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Saipta Metalhrgica et Materialia, Vol. 32, No. 3, pp. 423-426,1995Copyright B1994 Elsevier Science Ltdprinted io the USA. All rights lWerved0956-716X/95 $9.50 + .W

INTERCRYSTALLINE DENSITY OF NANOCRYSTALLINE NICKELT.R. Haasz, K.T.Aust, G. Palumbo, A.M. El-Sherik3 and U. Erb3

Department of Metallurgy and Materials ScienceUniversity of Toronto, Toronto, Canada, M5S lA4

Ontario Hydro Research Division, 800 Kipling Ave., Toronto, Canada, MSZ 5S43Department of Materials & Metallurgical Engineering, Queens University, Kingston, Canada, K7L 3N6

(Received August l&1994)(Revised September 9,1994)

IntroductionMost methods currently available for the synthesis of nanostructured materials result in considerable residual

porosity. Studies concerned with the novel structures and properties of these materials (1, 2) are thuscompromised by the intrinsically high levels of porosity. As recently shown by Kristic et al. (3) porosity canhave a significant effect on fundamental materials properties such as Youngs modulus. One of the mostpromising techniques for the production of fully dense nanocrystalline materials is electrodeposition. In the presentwork, the residual porosity and density of nanostructured nickel produced by the electrodeposition method isassessed and discussed in light of the intrinsic intercrystalline density of nickel

ExperimentalDensity measurements and calculations presented in this work are based upon Archimedes Principle. The

nanocrystalline nickel studied in this investigation was produced by electrodeposition with a purity of 99.9 wt.%(4). The average grain sizes of the electrodeposited material used were 1 +l nm and 18s nm. Thenanocrystalline materials were in the form of 10 mmx 10 mmx0.3 mm sheets. In order to calculate intercrystallinedensity, 99.999 wt.%f pure single-crystalline nickel was also used. The single-crystalline nickel was in the formof a 1.5 mm thick, 10 mm in diameter disk.

The experimental technique used to measure sample weights was developed with the goal of ensuring precisionand accuracy. The technique addressed minimizing: (i) reference medium evaporation, (ii) gas absorption bythe reference medium, (iii) gas adsorption by materials immersed into the reference medium and (iv) surfacecontamination.

All samples studied in this investigation were lightly polished with 240 grit Sic paper and cleaned in ultrasonicbaths of trichloroethane, methanol and distilled water, respectively. By means of a Mettler AB163 analyticalbalance, five atmospheric weight measurements were taken of each sample to establish the weight of each samplein air (w& To accommodate buoyant weight measurement, the AB163 balance was fitted with an archattached to the weighing pan of the balance and a beaker filled with Edwards 704 Silicone fluid was placed onan elevated stand below the arch. Utilizing this configuration, sample buoyant weights were determined bymeasuring: (i) the combined buoyant weight of each sample and a clasping device (wcn+& and (ii) the buoyantweight of the clasping device (w,& itself, Values for both weight configurations were established by performingfive weight measurements of each weight configuration. To minimize absorbed gas content in the referencemedium and adsorbed gas content on all immersed materials, all buoyant weight measurements were precededby a vacuum residency period. In order to facilitate this operation, the clasping device and/or sample washooked upon a rest arm attached to the beaker and the complete unit was placed in a 10. Pa vacuum chamber untilthe majority of these gases were removed.

?Purity levels are based upon metallic and non-metallic contributions

423


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424 DENSITY OF Ni Vol. 32, No. 3

Following adjustment of all raw weight data to account for effects due to evaporation, impurities and oxidefilms, specific density (sp) was calculated according to Archimedes Principle. Also, the error associated with eachspecific density calculation (asp), based upon a theory proposed by Doebelin (5) was determined to be,

where each term preceded by a A corresponds to the + error associated with the subscript quantity, To avoidsources of error associated with the chosen reference medium, all material densities were left as specific densities.

Results and DiscussionTable 1 shows the effect of grain structure and porosity on specific density. Decreases in specific density

associated with decreases in grain size are representative of the larger intercrystalline component found innanostructured materials (6) and any possible residual porosity. The apparent increase in specific density withdecreasing nanocrystalline grain sizes (i.e. 18tinm to 1 +lnm) may be due to porosity. The amount of porositycan be approximated by using two sources of reference: (i) the density difference between the single-crystallinestructure and the nanocrystalline structures and (ii) the density difference between the two nanocrystallinestructures, The former establishes an ultimate nanocrystalline porosity content of approximately 0.6 % and thelatter establishes a minimum nanocrystalline porosity content of approximately 0.3 % with respect to volume.However realistically, porosity contents are expected to be less than the ultimate porosity predicted because of thefinite excess volume associated with the intercrystalline regions in the nanocrystalline samples.

TABLE I.Specific density results with respect to Edwards 704 Silicone fluid for

different Ni grain structures.

IIrain Structure I Specific Density[Dimensionless] IIII Single crvstal 18.387M.002 II11&l nm 8.345~0.02

18s nm 8.32kO.02

Porosity content calculations can be further refined by comparing the magnitude of each specific densitydifference with the corresponding structural difference. The resulting outcome suggests that the magnitude of thetransition from the 1151 nm grained material to the 1852 nm grained material is half as profound as the transitionfrom the single crystalline material to the nanocrystalline materials (i.e. the 1If1 nm and the 18+2 nm grainedmaterials). Also, the decrease in grain size from 18152 nm to 1 If1 nm produces a decrease in free volume.Conceptually, this is difficult to accept. A more reasonable explanation assumes that the 18ti nm grained materialis virtually free of porosity and the 1 l+l nm grained material contains 0.3M.l % porosity.Intercrvstalline Density

Intercrystalline density (p,J can be viewed as crystalline density (pcRYs.) less the mass density equivalent ofintercrystalline free volume density (pIc - voL,MASSQv, This is mathematically expressed as,

PIG= kRYs.-Pn: Pm VOL MASS eqv (2)However in order to evaluate equation (2), an expression for pFREEoL.MASSev, must be introduced. This quantitycan be described by the ratio below.


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Vol. 32, No. 3 DENSITYOF Ni 425

prcme nn. 8u.w PQ. = Intercr)rstauinr jke v&me maw equivalentInterc~lline volume= hsvG~cm..-P,)

u;J(3)

where,PSINGLERYS single crystalline density of a specified chemistryopoLY= polycrystalline density of identical chemistryf, = intercrystalline volume fractionEquation 3 can be further simplified by utilizing intercrystalline theory proposed by Palumbo et al. (6). The

authors describe intercrystalline volume fraction as,frc = l- (1 -;i (4)Ani

where,A = grain boundary thicknessdAW = average grain size.

Therefore, the intercrystalline density of a nanocrystalline material can be expressed as,PIG= PsIhw.ecm. - PsLwLe ws. - PNANG.l- (1 - F (5)

AVG

Alternatively, intercrystalline density can be expressed in terms of a reference medium, i.e.,%2 = sh,MGLECiWshLVGLEC,WS. - %VNO.l- (1 - y-)3 (6)

AVGwhere,

so, = the specific density of the subscript quantity x with respect to a chosen reference mediumHowever, to effectively distinguish between crystalline and intercrystalline density, intercrystalline density (prc)has been presented in terms of a percent reduction of crystalline density (IS%), i.e.,

6% = SPCRYS. SPICXlOOk (7)SPCRYS.Errors associated with 6% were formulated by applying Doebelins technique (described earlier in relation to

specific density) to expression 7. Following substitution of the specific densities determined in this investigation,intercrystalline densities of the 1 &l nm and 1W2 nm samples, expressed in terms of crystalline percentreductions, were calculated to be 2.3k1.8 % and 5.1+3.8 %, respectively. All calculations were based upon a grainboundary thickness of lf0.5 nm.

The calculated crystalline density reductions (6%) cannot be assumed to be absolute or without limitations.The proposed intercrystalline theory underlying the derivation of intercrystalline density reduction presented inthis paper assumes two material characteristics and thereby imposes two limitations: (i) negligible vacancy andlattice dislocation content and (ii) negligible porosity content.

Vacancy and lattice dislocation content becomes a concern when it imposes an effect on the free volume ofthe material as a whole. This factor can be eliminated by examining one nanocrystalline grain within ananocrystalline structure. Consider a grain in the form of a tetradaidecahedron with a grain size of 10 nm. If weassume that the surfaces of the tetradaidecahedron are comprised of low angle interfaces, each surface can bedescribed as a collection of dislocations (7). Also, due to the limited size of a 1Onm grain, the probability offinding a dislocation within the grain becomes quite low. Hence, when all sides of the tetradaidecahedron are


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426 DENSITYOF Ni Vol. 32, No. 3

considered, the contribution of intra-granular dislocations becomes minimal. Furthermore, when larger angledboundaries are considered, the intra-dislocation content becomes even less pronounced due to the enhanced freevolume associated with larger angled boundaries. Therefore, nanocrystalline materials can be considered todisplay minimal intra-granular dislocation effects.

However because of the suspected porosity contents in the 18+2 nm grained samples (0.3fO.l %), porositycannot be assumed to be negligible and therefore, the calculated 6% value above tends to be inflated. A moreaccurate 6% value would be 3.613.6 %. Due to the considerable overlap of both nanocrystalline 6% values, the1 +l nm and 18s nm grained materials were chosen to be described by a common 6% value, i.e. 2.3Ifil.S %.

Although the error associated with the calculated 6%++value may appear to cloud the result, it in fact describesthe nature of the intercrystalline component: an intricate network of interfaces connected via triple lines andquadruple nodes with each single defect possessing its own specific structure.

ConclusionsWith the use of Archimedes Principle, a maximum porosity of 0.3?0.1 % was found in the electrodeposited

nanocrystalline nickel studied. Moreover, the intercrystalline density of these materials was calculated to be2.31tl.8 % less than the density of single crystalline nickel. These calculations are believed to be void of anysignificant porosity, intra-granular dislocation and impurity effects.

AcknowledgmentsHaasz would like to thank Mr. N. Wang and Dr. A.A. Haasz for useful discussions dealing with structure theory

and experimental technique. Special thanks are also extended to the Natural Sciences and Engineering ResearchCouncil of Canada for financially supporting this endeavour.

References1. M.J. Aus, B. Szpunar, U. Erb, A.M. El-Sherik, G. Palumbo and K. T. Aust, J. Appl. Phys., 75(7), 3632 (1994).2. A.M. El-Sherik, U. Erb, G. Palumbo and K.T. Aust, Scripta Metall. Mat., 27, 1185 (1992).3. V. Kristic, U. Erb and G. Palumbo, Scripta Metall. Mat., 29, 1501 (1993)4. A.M. El-Sherik and U. Erb, U.S. Patent Pending (1993).5. Ernest 0. Doebelin, Measurement Systems: Application and Design, p. 59, McGraw-Hill Book Company, NewYork (1966).6. G. Palumbo, S.J. Thorpe and K.T. Aust, Scripta Metall. Mat., 24, 1347 (1990).7. Robert E. Reed-Hill, Physical Metallurgy Principles - 2nd Edition, p. 214, PWS-KENT Publishing Company,

Boston (1973).

++A11rrors assigned to variables describing 6% have been chosen to represent a minimum confidence intervalof 80%. Therefore, 6% errors also describe the degree of variation of 6%

[]-'intercrystalline density of nano crystalline nickel' erb

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