1470
Chem. Mater. 2008, 20, 1470–1475
Effect of Gas Adsorption on the Elastic Properties of Faujasite Films Measured Using Magnetoelastic Sensors Theodoros Baimpos,† Ioannis G. Giannakopoulos,† Vladimiros Nikolakis,*,† and Dimitris Kouzoudis*,‡ Foundation for Research and Technology Hellas, Institute of Chemical Engineering and High Temperature Chemical Processes, P.O. Box 1414, GR-26504 Patras, Greece, and Department of Engineering Science, UniVersity of Patras, GR-26500 Patras, Greece ReceiVed September 17, 2007. ReVised Manuscript ReceiVed October 30, 2007
A novel method is presented for measuring the elastic properties of zeolite films using magnetoelastic sensors. The method is relatively simple and can be used to determine the effect of adsorbed molecules on the mechanical properties of the zeolite film. It is demonstrated using NaX films grown on Metglas ribbons. Measurements were taken as a function of temperature (10-40 °C) and gas (CO2, N2, vacuum) for a range of sensors with different faujasite masses (0.25-8.0 mg). For each temperature and gas, the modulus of elasticity of the faujasite crystalline film was extracted by analyzing the measurements as a function of film thickness. For all temperatures examined, it was found that the elastic modulus of the NaX films in vacuum was 38.7 ( 1.7 GPa. On the other hand, in the presence of N2, it varied between ∼24 and ∼38 GPa and, when in contact with CO2, between ∼39 and ∼52 GPa. Our results show for first time that the adsorption of gases in zeolites might induce changes in the elastic properties of the zeolite crystals. Finally, even though this method was applied to a zeolite film, it is quite general, and it can be used for measuring the elastic modulus of any microporous or mesoporous film.
Introduction During the last decades, there has been an increasing interest in developing novel zeolite based applications. Most efforts have been focused on the synthesis of permselective zeolite membranes for the separation of gas or liquid mixtures.1–3 However, there has been a considerable amount of work toward the development of other types of applications4 such as modified electrodes,5 optical devices,6 films of low dielectric constant for the replacement of dense silica in the semiconductor industry,7 and gas sensors.8 A solid knowledge of the zeolite crystal and film mechanical properties would be of great help in the design of novel applications (especially in the case of permselective membranes, gas sensors, and low dielectric constant films) 7,9,10 as well as for the performance improvements in traditional * Corresponding authors. Tel: ++30-2610965242(V.N.) or ++302610996880(D.K.). Fax: ++30-2610965223 (V.N.) or ++30-2610996880 (D.K.). E-mail:
[email protected] (V.N.) or
[email protected] (D.K.). † Institute of Chemical Engineering and High Temperature Chemical Processes. ‡ University of Patras.
(1) Caro, J.; Noack, M.; Kolsch, P.; Schafer, R. Microporous Mesoporous Mater. 2000, 38, 3. (2) Julbe, A. Stud. Surf. Sci. Catal. 2005, 157, 135. (3) Tavolaro, A.; Drioli, E. AdV. Mater. 1999, 11, 975. (4) Bein, T.; Mintova, S. Stud. Surf. Sci. Catal. 2005, 157, 263. (5) Walcarius, A. Anal. Chim. Acta 1999, 384, 1. (6) Schulz-Ekloff, G.; Wohrle, D.; van Dufel, B.; Schoonheydt, R. A. Microporous Mesoporous Mater. 2002, 51, 91. (7) Li, Z.; Johnson, M. C.; Sun, M.; Ryan, E. T.; Earl, D. J.; Maichen, W.; Martin, J. I.; Li, S.; Lew, C. M.; Wang, J.; Deem, M. W.; Davis, M. E.; Yan, Y. Angew. Chem., Int. Ed. 2006, 45, 6329. (8) Xu, X. W.; Wang, J.; Long, Y. C. Sensors 2006, 6, 1751. (9) Jeong, H. K.; Lai, Z.; Tsapatsis, M.; Hanson, J. C. Microporous Mesoporous Mater. 2002, 84, 332. (10) Lassinantti Gualtieri, M.; Andersson, C.; Jareman, F.; Hedlund, J.; Gualtieri, A. F.; Leoni, M.; Meneghini, C. J. Membr. Sci. 2007, 290, 95.
zeolite based applications (i.e., zeolite catalysts).11–13 For example, several recent publications in the field of zeolite membranes have mentioned that the enhancement or loss of the membrane permselective performance might be attributed to changes of the zeolite polycrystalline film mechanical properties as a result of the adsorption of gas or vapor molecules.14,15 Furthermore, the formation of cracks during calcination of MFI membranes has been attributed to the development of thermal stresses between the R-Al2O3 support and the zeolite film.9,10,16,17 These stresses are observed because the membrane support undergoes thermal expansion as the temperature increases while, on the other hand, the MFI crystals contract as a result of the removal of the template.10,16 Finally, a prerequisite for the utilization of zeolite layers as low dielectric constant films in the semiconductor industry is their ability to withstand the chemical and mechanical conditions encountered during the fabrication processes.7 Thus, knowledge of the elastic constants of zeolite films is of great practical interest.
(11) Brabec, L.; Bohac, P.; Stranyanek, M.; Ctvrlik, R.; Kocirik, M. Microporous Mesoporous Mater. 2006, 94, 226. (12) Lin, J.; Shu, X. F.; Dong, J. X. Mater. Lett. 2005, 59, 1595. (13) Wang, Z.; Lambros, J.; Lobo, R. F. J. Mater. Sci. 2002, 37, 2491. (14) O’Brien-Abraham, J.; Kanezashi, M.; Lin, Y. S. Microporous Mesoporous Mater. 2007, 105, 140. (15) Yu, M.; Amundsen, T. J.; Hong, M.; Falconer, J. L.; Noble, R. D. J. Membr. Sci. 2007, 298, 182. (16) Dong, J.; Lin, Y. S.; Hu, M. Z.-C.; Peascoe, R. A.; Payzant, E. A. Microporous Mesoporous Mater. 2000, 34, 241. (17) Geus, E. R.; Bekkum, H. Zeolites 1995, 15, 333. (18) Colligan, M.; Forster, P. M.; Cheetham, A. K.; Lee, Y.; Vogt, T.; Hriljac, J. A. J. Am. Chem. Soc. 2004, 126, 12015.
10.1021/cm7026509 CCC: $40.75 2008 American Chemical Society Published on Web 01/04/2008
Elastic Properties of Faujasite Films
Chem. Mater., Vol. 20, No. 4, 2008 1471
The knowledge of the mechanical properties of zeolites is scarce 7,9,11–13,18–20mainly because of the difficulty of measuring these properties.12 The elastic modulus (frequently also referred as Young modulus) of zeolites can be measured using either mechanical (nanoindentation, microdeformation, or three point bending) 11,12,19,21,22 or spectroscopic methods (Brilllouin or Synchrotron X-ray spectroscopy).10,18,23 Unfortunately, the mechanical methods often require samples having sizes of at least several millimeters that are much larger than the usual maximum size of the synthetic zeolite crystals (100-200 µm). In the case of spectroscopic techniques, the experimental procedures and the analysis of the data are rather complicated. Magnetoelastic materials are usually made of amorphous metallic alloys or composites of rare-earth elements. They have been used in the past to measure the mechanical properties of nonporous silver and aluminum films 24,25 that do not adsorb gases. Recently, Zhang et al.26 have shown that water adsorption in thin SiO2/Pt-TiO2 films deposited on magnetoelastic sensors induces large changes in the effective Young modulus of the composite. A method for eliminating the effect of such changes, from the resonant frequency shifts, has also been proposed. However, the main applications of magnetoelastic materials include oscillators, microactuators, filters, and sensors. Information concerning the principles of operation, as well as the design and application of magnetoelastic sensors, can be found in two review papers by Grimes et al.27,28 and in the references mentioned therein. Sensing applications utilize the inherent property of magnetoelastic materials to vibrate under the influence of an alternating magnetic field. The vibrations result in the generation of both magnetic and acoustic flux, which can be detected by an appropriate microphone or pickup coil. The measured flux passes through a maximum when the strip is vibrating at its resonance frequency, which depends on the strip’s physical dimensions and the material’s properties. Metglas is a common magnetoelastic material that can be set to resonate under an external alternating magnetic field. The following expression for the resonance frequency
E E 1 ≈ (1) 2 2L F F(1 - ν ) is commonly found in the literature 25–29 for a flat ribbon of length L, density F, Young modulus (elastic modulus) E, and Poisson ratio ν. The Poisson ratio is defined as the ratio of f0 )
1 2L
(19) Johnson, M.; Li, Z.; Wang, J.; Yan, Y. Thin Solid Films 2007, 515, 3164. (20) Lethbridge, Z. A. D.; Williams, J. J.; Walton, R. I.; Smith, C. W.; Hooper, R. M.; Evans, K. E. Acta Mater. 2006, 54, 2533. (21) Lethbridge, Z. A. D.; Williams, J. J.; Walton, R. I.; Smith, C. W.; Hooper, R. M.; Evans, K. E. Acta Mater. 2006, 54, 2533. (22) Wang, Z.; Lambros, J.; Lobo, R. F. J. Mater. Sci. 2002, 37, 2491. (23) Sanchez-Valle, C.; Sinogeikin, S. V.; Lethbridge, Z. A. D.; Walton, R. I.; Smith, C. W.; Evans, K. E.; Bass, J. D. J. Appl. Phys. 2005, 98. (24) Schmidt, S.; Grimes, C. A. Sens. Actuators, A 2001, 94, 189. (25) Schmidt, S.; Grimes, C. A. IEEE Trans. Magn. 2001, 37, 2731. (26) Zhang, R.; Tejedor, T. I.; Grimes, C. A.; Anderson, M. A. Anal. Chem. 2007, 79, 7078. (27) Grimes, C. A.; Mungle, C. S.; Zeng, K.; Jain, M. K.; Dreschel, W. R.; Paulose, M.; Ong, K. G. Sensors 2002, 2, 294. (28) Grimes, C. A.; Ong, K. G.; Loiselle, K.; Stoyanov, P. G.; Kouzoudis, D.; Liu, Y.; Tong, C.; Tefiku, F. Smart Mater. Struct. 1999, 8, 639. (29) Stoyanov, P. G.; Grimes, C. A. Sens. Actuators, A 2000, 80, 8.
the lateral (normal to the applied load) and axial strains.30 Usually the (1 - ν2)–1/2 term is ignored for simplicity because it amounts only to a 3% correction of f0 for typical values of ν ) 0.25. In the present publication, a novel method is presented that can measure the elastic properties of zeolite polycrystalline films nondestructively as a function of temperature and gas phase composition. It is also shown for the first time that the adsorption of gases in the zeolite framework can induce changes in the elastic modulus of the zeolite film. The measurements were taken using NaX faujasite-Metglas composite strips that work as remote gas sensors.31 In particular, the resonance frequency of the strips was measured as a function of temperature (10-40 °C) and gas (CO2, N2, vacuum) for a range of sensors with different faujasite masses (0.25-8.0 mg). For each temperature and gas, the modulus of elasticity of the faujasite crystalline film was extracted by analyzing the measurements as a function of the film thickness. Experimental Section Preparation of the Faujasite-Metglas Composite. Zeolite films were synthesized on the surface of 6 mm × 20 mm × 28 µm Metglas 2826MB (Fe40Ni38Mo4B18, F ) 7.43 g/cm3) magnetoelastic strips (Allied Signal) using the following method. Initially, a twostep cleaning procedure was used to remove any contaminants from the surface of the Metglas strip. The strip was first placed in a CH3CCl3 solution that was sonicated for 10 min at 60 °C and then washed with methanol. Finally, after repeating the procedure several times, the strip was dried in an oven at 60 °C for 20 min. An aqueous suspension of NaY (Aldrich) crystals (∼10 g/L) was used to seed both sides of the cleaned strip using a dip coating technique. The seeded strip was then placed at the bottom of a polypropylene bottle and was hydrothermally treated at 85 °C with a gel having a molar composition of 4.17 Na2O:1.0 Al2O3:10 (triethanolamine):1.87 SiO2:460 H2O. This solution is known to result in the nucleation and growth of NaX (F′ ) 1.57 g/cm3) crystals as well as NaX membranes.32 Details about the preparation of the synthesis gel can be found in earlier publications.31 After hydrothermal treatment (20-170 h), the composite was cooled, washed several times with distilled water, and calcined at 280–300 °C for 24 h. Characterization Methods. The zeolite-Metglas composite was characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD patterns were collected on a Siemens D-500 system using Cu KR radiation. SEM images were obtained using a LEO SUPRA 35VP field emission microscope operated at 20 kV in the variable pressure mode. The X-ray spectra of our samples can be found in a previous publication.31 Measurement of Adsorption Isotherms. The adsorption equilibria of CO2 and N2 in the faujasite-Metglas strip were measured as a function of temperature and pressure (0–1 bar) using a VTI GHP-100 gravimetric high pressure sorption analyzer (Rubotherm). The amount of faujasite deposited on the Metglas strip was determined by weighing the strip before and after synthesis. Before the experiments, the samples were degassed for 6 h under vacuum (30) Callister, J. W. D. Materials Science and Engineering. An Introduction, 3rd ed.; Wiley & Sons: New York, 1994. (31) Giannakopoulos, I. G.; Kouzoudis, D.; Grimes, C. A.; Nikolakis, V. AdV. Funct. Mater. 2005, 15, 1165. (32) Nikolakis, V.; Xomeritakis, G.; Abibi, A.; Dickson, M.; Tsapatsis, M.; Vlachos, D. G. J. Membr. Sci. 2001, 184, 209.
1472
Chem. Mater., Vol. 20, No. 4, 2008
Baimpos et al.
Figure 1. Schematic drawing of the apparatus for the sensing experiments.
at 280–300 °C. The individual points of the isotherm were determined twice, first by raising the pressure in incremental steps (adsorption stage) and then by decreasing the pressure in the same way (desorption stage). Sensing Experiments. The resonance frequency of the sensors was measured using a magnetoelastic resonator (Sentech Corp.). The sensor was placed in a glass tube open at both ends. A sensing coil was connected to the magnetoelastic resonator and was wrapped around the cell, which was finally placed in a water bath. The resonator excited the sensor with a time-varying magnetic field and recorded the induced voltage on the detection coil as a result of the response of the sensor. For the measurements in vacuum, one end of the cell was sealed and the other one was connected to a vacuum pump. The pressure was measured using an Omega OM-CP-PRTEMP1000 pressuretemperature recorder. For the experiments in N2 or CO2, two AERA 7700C mass flow controllers were used to regulate the flow rates of CO2 (100%, Aeroscopio) and N2 (grade 5, Messer) through the cell over the sensor. A diagram of the experimental setup used is shown in Figure 1.
Table 1. Characteristics of the Five Faujasite-Metglas Sensors Used in This Study sensor no.
faujasite mass [mg]
faujasite thickness [µm]
composite thickness [µm]
relative thickness h′/h
1 2 3 4 5
0.25 2.5 3.9 5.1 8.0
1.9 12 18.4 22.4 35
29.9 40 46.4 50.4 63
0.067 0.429 0.657 0.8 1.25
It is clear that the faujasite film was grown on both sides of the ribbons. For each sensor, the zeolite film thickness was estimated by subtracting the thickness of the Metglas strip (28 µm) from the thickness of the composite sensor estimated from the SEM images (Table 1).
Results and Discussion The resonance frequency of the bare Metglas ribbons with an average mass of 28 mg was measured to be equal to 113 ( 0.1 kHz. From eq 1 the elastic modulus of the ribbons was calculated as E ) 151 GPa, which is a typical value for Metglas. Equation 1 is of general validity and has been used by many authors in the past27 to calculate mass loads in the microgram range from the measured resonance frequency data. Nominally the two assumptions made are that (a) the modulus E is taken as a constant and (b) the ribbon is stress free, except for the dynamic stresses that are developed during vibrations. Under these assumptions, the resonance frequency in eq 1 is a function of the mass load on the sensor, indirectly through the density F. In the current work, five sensors were made by synthesizing faujasite films of different thicknesses on the 28 mg Metglas ribbons, as shown in Table 1. The mass of each film was calculated from the difference of the ribbon’s weight before and after the synthesis. Figure 2a shows the SEM micrographs of the five sensors, and Figure 2b shows a schematic representation of the layer layout in these sensors.
Figure 2. (a) SEM micrographs of the five sensors and (b) schematic representation of the layer layout in the sensors.
Elastic Properties of Faujasite Films
Chem. Mater., Vol. 20, No. 4, 2008 1473
Figure 4. Adsorption isotherms of CO2 (squares) and N2 (stars) measured in the faujasite-Metglas composite strip. Table 2. Parameters of the CO2 (Eq 3a) and N2 (Eq 3b) Isotherms Figure 3. (a) Variation of sensor no. 3 resonance frequency with temperature when the sensor is placed in vacuum (circles), in N2 (stars), and in CO2 (squares). (b) The corresponding expected curves from eq 2 and the adsorption isotherms, vacuum (solid line), N2 (dashed line), and CO2 (dotted line).
Equation 1 has been derived for a single uniform layer and needs to be modified when three layers are present. Schmidt and Grimes24,25 have derived a corresponding expression for this case (eq 20 therein) that under a minor manipulation leads to
E′′ E + χE′ 1 1 ) (2) 2L F′′ 2L F + χF′ where h, F, and Ε are the thickness, density, and elastic modulus of the Metglas ribbon, the primed variables are the corresponding quantities of the faujasite film, and χ equals h′/h (relative film thickness). The effect of the temperature on the resonance frequency of sensor no. 3 when it is placed in CO2, N2, or vacuum (2.3 mbar) is shown in Figure 3a. The corresponding graphs for the other sensors are provided as Supporting Information. Figure 3b shows the expected curves with the adsorbed mass estimated from the adsorption isotherms (see below) and with E′ calculated from the vacuum data. Obviously there is a discrepancy between part a and part b of Figure 3. The three curves fC, fN, and fV of Figure 3a corresponding to CO2, N2, and vacuum, respectively, show some distinct features: (a) The fV curve is higher than the other three curves. This is in accordance with eq 2 since in vacuum there is no adsorbed mass, and the denominator has its minimum value. Also, the curve shows a weak temperature variation, which indicates that the sensor itself is practically temperature insensitive in the absence of gases, the thermal stresses are small, and the E and E′ depend only slightly on temperature. (b) The fN curve increases with temperature reaching an almost constant value at ∼30 °C. Qualitatively, this behavior is expected because the amount of the adsorbed gas decreases as the temperature increases. Thus, according to eq 2, the resonance frequency should increase with temperature. f)
CO2 N2
qs ) 5.15 [mol (kg of b0 ) 0.00389 faujasite)-1] [kPa-1] K0 ) 0.03897 [mol (kg of faujasite)-1 kPa-1]
-∆H ) 32.44 [kJ/mol] -∆H ) 16.75 [kJ/mol]
(c) The fC curve initially decreases with temperature reaching a minimum at ∼25 °C. Then, it increases again and approaches the fN. It is noteworthy that there is a temperature ∼20 °C below which fC is higher than fN. As seen in Figure 3b, eq 2 cannot predict the measurements assuming a constant E′′ because at all temperatures larger amounts of CO2 than of N2 are adsorbed in the faujasite crystals. The discrepancy between part a and part b of Figure 3 implies either that the elastic modulus E′ of the faujasite crystal changes with adsorption or that mechanical stresses are present in the sensor. As it will be shown later, even if stresses are present in the sensors, their typical magnitudes are not capable of producing the observed deviations. Thus, we are left only with the first possibility. Then, we can work backward and calculate E′ via eq 2 from the measured resonance frequency data for different temperatures and gases. Large changes in the mechanical properties of materials as a result of adsorption is not a new phenomenon, and it can be seen even in every day life situations like, for example, when sheets of paper get wet or sponges adsorb water. However, to the best of our knowledge, such behavior has never been reported for zeolites. To extract E′ from eq 2, the density F′ of the faujasite film has to be calculated at different temperatures. This density is equal to (mFAU + mAD)/VFAU where mFAU and VFAU are correspondingly the mass and the volume of the film and mAD is the mass of the adsorbed gases. To calculate mAD for each gas as a function of temperature, the adsorption isotherms of CO2 and N2 at three temperatures were measured on faujasite-Metglas strips as shown in Figure 4. By fitting the experimental data, the parameters of the
1474
Chem. Mater., Vol. 20, No. 4, 2008
Baimpos et al.
Figure 5. Methodology for measuring the elastic properties of zeolite films using magnetoelastic sensors.
Langmuir isotherm for CO2 and the Henry constant for N2 were determined using -∆H P ( RT ) q)q -∆H 1 + b exp( P RT ) b0 exp
s
(3a)
0
for CO2 and P ( -∆H RT )
q ) K0 exp
(3b)
for N2, where P is the gas partial pressure, R is the gas constant, T the temperature in kelvin, and the parameters qs, b0, ∆H, and K0 are given in Table 2. Then, the adsorbed mass mAD is calculated from mAD ) qmFAU. The degassing temperature (280–300 °C) used is relatively low and might not ensure the complete dehydration of the crystals. However, the estimated heats of adsorption for CO2 and N2 are within the limits of previously reported values,33–38 indicating that the effect of residual water, if any, is very small. The specific choice of the degassing temperature was made because the Curie temperature of Metglas 2826MB is ∼353 °C.39 The procedure used to calculate the elastic modulus of the faujasite film as a function of gas and temperature is shown in Figure 5. The effective density F′′ ) F + χF′ was calculated using the data from the adsorption isotherms and the Metglas density. The effective elastic modulus E′′ ) E + χE′ is (33) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. J. Chem. Eng. Data 2004, 49, 1095. (34) Dunne, J. A.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1996, 12, 5896. (35) Hyun, S. H.; Danner, R. P. J. Chem. Eng. Data 1982, 25, 196. (36) Lee, J. S.; Kim, J. H.; Kim, J. T.; Suh, J. K.; Lee, J. M.; Lee, C. H. J. Chem. Eng. Data 2002, 47, 1237. (37) Park, Y. J.; Lee, S. J.; Moon, J. H.; Choi, D. K.; Lee, C. H. J. Chem. Eng. Data 2006, 51, 1001. (38) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley-Interscience Publishers: New York, 1984. (39) http://www.metglas.com/products/page5_1_2_7.html (accessed Oct 29, 2007).
calculated from eq 2 and, when plotted against χ, should give a straight line with the slope equal to E′ and the intercept equal to E. Figure 6 shows such plots for vacuum, N2, and CO2. Different symbols represent different temperatures, and the straight line is a linear fit to the 20 °C data. The strong linearity of the data is a solid proof of the expression E′′ ) E + χE′ and the validity of eq 2. Table 3 shows the average Metglas elastic modulus that was calculated from the intercepts of Figure 6. The N2 and vacuum values are very close to the measured value of 151 GPa for the bare Metglas ribbon. The CO2 value is lower by about 10 GPa. From the slopes of the plots of Figure 6 at different temperatures the E′ of the faujasite film is calculated and plotted versus temperature in Figure 7. It is clear that the effect of temperature in E′ when the sensor is in vacuum is rather small. The average slope of all the fits for vacuum is E′ ) 38.7 ( 1.7 GPa. Also, the results show that the elastic modulus depends strongly on the adsorbed gas. The values of E′ for vacuum and N2 are almost the same at most temperatures (with the exception of N2 at 10 °C). On the other hand, the E′ for CO2 has a strong temperature dependence. It ranges from 39.1 to 52.4 GPa approximately, with the minimum appearing at 25 °C. The average standard deviations of E′ from linear regressions on Figure 6 are between 5.5 and 7.3. The errors can be attributed mainly to the existence of areas with nonuniform film thickness and to the existence of film defects (pinholes/cracks). As it was shown above, E′ derived from the measurements in vacuum does not show any temperature dependence. Thus, the observed changes must be entirely due to the changes of the mechanical properties of the faujasite crystal as a result of the different amounts of adsorbed CO2. Colligan et al.18 have measured the Young modulus of powder faujasite crystals by applying external pressure and measuring the changes of the unit cell volume from synchrotron powder XRD. Depending on the liquid used to transfer the pressure to the crystals (water/alcohol vs silicon oil) and for pressures less than 4 GPa, they reported that the Young modulus of siliceous faujasite crystals varies between 38 and 64 GPa and that of NaX crystals between 35 and 89 GPa. Despite the totally different methodology used, their reported results are very close to the ones in the current work. The observed changes in the Young modulus can be attributed to the interactions of CO2 within the crystalline framework. Several researchers have argued that when molecules such as CO2 are adsorbed in the Na+ form of faujasite, they result in the movement of the crystal cations to new crystallographic positions.40 Such phenomena are expected to influence the mechanical properties of the zeolite film. It has also been reported that the adsorption of CO2 in faujasite induces deformation of the crystal framework.41 Furthermore, the adsorption induced strain had an extremum at ∼25 °C when the CO2 pressure was 1 atm.41 This is the same temperature where the minimum of E′ occurs in Figure 7. It was mentioned earlier that the mechanical stresses in the sensor are not capable of producing the observed (40) Plant, D. F.; Maurin, G.; Jobic, H.; Llewellyn, P. L. J. Phys. Chem. B 2006, 110, 14372. (41) Pulin, A. L.; Fomkin, A. A.; Sinitsyn, V. A.; Pribylov, A. A. Russ. Chem. Bull. 2001, 50, 60.
Elastic Properties of Faujasite Films
Chem. Mater., Vol. 20, No. 4, 2008 1475
Figure 6. Dependence of the effective modulus of elasticity E′′ with the relative thickness x ) h′/h for the five sensors in (a) vacuum, (b) N2, and (c) CO2 (the lines shown are the linear fits of the 20 °C data). Table 3. Young Modulus of Metglas Calculated from the Composite Sensor under Different Conditionsa E [GPa]
bare Metglas
vacuum
CO2
N2
151
147 (1)
139.3 (1.3)
146.6 (1.1)
a The values shown are the average of all temperatures. The numbers in parentheses are the standard deviations.
It should also be noted that, at all the temperatures examined, cyclic changes of CO2 and N2 were performed, and the resonance frequency data always assumed two distinct values within error. That proves that irreversible adsorption of CO2 does not take place in the faujasite film. Finally, it must be pointed out that the proposed methodology is general, and it can be applied, in principle, to measure the elastic properties not only of zeolite films but also of other microporous materials as well. It might also become a useful tool for measuring differences in the elastic moduli of zeolites along different crystallographic directions. In addition to the scientific merit, such measurements are expected to provide further insight into the understanding of crack formation in zeolite membranes. However, such a study has a prerequisite, the identification of the appropriate conditions and procedures for the synthesis of zeolite films with different orientations on Metglas sensors. Conclusions
Figure 7. Dependence of the faujasite film modulus of elasticity E′ with temperature for vacuum (open circles), CO2 (squares), and N2 (stars) for the small temperature dependence of the vacuum data.
deviations of Figure 6. Dey and Addy42 have treated the case of the reflection and refraction of elastic plane waves on the interface of two media when there is an initial stress S present in the system. They have shown that an extra S term appears in the velocity of the longitudinal waves which in our case leads to a modified resonance frequency of
E″ + S 1 (4) 2L F″ Even for interfacial stresses as high as S ) 100 MPa, the relative change in the resonance frequency ∆f/f is about 5 × 10-4 for a typical value of E′′ ) 100 GPa, an extremely low value. Thus, the effect of internal stresses is negligible for most practical cases. This is in agreement with the analysis of Liu et al.43 who have studied Love waves in a layered piezoelectric structure. However, we cannot completely eliminate the possibility that stresses are developed in the sensor because of adsorption. The existence of such stresses might explain the differences of E in Table 3. f)
(42) Dey, S.; Addy, S. K. Internat. J. Non-Linear Mech. 1977, 12, 371. (43) Liu, H.; Wang, Z. K.; Wang, T. J. Int. J. Solids Struct. 2001, 38, 37.
In this article, a relatively simple nondestructive methodology is proposed for measuring in situ the elastic properties of zeolite films (Figure 5). Following this methodology, we have shown that not only the mass but also the mechanical properties of a faujasite crystal change with gas adsorption. The Young modulus of our faujasite films in vacuum (2.3 mbar) was found to be 38.7 ( 1.7 GPa. When the sensor was in contact with N2, the elastic modulus varied between ∼24 and ∼38 GPa and, when in contact with CO2, between ∼39 and ∼52 GPa. These variations have been observed in the temperature region 10-40 °C. To the best of our knowledge, this is the first time that the effect of gas adsorption on the elastic properties of faujasite films is measured. Acknowledgment. We acknowledge the financial support from the Operational Programme “Competitiveness” of the European Union (75%) and from The General Secretariat for Research and Technology of Hellenic Ministry of Development (25%) through the Project No. 05-NON-EU-152. We also thank Prof. M. Tsapatsis and J.A. Sheffel from the University of Minnesota for providing access to the magnetic microbalance facility. Supporting Information Available: The resonance frequency measurements of sensors 1, 2, 4, and 5 as a function of temperature and environment can be found as Supporting Information (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM7026509
