.

Lewis and Bentz investigated the per-colative nature of the binder phase in MDFcement using a hard core/soft shell contin-uum percolation model. Both the bulkPVA matrix and the interphase regionswere found to form percolative pathwaysthrough the model system. This systemconsisted of a three-dimensional cube,150 /am on a side, into which 3.005 X 105

spherical particles (hard cores) were placedat random locations, producing a volumefraction of unreacted cement equal to 0.666(refer to Table I). A soft shell (or interphaseregion) surrounded each particle as shownin Figure 5. The "best" shell thickness wasdetermined to be 300 nm (see Table II);however, even at the minimum thicknessevaluated (100 nm), the interphase regionswere found to percolate. The resolutionrequired to detect percolation of the bulkPVA regions was found to be 500 nm (asshown in Table III) for the system basedon the "best" thickness, which correspondsto the maximum pore diameter observedexperimentally in MDF cements.15 Thecombination of direct microstructuralobservations and computer modeling is apowerful approach to understanding thedistribution and percolative nature of thebinder regions in MDF cement. We willnow examine the influence of these re-gions on the bulk properties of these com-posite materials.

Influence of Microstructure onBulk Properties

The bulk properties of MDF cementsdiffer dramatically from those of conven-tional cement pastes due to their uniquemicrostructure. The bulk polymer matrix,interphase regions, and absence of largepores are among these unique microstruc-tural features, each of which plays animportant role in determining the perfor-mance properties of MDF cements. In thissection, we will discuss the influence ofmicrostructure on the mechanical andelectrical properties, as well as on themoisture sensitivity of MDF cements.

The flexural strengths of MDF cementslie in the range 150-250 MPa.51213 Thesevalues represent a tenfold increase relativeto the strength of conventional cementpastes. Initially, these high strengths wereattributed solely to the absence of largevoids (or defects) in the MDF microstruc-ture resulting from the filling of pores bythe polymeric constituent.2 However, itis now known that the percolative inter-phase network also contributes to thesehigh strengths.8<"5 On exposure to highrelative humidities (>60%) or immersion

Figure 5. Two-dimensional view of the model MDF cement system, where the unreactedcement grains are shown in red, the interphase regions are shown in green, and the bulk PVAmatrix is shown in black.

0.1

0.08 "u

3 'So

£"0 0.06

"3 S 0.04

u

0.02

0

. Interphaseregion

. PVA Matrixregion

\

Binder Removed

0.001 0.01 0.1Pore Diameter (|im)

10

Figure 6. Pore development of heat-treated MDF cements as a function of binder removed.

MRS BULLETIN/MARCH 1993 75

Page 5: Microstructure- Property Relationships in Macro-Defect ... · Microstructure-Property Relationships in Macro-Defect-Free Cement ference between the thic PVk A coating on Cu (representative

Microstructure-Property Relationships in Macro-Defect-Free Cement

in water, the f lexural strength of MDF ce-ments decreases with time to a final valuethat is approximately 50% of their initialstrength.

The electrical properties of MDF ce-ments make them attractive for "low-end"packaging applications. The dielectricconstant (e) of these composites at 1 MHzand 0% relative humidity ranges between8 and 10.14 The dissipation factor (tan 8)and dc resistivity under the same condi-tions range between 0.01-0.05 and 1012-10B

fl-cm, respectively. Again, on exposure tomoisture, these values change dramati-cally. For example, the dielectric constantand dissipation factor have been observedto increase by 50% or more, while the dcresistivity has been observed to decreaseby several orders of magnitude.

The moisture sensitivity of MDF ce-ments and its deleterious impact on theirperformance properties has preventedcommercialization of these materials. It iswell known that the hydrophilic nature ofthe polymeric phase in MDF cement con-tributes to this phenomenon. The binderphase is complex, however, consisting ofboth interphase and bulk PVA regions,each of which could provide a pathwayfor moisture transport from the cementsurface to its interior. In recent work, theinfluence of these different regions on themoisture absorption kinetics and flexuralstrength of MDF cement was elucidatedby selectively varying their respective dis-tributions within the microstructure.15

The binder distribution in MDF cementwas varied by heating samples to tempera-tures between 125°C and 400°C. Chemi-cally bonded water is removed from thesystem at 125°C, while PVA and water areremoved simultaneously at higher tem-peratures. A bimodal pore-size distribu-tion develops as increasing amounts ofbinder are removed from the composite,as shown in Figure 6. The fine-scale poros-ity (pore diameter <15 ran) corresponds topores developed in the interphase regions,while large-scale porosity (pore diameter>30 nm) corresponds to pores developedin the bulk PVA regions. Exposing theseheat-treated MDF cement samples to 100%moisture showed that only the develop-ment of large-scale pores led to significantdifferences in moisture absorption behav-ior relative to standard MDF cement (referto Figure 7). This indicates that the inter-phase regions are highly resistant to mois-ture transport, and that the predominanttransport pathway is through the bulkPVA matrix. Additionally, Figure 8 depictsthe flexural strength of these samples as afunction of the extent of binder removed.The strength does not decrease signifi-cantly until a percolative network of large-

scale porosity is formed (i.e., =30% binderremoved) within the MDF composite. Theseobservations suggest that to improve themoisture resistance of this system, onemust either reduce the "bulk" polymercontent in the binder phase or chemicallymodify this region to increase its resis-tance.1115 We are currently investigating themerit of each of these approaches.

Concluding RemarksIn the past decade, much attention has

been given to understanding the micro-

1 2

1 0

8 "

I 6

t'53 4

structure and microchemistry of MDFcements and their relation to the uniqueproperties of this materials system. As wehave discussed in this article, the MDFcement microstructure is a complex one,differing significantly from conventionalcement pastes. The key microstructuralfeatures are the bulk polymer matrix, theinterphase regions, and the absence oflarge pores, which contribute to the supe-rior mechanical properties of these com-posites. However, the hydrophilic natureof the polymeric constituent currently limits

1 0 0 %

- A'

: * .•-•*;: •

RH

A A A

/

1 i

.--A' -#"

i i 1 i i

% Binder Removed / Temp CO

. A---"A 75%/400°C. A - - ' : * - . - - - • - - - • 55%/300°C

-;.---t-'- - 28% / 175°C

„ c, 0%/ 25°C^ - e — e — ^ ^ 8 13% / I25°c

10 15 20Time (days)

25 30

Figure 7. Moisture absorption behavior of heat-treated MDF cements a.s a function of binderremoved.

CO

•B00

i

250.00

200.00

150.00

100.00

50.00

0.00

Heat-treated MDF Cement I

onset of percolationof large scale pores

• • -

20 40 60 80Binder Removed (Vol%)

100

Figure 8. Flexural strength of heat-treated MDF cements as a function of binder removed.

76 MRS BULLETIN/MARCH 1993

Page 6: Microstructure- Property Relationships in Macro-Defect ... · Microstructure-Property Relationships in Macro-Defect-Free Cement ference between the thic PVk A coating on Cu (representative

Microstructure-Property Relationships in Macro-Defect-Free Cement

the use of MDF cements in commercialapplications. The low processing costs andinherent flexibility has sustained industrialinterest in MDF cements. Current researchefforts focus on exploiting the flexibleprocessing route to produce noncement-based materials, e.g., ceramics, and onimproving the moisture resistance ofcement-based MDF composites.

AcknowledgmentsWe gratefully acknowledge the funding

provided by the NSF Center for AdvancedCement-Based Materials and by the AFOSRthrough Contract No. F49620-87-C-0023and Grant No. 90-0242. In addition, wewould like to thank Professor J.F. Youngfor many fruitful discussions.

References1. J.D. Birchall, A.J. Howard, and K. Kendall,U.S. Patent No. 4,410,366 (1983).2. K. Kendall, A.J. Howard, and J.D. Birchall,Philos. Trans. R. Soc. London, Ser. A 310 (1983)p. 139.3. R.N. Edmonds and A.J. Majumdar, /. Mater.Sci. 24 (1989) p. 3813.4. J.D. Birchall, A.J. Howard, and K. Kendall,Ceram. Soc. Proc. 32 (1982) p. 25.5. N. M. Alford and J.D. Birchall, in Very HighStrength Cement-Based Materials, edited by|.F. Young (Mater. Res. Soc. Symp. Proc. 42,Pittsburgh, PA, 1985) p. 265.6. D.M. Roy, Science 35 (1987) p. 651.7. J.D. Birchall, Philos. Trans. R. Soc. London,Ser. A 310 (1983) p. 31.8. O.O. Popoola, W.M. Kriven, and J.F. Young,/. Ultramicrosc. 37 (1991) p. 318.9. O.O. Popoola and W.M. Kriven, /. Mater. Res.7 (1992) p. 1545.10. S.A. Rodger, S.A. Brooks, W. Sinclair,G.W. Groves, D.D. Double, /. Mater. Sci. 20 (1985)p. 2853.11. C.S. Poon, L.E. Wassell, and G.W. Groves,Mater. Sci. Tech. (1988) p. 993.12. CM. Cannon and GW Groves, /. Mater. Sci.21 (1986) p. 4009.13. P.P. Russell, J. Shunkwiler, M. Berg, andJ.F. Young, Ceram. Trans. 16 (1991) p. 501.

Is the MRS Bulletin inyour institutional library?Recommend it to yourlibrarian today...

Circle Reader ServiceCard No. 125 and we'llsend you an MRS JournalRecommendation Formright away.

14. G.V. Chandrashekhar , E. Cooper, andM.W. Shafer, /. Mater. Sci. 24 (1989) p. 3356.15. J.A. Lewis, M.A. Boyer, and S. Covey, /. Am.Ceram. Soc, submitted for publication.16. O. Popoola, W.M. Kriven, and J.F. Young,Electron Microscopy 1990 4 p. 1068; Proc. 12thIntl. Congr. Electron Microsc, Seattle, WA (SanFrancisco Press, 1990).17. O.O. Popoola, W.M. Kriven, and J.F. Young,/. Am. Ceram. Soc. 74 (1991) p. 1928.18. M.A. Gulgun, I. Nettleship, O.O. Popoola,W.M. Kriven, and J.F. Young, in Advanced Ce-mentitious Systems: Mechanisms and Properties,

edited by F.P. Glasser, PL. Pratt, T.O. Mason,J.F. Young, and G.J. McCarthy (Mater. Res. Soc.Symp. Proc. 245, Pittsburgh, PA, 1992) p. 199.19. O.O. Popoola, WM. Kriven, and J.F. Young,in Advanced Cementitious Systems: Mechanismsand Properties, edited by EP. Glasser, P.L. Pratt,T.O. Mason, J.F. Young, and G.J. McCarthy(Mater. Res. Soc. Symp. Proc. 245, Pittsburgh,PA, 1992) p. 283.20. Microbeam Analysis-1991, edited byD.G. Howitt (San Francisco Press) p. 167.21. J.A. Lewis and D.P. Bentz, /. Am. Ceram.Soc, submitted for publication. D

MBE Technology Leadership

WORLD record GaAs mobility, HEMT mobility, bestlattice-matched quaternary material or

just innovative research - it all happenswith Intevac MBE Equipment.

Intevac has a 18 year history ofsuccess in the MBE, CBE and GasSource MBE equipment business.Let our expertise support yourdevice research and production.

For systems, sources, facts andfigures, contact Intevac's MBEmarketing department at(408) 986-9888.

POINT", a powerful new spectrometer system forautomated substrate temperature measurement.

Rapid response Group V source

vacMBE Equipment Division

Corporate Headquarters: 3550 Bassett Street, Santa Clara, California 95054.

Phone 408-986-9888. Fax 408-727-7350

European Headquarters: Oberachweg 3B, 8183 Rottach-Egern, Germany.

Phone 08022 67 03 11. Fax 08022 67 00 12

Please visit Booth No. 513 at the MRSEquipment Exhibit in San Francisco, April 13-15,1993.

Circle No. 38 on Reader Service Card.

MRS BULLETIN/MARCH 1993 77