292 Int. J. Nanotechnology, Vol. 2, No. 3, 2005 Application of the tandard poroimetry method for nanomaterial Y.M. Volfkovich and A.V. Sakar* Porotech Ltd., 100 Cater Avenue, Vaughan, L4L 5Y9 Ontario, Canada E-mail: porotech@magma.com *Correponding author A.A. Volinky Department of Mechanical Engineering, Univerity of South Florida, 4202 E Fowler Ave ENB118, Tampa, FL 33620, USA Fax: (813) 974-3539 E-mail: volinky@eng.uf.edu Webite: http://www.eng.uf.edu/~volinky Abtract: A new method of Standard Poroimetry (MSP) for invetigation of any type of porou material, including oft, frail, amalgamated material, film and powder ha been developed. The method i relatively imple and non-detructive and can be ued for meaurement in a wide range of pore ize from 0.3 nm to 3 10 6 nm. The method i currently being ued to tudy porou material uch a carbon nanotube, membrane, thin film, ceramic, electrode for batterie, paper etc., and can be ued in other application, including nanotechnology. The MSP employ numerou manual operation. In order to avoid them, the Automated Standard Poroimeter (ASP) ha been developed baed on the MSP by POROTECH, Ltd. Keyword: porou tructure; poroimetry; capillary equilibrium; carbon nanotube; automated poroimeter; porou ize ditribution; pecific urface area; electrode; non-detructive method; non-toxicity; non-amalgamation. Reference to thi paper hould be made a follow: Volfkovich, Y.M., Sakar, A.V. and Volinky, A.A. (2005) Application of the tandard poroimetry method for nanomaterial, Int. J. Nanotechnology, Vol. 2, No. 3, pp.292 302. Biographical note: Yury M. Volfkovich i the Chief Scientit at C&T Lab Inc., Toronto, ON, Canada and imultaneouly he i a Profeor, Head of the laboratory at Frumkin Intitute of Electrochemitry of Ruian Academy of Science (FIE), Mocow, Ruia. He obtained hi PhD Degree in 1969 from FIE, thei title: Micro- and Macrokinetic of Electro-oxidation of Methanol on Smooth and Porou Electrode. He obtained hi Dr.Sc. Degree in 1983, thei title: Macrokinetic of Electrochemical Cell with Capillary Membrane and Method of Standard Poroimetry. Hi reearch interet are Method and device development for invetigation of porou tructure. He authored over 300 cientific publication and hold more than 40 patent. Copyright 2005 Indercience Enterprie Ltd.
Application of the tandard poroimetry method for nanomaterial 293 Alexander V. Sakar i a Reearcher at the Porotech Ltd. Company, Canada. He obtained hi PhD Degree in Engineering from the Mocow Power Engineering Intitute, Technical Univerity in 1985. He i an expert in ma- and heat-tranfer. Hi current reearch involve the development of the Method of Standard Poroimetry and device. He tudied a lot of different porou material uch a carbon nano-tube, activated carbon, membrane; including nano-membrane, thin film, ceramic, and electrode for batterie. In 2003 he demontrated the new Automated Poroimeter baed on the Method of Standard Poroimetry. He participated in many conference and exhibit. Alex A. Volinky i an Aitant Profeor at the Univerity of South Florida, Mechanical Engineering Department. PhD thei title (Univerity of Minneota, Material Science, 2000): The Role of Geometry and Platicity in Thin, Ductile Film Adheion. Dr. Volinky held an Engineering Material Senior Staff Member potion at Motorola Proce and Material Characterization Lab prior to joining USF. There, he conducted principal reearch employing XRD, SEM, FIB, FA analytical technique for advanced technologie development. Profeor Volinky reearch interet are thin film proceing, mechanical propertie and characteriation; thin film poroity, adheion and fracture. Profeor Volinky reearch wa recognied by national and international award. 1 Introduction The knowledge of tructural and urface propertie of porou material i important in many application, product and procee, uch a electrochemical device (electrode, eparator, membrane, catalyt, active component in batterie, fuel cell, electrolyer, enor, etc), porou thin film, carbon nanotube, ceramic, powder-metallurgical product, oil, paper, oil- or ga-bearing trata and rock, contruction material, etc. The porou tructure can be characteried by integral or differential curve of pore volume ditribution v. pore radiu (poroimetric curve or porogram). The following method for meauring porogram are well known and documented: mercury poroimetry mercury intruion into a non-wettable porou material [1] mall-angle X-ray cattering [2] Electron, Atomic Force and Tunnel Microcopy; centrifugal poroimetry [3] diplacement of wetting liquid from the pore volume by ga preure [4] capillary condenation [5], and other. Each of thee method ha advantage and limitation. For example, mall-angle X-ray cattering can be ued only for pore radii ranging from 2 nm to 50 nm and often lead to ambiguou reult. Centrifugal poroimetry, optical microcopy and diplacement method are practically uele for r < 10 3 nm. Meaurement by electron microcopy are aociated with difficultie in ample preparation and reult interpretation. The method of capillary condenation can be ued only in the pore ize range from 0.3 nm to 50 nm.
294 Y.M. Volfkovich, A.V. Sakar and A.A. Volinky The method of mercury poroimetry (MMP) provide the widet range of meaurable pore radii (from 2 nm to 10 5 nm). A great diadvantage of thi method i the neceity to apply high preure of mercury (up to thouand of atmophere), which can lead to ample deformation or even detruction, a well a the ditortion of porogram [5 7]. Other drawback of thi method are: mirepreentation of the reult due to amalgamation of mot metal [8] different value of the mercury wetting angle for different material [7] complexity of the equipment and toxicity of mercury. All method of microcopy practically give only half-quantitative information. Mot of the above-mentioned diadvantage are eliminated with the ue of the MSP. In addition, it enable meaurement in a large range of pore ize in different kind of material, including oft or frail material, thin film, a well a material forming amalgam [9 20]. 2 Principle of the method The method i baed on the law of capillary equilibrium. If two (or more) porou bodie partially filled with a wetting liquid are in a tate of capillary equilibrium, then the capillary potential ψ i for thee bodie are: ψ = ψ = ψ ψ (1) 1 2 i, where ψ i i the capillary potential for the ith body. The MSP meaure the dependence of relative liquid content equilibrium, i.e., the liquid volume (V t ) in the tet ample a a function of the liquid volume (V ) in the tandard ample: V = φ( V ). (2) t Prior to the meaurement, the integral liquid ditribution in term of ψ for the tandard ample i etablihed, V = f ( ψ ). (3) From equation (1 3), the liquid ditribution in term of ψ for the tet ample i obtained: V = φ[ f ( ψ)], (4) t with the equilibrium tate being attained by liquid and vapour flow. Thee tranport effect are caued by gradient of the capillary potential formed by: the capillary preure p c and the relative vapour preure of the liquid: p = p /p o (where p deignate the vapour preure of the liquid in the ytem and p o i the aturated vapour preure). The capillary preure can be repreented by the Laplace equation: p = 2σ co θ/ r, (5) c m where σ i the urface tenion of the liquid, θ the wetting angle and r m i the maximum radiu of pore filled with liquid. Uing the value of p c a a capillary potential, one can derive the radial pore ditribution function uing equation (4) and (5):
Application of the tandard poroimetry method for nanomaterial 295 V = φ[ f ( 2σ co θ/ r)] F( r). (6) t Employing a meauring liquid with θ ~ 0 (a fully wetting liquid) from equation (6) one obtain: V = φ[ f ( 2 σ / r)] F( r). (7) t An example of liquid ditribution in two porou bodie i given in Figure 1. Figure 1 Example for determination of pore ize ditribution curve by MSP (1) V v. V t ; (2) pore ize ditribution curve for the tandard ample; and (3) for the tet ample On the left, curve 1 repreent the experimental dependence of the volume V in the tandard ample on the volume V t in the tet ample for different value of V o (V o = V + V t ). On the right, the integral pore ize ditribution curve (pore volume a a function of log r) are hown. Curve 2 i the known curve for the tandard ample. Let u aume, for implicity, that the wetting angle of the liquid for both bodie are equal. For a certain total volume of liquid V o, the volume of liquid in both bodie V and V t are repreented by the coordinate of point C. Thi point correpond to point D on curve 2 and to a certain value r of the maximum radiu of filled pore. In the cae of capillary equilibrium (under the aumption made), the maximum radiu of filled pore in the tet ample will be the ame. A in thi example, the volume of liquid i repreented by point B (the line i drawn at an angle of 45 o ) and the point E i on the pore ize ditribution curve for the tet ample. Thu, by changing the value of the total liquid volume V o, the overall ditribution curve 3 for the tet ample can be determined. The pecific urface area (S), which i an important parameter of the porou tructure, can be obtained from the integral pore radiu ditribution curve by uing:
296 Y.M. Volfkovich, A.V. Sakar and A.A. Volinky S = 2 (1/ r)(d V/d r)d r. (8) 0 3 Experimental The amount of liquid in the ample i determined by weighing. Hydrocarbon (octane, decane, etc.) are uually ued a a working liquid, which completely wet mot of the material (θ ~ 0 o ). Thu, the aumption of equal wetting angle i fulfilled. In ome cae, other liquid are ued, e.g., water. The porou tandard and the tet ample are prepared in the hape of dic 0.1 3 mm thick. The tandard are wahed, dried and weighed. Then, they are filled with the liquid (under vacuum). The tack of dic i aembled in a pecial clamping device (Figure 2), in which they are tightly attached to each other for attainment of capillary equilibrium. In the cae of eaily compreible (oft) material, the preure mut be controlled and the clamping device allow it. In the aembly, the tet ample (1) i uually placed between two tandard ample (2). A mall portion of the liquid i evaporated off thi aembly through an open urface (3) by heating and/or vacuum treatment with the flow of a dry inert ga. When a certain amount of liquid i removed, the open urface of the ample i hermetically covered and left unditurbed for a certain time (1 30 minute), allowing a new capillary equilibrium to be etablihed. Subequently, the tack i diaembled; the ample are placed into individual vial and weighed. Then, the tack i reaembled and all operation are repeated everal time until the liquid from the tet ample i completely evaporated. Figure 2 Clamping device aembly: (1) tet ample; (2) tandard ample; (3) open tack urface Other poibility for changing the total volume of the liquid i by repeated contact between the tet ample and tandard ample containing different amount of liquid. The attainment of capillary equilibrium can be controlled by uing two tandard ample, one of which i placed at the open urface of the tack (where the liquid evaporate) and the other one at the cloed urface. If during the experiment the amount of liquid in both tandard ample correpond to their known ditribution curve, then equilibrium i etablihed throughout the whole tack. In the cae of ample with mall pore (<10 nm), the tack diaembly i performed in a dry box to prevent adorption of
Application of the tandard poroimetry method for nanomaterial 297 humidity from the air. When ample with large pore (>10 4 nm) are analyed, the tack i diaembled in a box aturated with the meauring liquid vapour to avoid drying of the tack. Uing thi method, porogram can be meaured for powdered material a well. Thee material are placed between two heet of filter paper. Other ample of filter paper with no powder are alo aembled in a tack. The pore volume ditribution curve for the powder can be determined by ubtracting the porogram of the empty filter paper ample from the one of the tet ample. Capillary equilibrium between different porou ample can alo be etablihed through a vapour phae (without direct contact with the ample), the o-called method of contactle tandard poroimetry. However, in thi cae, the time for etablihing equilibrium increae coniderably. The pore ize ditribution of the tandard ample i determined by other method, e.g., by mercury poroimetry. The material ued mut be ufficiently hard and mut not be deformed and amalgamated by mercury, uch a nickel or ceramic material. The tandard ample mut poe ufficiently high pore volume in the pore ize range of interet to allow accurate meaurement of the ma increae during flooding of thee pore. The time needed for the meaurement of a porogram depend on the pore propertie of the ample (which determine the time of etablihing an equilibrium) and varie between 2 and 10 hour. 4 Reult and dicuion MSP with appropriate tandard ample can be ued for meaurement of pore ize in the range from 0.3 nm to 3 10 6 nm. The accuracy of MSP depend primarily on the accuracy of meauring the pore ize ditribution curve for the tandard ample. The accuracy of the method of Mercury Poroimetry (MMP) i about 1% of the total pore volume. A hown by our previou experiment, under uitable condition, the error (non-reproducibility) of the MSP i <1%. The enitivity of the method i illutrated in Figure 3, which repreent the reult for everal practically monolithic ample of carbonate rock (poroity <1%). Related to the total volume of the ample, the enitivity in thi cae wa 0.05 0.07% [19]. Figure 3 Integral porogram for four different carbonate rock from the Vuktyl region [19]
298 Y.M. Volfkovich, A.V. Sakar and A.A. Volinky Porogram obtained by MSP and MMP for different type of electrode are hown in Figure 4. The reult obtained by thee two method how good agreement, although uch a good concurrence i poible only for ufficiently firm and not amalgamated ample. Figure 4 Integral porogram obtained by MSP and MMP for porou electrode prepared from a mixture of carbonyl and Raney nickel (1), carbonyl nickel (2) and titanium (3) [18]. Here, V/V o i the relative pore volume, where V i the pore volume and V o the total volume of the tet ample Sometime MMP i ued for examining ample with low mechanical trength [6,7]. To reveal the influence of high mercury preure, the reult obtained by MMP and MSP were compared (Figure 5). A an example, differential porogram are hown for a fibrou gla battery eparator. It i een that the change in MMP curve take place due to ample deformation. The poition of the maxima on thee curve are different. Figure 5 Differential porogram obtained by MSP (1) and MMP (2) for a gla eparator A imilar picture (Figure 6) wa obtained after meauring a Pb-electrode porou tructure by both method (MMP and MSP). The majority of metal, including Pb, are prone to amalgamation, which explain the ditinction in thi cae. Thu, ММР doe not alway give a correct reult, a oppoed to MSP.
Application of the tandard poroimetry method for nanomaterial 299 Figure 6 Differential porogram obtained by MSP (1) and MMP (2) for Pb battery electrode The method of tandard poroimetry allow meauring the ample poroity at fixed compreion level, i.e., under condition in which they are commonly ued in variou device. Porogram at different compreion level reveal additional information of their propertie and porou tructure. It hould be alo noted that MSP allow repetitive meaurement of the ame ample at different external loading condition. Indeed, even for oft or frail material, there i no deteriorative influence of the meaurement on the ample (in contrat to MMP) and after drying up the latter fully retain it previou tructure. We have applied MSP to meaure the porou tructure of carbon nanotube powder. Integral porogram of two type of carbon nanotube powder are hown in Figure 7. The reult of mathematical data proceing, obtained with the Porotech TM pecial oftware, are given in Figure 8 and 9. After the pecial treatment of carbon nanotube, the pecific urface area wa increaed (Figure 9), in pite of overall decreaed poroity (Figure 7). Thi happened due to increaed volume of nanopore in the material and decreaed volume of large pore (Figure 8). Thu, MSP allow fixing all tructural change of porou material in a wide range of pore ize, while giving concrete technological recommendation. In thi cae, treatment wa ueful from the tandpoint of a pecific urface area. Figure 7 Integral porogram obtained by MSP for two type of carbon nanotube powder
300 Y.M. Volfkovich, A.V. Sakar and A.A. Volinky Figure 8 Differential porogram obtained by MSP for two type of carbon nanotube powder Figure 9 Ditribution of pecific urface area of the pore radiu for two type of functional carbon nanotube powder 5 Automated tandard poroimeter In order to avoid numerou manual operation (multiple aembly and diaembly of tack, weighing of the individual tandard and ample), the Automated Standard Poroimeter (ASP), baed on the Method of Standard Poroimetry wa developed (Figure 10). Figure 10 Typical automated tandard poroimeter (manufactured by POROTECH, Ltd.)
Application of the tandard poroimetry method for nanomaterial 301 The Automated Standard Poroimeter conit of a clamping device for making contact between the tandard and the ample(), ample holder and an automatic manipulator for moving ample, drying device, a well a an automatic flow-meter valve for drying ga, a weighing tation and a computer. 6 Concluion A poroimetric method ha been developed, which allow tudying all type of porou material, including oft and frail material, a well a powder. Thi method i nondetructive and quantitative, allowing repetitive meaurement of the ame ample. The method can be ued for a wide range of pore ize from 0.3 nm to 3 10 6 nm, which make it pecifically attractive in nanotechnology application. The method of tandard poroimetry i now widely ued in many cientific and indutrial field, including nanotechnology. Reference 1 Drake, I.C. (1949) Pore-ize ditribution in porou material, Ind. Eng. Chem., Vol. 41, No. 4, pp.780 785. 2 Dubinin, M.M. and Plavnik, G.M. (1968) Microporou tructure of carbonaceou adorbent, Carbon, Vol. 6, No. 2, pp.183 192. 3 Miklo, S. and Pohl, A. (1970) Application of centrifugal poroimetry, Bergakademie, Vol. 22, pp.97 116. 4 Svata, M. and Janta, I. (1965) Method of poroimetry baed on diplacement of wetting liquid from pore by ga preure, Coll. Czech. Chem. Commun., Vol. 30, pp.2455 2463. 5 Gregg, S.J. and Sing, K.S.W. (1967) Adorption, Surface Area and Poroity, Academic Pre, New York. 6 Roi, G. and Uai, G. (1970) Deformadion of porou tructure during mercury poroimetry, Quad. Ind. Chim. Ital., Vol. 6, pp.134 148. 7 Sarakhov, A.I. (1963) About accuracy of mercury poroimetry, Zhurn. Fyz. Khimii, Vol. 37, pp.465 467. 8 Gave (1969) Corroion and Wettability of Metal by Mercury, Nauka Khimiya Publ., Mocow, in Ruian, p.208. 9 Volfkovich, Y.M. and Bagotzky, V.S. (1994) The method of tandard poroimetry: 1. Principle and poibilitie and The method of tandard poroimetry 2. Invetigation of the formation of porou tructure, J. Power Source, Vol. 48, No. 3, pp.327 338 and pp.339 348. 10 Volfkovich, Y.M., Luzhin, V.E., Vayulin, A.N., Shkolnikov, E.I. and Blinov, I.A. (1984) Application of the tandard poroimetry method for invetigation of ion-exchange membrane porou tructure, Elektrokhimiya, Vol. 20, pp.656 664. 11 Volfkovich, Y.M. and Soenkin, V.E. (1978) Relation between the electrochemical and capillary characteritic of electrochemical cell with capillary membrane, Elektrokhimiya, Vol. 14, pp.652 664. 12 Volfkovich, Y.M., Levi, M.D., Zolotova, T.K. and Piarevkaya, E.Y. (1993) Porou tructure of electroyntheized poly (p-phenylene) film characterized by the tandard poroimetry technique, Polymer Communication, Vol. 34, pp.2443 2455.
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