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Permeability and thermal conductivity of porous metallic matrix hydride compacts
Journal of the Less-Common
Metals, 153 (1989)
PERMEABILITY AND THERMAL CONDUCTIVITY METALLIC MATRIX HYDRIDE COMPACTS E. BERSHADSKY,
Y. JOSEPHY
August
6, 1988;
OF POROUS
and M. RON
Department of Materials Engineering, Haifa (Israel) (Received
65
65 - 78
Technion-Israel Institute of Technology,
in revised form December
6, 1988)
Summary PMH compacts with various contents of aluminium matrix were prepared by a method including a final step of sintering under high hydrogen pressure. The gas permeability was determined as a function of the weight fraction of the aluminium matrix, f. A correlation between the microstructure, morphology and porosity of the PMH compacts and the coefficient of permeability (x’ was found. A tridimensional aluminium matrix was found to form in compacts having fractional aluminium contents in the range 0.15 - 0.22, and to be followed by a steep decrease of c!. On the other hand, the effective thermal conductivity k eff of the PMH compacts is known to increase with the increasing aluminium content. Major requirements from PMH compacts are high hydrogen and thermal yields, which are contingent upon high heat and mass transfer rates prevailing simultaneously. As the heat and mass transfer rates depend on keff and (Y’ respectively, it is concluded that these quantities must be optimized in a range where keff as well a’ are sufficiently high. The permeability results were interpreted also in terms of dimensionless parameters, such as the friction factor Cf and Reynolds’ number Re. From the results it is seen that the transition from the D’Arcy region to the highvelocity region is quite smooth and gradual as expected of porous materials.
1. Introduction Many applications of metal hydrides for hydrogen energy storage require high hydrogen flow rates. These are contingent upon rapid kinetics, high heat and high Hz mass transfer rates. It has been concluded that the poor heat transfer of a container filled with a hydride powder bed is mainly due to the low effective thermal conductivity keff of the bed [l, 21. Likewise, the H, mass transfer can be substantially hindered by selfdensification and clogging of fine powder metal hydride beds. In order to relieve the heat and mass transfer problems, we have suggested converting powder metal hydrides into composite compacts 0022-5088/89/$3.50
@ Elsevier
Sequoia/Printed
in The Netherlands
66
dubbed PMH (porous metallic-matrix hydrides) [3 - 61. The PMH compacts have a number of advantages in comparison to powder metal hydride beds. (i) The compacts have an effective thermal conductivity keff increased by a factor of 10 - 50 in comparison to the fine powder of the same metal hydride [ 71. (ii) They form a stable structure of constant porosity. (iii) The handling of large amounts of compacts is easier than that of equivalent amounts of fine powder. The microstructure of the PMH compacts consists of a sufficiently porous, metallic tridimensional matrix with incorporated metal hydride particles. The PMH compacts were produced by admixing fine powders of a metal and of a metal hydride and consolidating the mixture by methods of powder compaction. The process of preparation of PMH compacts is accomplished by a step of sintering at an elevated temperature T, under a pressure of hydrogen exceeding P,,(T,) - the equilibrium pressure at the sintering temperature [ 6,7]. By this process a porous structure is produced possessing considerable strength and stability upon cyclic hydrogen sorption. The thermal conductivity keff of powder metal hydride beds, as well as that of PMH compacts, has been the subject of several studies [ 2, 7 - 91. The dependence of k eff on the porosity is commonly expressed by a factor 1 - E, where E is the fractional porosity [7]. Only a few studies have been devoted to the Hz mass transfer of a metal hydride bed and its dependence on the porosity, density or morphology of the bed [9 - 111. Groll and Nonnenmacher [lo] have studied the dependence of the H, flow on the packing density of a bed of LaNisH, in a tubular shape reactor. Eaton et al. have studied various compacts for their mass transfer [ 91. The hydrogen and thermal yields of PMH compacts with an aluminium matrix have been studied by Josephy et al. [7]. They found that increasing weight fraction of the aluminium matrix up to f = 0.18 caused the hydrogen flow rates to increase, whereas further increasing of f resulted in a decrease of the flow rate. It was suggested that raising the amount of aluminium matrix increases the thermal conductivity keff but concurrently lowers the porosity of the compacts, which results in increased resistance to the hydrogen flow. In the present investigation, the permeability was determined for a series of LaNi,H, PMH compacts, with various contents of aluminium matrix. A correlation between the microstructure, morphology and porosity of the PMH compacts and the coefficient of permeability CX’was found. Also the viscous and inertial resistance coefficients were determined as functions of the open porosity fraction. The behaviour of the friction factor Ct us. Reynolds’ number is shown to be smooth in the transition range between low and high flow velocities. 2. Experimental
procedures
The starting materials used were aluminium powder and LaNis alloy. The alloy in the’form of chunks was supplied by Ergenics Co., U.S.A. The
67
aluminium powder was supplied by Alcoa Co. under the trade mark Alcoa # 1401, and a nominal size of 6 - 9 pm. The powder was produced by atomizing and was of 99.5% purity. The preparation of the PMH samples consisted of the following main steps. (a) grinding the LaNi, to a fine powder; (b) mixing of LaNi, and aluminium powders; (c) activating the metal hydride powder; (d) cold compaction; (e) high-hydrogen-pressure sintering. During the grinding and mixing the powders were immersed in a protective liquid. The final product was kept in a protective liquid. The LaNi, was ground in a mortar-type grinder to fine powder. A particle size distribution histogram for the LaN& powder is shown along with one for the aluminium powder in Fig. 1. An SEM micrograph of the two powders is shown in Figs. 2(a) and 2(b). The aluminium particles are seen to have a rounded shape whereas the LaNi, particles have rather sharp edges and faces characteristic of brittle crystalline materials. The two powders were mixed together within the mortar grinder. The LaNi, was activated by carrying out of about 30 hydrogen sorption cycles on the powder mixture under hydrogen pressures from sub-atmospheric pressure up to 30 atm and at temperatures from 25 to 70 “C. The preparation process has been described elsewhere [6,7]. A PMH compact of LaNi, with aluminium in the form of a cylindrical pellet is shown in Fig. 3. Testing of PMH compacts for mechanical stability was performed in specially designed reactors with transparent windows. The reactors were connected to a high-pressure vacuum apparatus and
1
9
12 15 18 21 24
PARTICLE SIZE RANGE (microns1
Fig. 1. Particle volume distribution of the LaNi5 and Al powders utilized for the preparation of PMH compacts.
(4
(b)
Fig. 2. Scanning electron micrograph of particles of (a) LaNi5 and (b) Al.
Fig. 3. Photograph
of (a) “green” and (b) sintered PMH compacts.
periodically charged and discharged with hydrogen. The formation of cracks or disintegration of the compacts was followed through the transparent windows. PMH compacts have been tested for over 33 000 cycles and found stable [ 111. Specimens of the series prepared in this investigation were tested for 20 000 cycles and only a few transverse cracks were detected. The durability of the PMH compacts, described above, is remarkable in view of the fact that the metal hydride component expands and contracts by ~20% in volume upon each sorption cycle. The measured density pmeas was determined by a gravimetric method of high precision [ 123. The PMH total porosity et was evaluated by means of the density difference E = Pcalc - Pmeas t Pcalc
(1)
where pCalC is the PMH density calculated from the known bulk densities of LaNi, and aluminium under the assumption of zero porosity. by immersing the comThe PMH open porosity E,, was determined pacts
in benzoyl
alcohol
and
evacuating
the
container.
The
low-surface-
69
OUT
Sample
Fig. 4. Schematic presentation of PMH compacts.
Flowmeter
of the apparatus for the measurement
of permeability
BakeWe,,? P.M.H. Compact
Fig. 5. Sample for permeability diameter = 11 mm.
measurement
embedded
in bakelite holder. Sample
tension and low-viscosity liquid penetrated into the open pores after air and other gases have bubbled out. Then the change in the weight of the sample was precisely determined. The determination of the coefficient of permeability CX’was carried out by using an apparatus schematically shown in Fig. 4 and specially prepared specimens as the one shown in Fig. 5. The specimens were prepared by placing PMH compacts into cylinders filled with a bakelite powder, which was subsequently polymerized, and the two parallel surfaces were machined. Steady state flow conditions were preserved by keeping Pi, and Pout constant. Helium gas was used for most of the permeability measurements. The flow velocities used in this investigation did not exceed 0.8 1 min-‘. A parabolic equation appropriate to describe the gas flow through the porous structure in the longitudinal direction was used [ 131 Pin2
-
Pout 2 = Ml&
+
M2Q2
(2)
with
and
where Pi, and Pout are the upstream and downstream pressures; cx is the viscous resistance constant; /3 is the inertial resistance constant; Q is the volume flow rate; L and A are the specimen length and cross-sectional area;
70
p is the gas viscosity; M is the molecular weight of the gas; pS is the gas density; and g, is the units conversion factor. The parameter known as permeability (a’) is determined in the range of low flow rates by D’Arcy’s equation and corresponds to (Y’= l/a. The values of (Y’ are average values obtained from measurement of 3 - 4 samples with standard deviation from 5 X lo-l2 cm2 for samples with low aluminium content to 0.7 X lo-l2 cm2 for samples with high aluminium content. The thermal conductivity measurements were carried out by using the comparative method in a vacuum chamber [ 14,161. The apparatus was designed and built in our laboratory. The standard deviations obtained for . . heff are marked m Fig. 17.
3. Results and discussion 3.1. Morphology, porosity and microstructure Compacts made of aluminium powder of 6 - 9 pm average particle sizes were prepared by cold compaction in order to examine its sinterability. The density of the aluminium compacts pcomp is shown in Fig. 6 us. cold compaction pressure PC,. The density of the aluminium compacts is seen to change from 0.975 to 0.985 of the bulk density, PA1= 2.7 g cmp3, with P,, changing in the range 5 ton crne2
I
lb
COLD COM?ACTION PREsSuRe. Pee [ton/cm21
Fig. 6. Density of compacted ium powder diameter 6 - 9 pm.
aluminium us. pressure of compaction.
Initial alumin-
71
Fig. 7. Effective density vs. pressure of cold compaction for PMH compacts in “green product” condition of various Al contents: *, 12 wt.% Al; A, 15 wt.% Al; q,18 wt.% Al; n, 25 wt.% Al. Fig. 8. Porosity vs. weight fraction of matrix of LaNiS with Al matrix PMH compacts; n, cold compacted; 0, sintered.
product condition) is shown in Fig. 7. The dependence of the density on PC, of these composite compacts is substantially different from that of the single component compacted aluminium. For a given weight fraction of aluminium f the measured density, pcomp, increases quite linearly with PC,, while for a given P,,, pcomp decreases with increasing f. The measured density pcomp depends not only on the relative fractions of the constituents (hydride and metal) but also on the fraction of porosity. For compacts cold-compacted at P,, = 10 ton cm-’ and for subsequently sintered compacts the total porosity fraction et is shown in Fig. 8. A substantial increase in porosity is seen to result from the high hydrogen pressure sintering process. The porosity increment, upon the high-hydrogenpressure sintering process, was found to be approximately 0.2V,/(l + 0.2VJ, where VA is the volume fraction of alloy LaNi,. Measured values of the total porosity of a sintered PMH and values calculated as a sum of et for the green product and the increment upon sintering are given in Table 1. As seen from Table 1, the measured values compare well with the calculated ones. The open porosity E,~ and the total porosity et were determined (see experimental procedures) and plotted US. PC, in Fig. 9 for compacts containing 15 wt.% of aluminium matrix, in the “green” product condition. The total (et), open (e,,) and closed (e,r) porosities plotted us. the weight fraction of the aluminium matrix f are given in Fig. 10, for a sintered PMH compact compacted with PC, = 10 ton cmp2. The open porosity, which is instrumental in the H, mass transfer, is seen to be at an almost constant
72 TABLE 1 Measured and calculated with Al matrix Volume fraction of LaNiS VA
values of the total porosity et for PMH compacts
Measured Et for green product
of LaNi,
Sintered compacts Calculated increment to total porosity et o.zv,
Total porosity Et Calculated
Me~ured
0.275 0.28 0.249 0.239 0.214 0.195
0.324 0.255 0.251 0.283 0.180 0.175
1. + 0.2VA 0.60 0.57 0.563 0.521 0.433 0.382
0.175 0.185 0.155 0.152 0.142 0.131
0.1 0.095 0.094 0.087 0.072 0.064
Fig. 9. The dependence of the total and open porosity of “green” PMH compacts LaNis with a 15 wt.% aluminium matrix on the compaction pressure.
of
Fig. 10. The dependence of the total (A) and open porosity (A) of a sintered PMH compact of LaNis and aluminium matrix on the Al content.
value of aO.22 for aluminium weight fractions of f < 0.18 and to fall off to a value of ~0.15 for f > 0.24. The particle volume distributions in Fig. 1 are seen to differ to some extent from each other. A micrograph of a compact of La& with 18 wt.%, in the “green” product condition is shown in Fig. 11. The microstructure is seen to be quite non-uniform. The non-uniformity of the microstructure seems to be related to imperfect admixing of the two powders rather than to the difference in their particle volume distributions.
73
Fig. 11. Metallographic cross-section of a compact of LaNis with 18 wt.% of the aluminium matrix. The light phase with a smooth surface is the LaNis, the light phase with pits on its surface is aluminium, while the dark phase represents the pores.
An SEM micrograph of a fracture surface of a PMH compact of LaNi, with 18 wt.% Al matrix, in sintered condition is shown in Fig. 12(a). An electron backscattering micrograph of the same surface is shown in Fig. 12(b). The two pictures show an area of a consolidated aluminium matrix in which indentations produced by hydride particles, which fell out upon the fracture of the compacts are clearly seen. All in all, it is evident that a tridimensional aluminium matrix forms in the range of aluminium contents of 0.15 < f< 0.22, followed by a decrease in the open porosity. 3.2. Permeability and thermal conductivity The permeability of the PMH compacts was determined as described in the previous section (experimental results) and a plot of (Pin2 - Pout*)/2 vs.
Fig. 12. (a) Scanning electron micrograph of fracture surface of a LaNis PMH compact with 18 wt.% aluminium matrix: (b) electron backscattering micrograph of the same surface: aluminium, dark phase; LaNis, light phase.
FLOW VEELOCITY.
”
~om,rsc,
a
Fig. 13. Graphs of (Pin* -P ,,ut*)/2 us. the gas flow velocity LJthrough PMH compacts of various Al contents: *, 12 wt.% Al; A, 13.5 wt.% Al; 0, 15 wt.% Al; n, 18 wt.% Al; 0, 25 wt.% Al. Fig. 14. The variation of the permeability coefficient (Y’ of sintered PMH compacts with the fraction of Al matrix.
the flow velocity is shown in Fig. 13 for a number of sintered PMH compacts. The corresponding permeability coefficient a’ is plotted us. the weight fraction of aluminium in Fig. 14. The permeability ~11’is seen to decrease steeply in the range of 0.15 < f < 0.22 in which the aluminium matrix forms. A correlation was found between the permeability coefficient CY’and the fraction of open porosity, E,~ as shown in Fig. 15, holding c! = AE,,,~
(3)
where A is a constant. The difference (Pin* -Pout 2)/2 is plotted us. the flow velocity in Fig. 16, for a number of 15 wt.% aluminium matrix compacts in various conditions: (i) a “green product” condition, measured with He gas; (ii) a sintered condition, measured with He gas; (iii) a sintered condition, measured with H2 gas. The corresponding permeabihties are marked in Fig. 16. The permeability measured with H, gas is lower than the one measured with He gas. The reason for this is that the LaNis particles absorb hydrogen, transform to hydrides and increase their volume, thereby reducing the porosity of the compact. The dependence of the thermal conductivity ketf of porous compacts on the thermal conductivities of its constituents is quite complex. It has been found that, for a series of PMH compacts, keff can be fitted to an
Fig. 15. The variation of the permeability porosity Cop.
coefficient
Q! with the fraction
of open
Fig. 16. Graph of (Pin2 -P 0ut2)/2 us. the gas flow velocity u for “green” (A), sintered (o), and sintered and hydrided (0) PMH compacts of La& with 15 wt.% of Al matrix.
empirical [16] :
formula,
which
is a modification
+ k2 1+
v-2 B,V*2 1
[15]
of the one used by Tye
(4)
where k, and k, are Et = 0 extrapolated values of the thermal conductivity of Al and LaNis compacts respectively; V, and I$, are the fractional volume of the Al and LaNiS respectively, with V, + V, = 1; and B, and B, are constants appropriately adjusted for each series of experimental measurements. For the curve shown in Fig, 17, B, = 60 and B, = 9. The constants B, and B2 represent the effect of the nature of the interfacial contact areas, which cannot be explicitly accounted for. Measured values of keff and (Y’are plotted us. the aluminium content in Fig. 17 for a series of LaNi, H,-aluminium matrix PMH compacts. As seen in Fig. 17, for obtaining concurrently high heat and H, mass transfer rates, values of cy’ and keff must be optimized and lie in the transition region of ~15-22 wt.% aluminium. Figure 18 shows the variation of the viscous and inertial resistance coefficients (Yand /3, with the fraction of open porosity, E,,. The corresponding relationships are ar = A1~,p-3
(5)
and fi = A2~,p-4.2
(6)
76
Fig. 17. The variation of the permeability and the thermal conductivity coefficients with the volume fraction of AI matrix for PMH compacts of LaNiS. Fig. 18. The variation of the viscous and inertial resistance coefficients a! and p with the fraction of open porosity.
where A, and A2 axe constants. It may be of use to describe the Pin2 - Pout2 vs. gas flow velocity by means of dimensionless parameters as follows [ 131 Cf, the friction factor, is given by
where u is the flow velocity (u = Q/A); Re, the Reynolds’ number, is given by Re=& o/J Cf is then given by 2 Cr= -+2 Re
(8)
(9)
The first term (Z/Re) corresponds to the D’Arcy regime of low flow velocities, while the second represents the regime of inertial high flow velocities. A plot of the friction factor Cf us. Re is shown in Fig. 19. The fit of the experimentally determined data points, to the curve, calculated co~esponding to eqn. (9), is seen to be quite good. The gradual transition from the low-velocity D’Arcy region, to the highvelocity inertial region, is typical of flow through porous structures.
77
Fig. 19. The friction factor Cf us. the Reynolds’ numbers for PMH compacts of various Al contents: A, 12 wt.% Al;., 13.5 wt.% Al; 0, 15 wt.% Al; m, X8 wt.% Al; 0, 25 wt.% Al.
4. Summary
and conclusions
A series of PMH compacts of LaNis with aluminium matrices were produced_with varying fractional contents of aluminium. The compacts were tested and found to be mechanically stable upon about 20 000 hydrogen sorption cycles in spite of the expansion in volume of about 20% of the LaNi, H, component with each cycle. The sintering process at high hydrogen pressure produces additional porosity, which renders the compact able to accommodate the volume expansion of the LaNisH, component and to be permeable to hydrogen gas flow. The total and open porosities, et and E,,~ respectively, were determined as a function of the ~um~ium matrix fractional content f_ Both et and E,,~ decrease with increasing f. The permeability factor or was determined as a function of f. cr’ is a gas mass transfer characteristic of a porous material, and its value was found to fall off quite steeply in the range of 0.16 < fG 0.20. Metallographic studies show that in that range of f a trid~ensional aluminium matrix forms upon the compaction process. A correlation between E,~ and QI’was found in the form of: o’ Q: eop3, The permeability results were interpreted also in terms of dimensionless parameters, such as the friction factor Cf and the Reynolds’ number Re. From the results plotted in Fig. 19, it is seen that the transition from the D’Arcy region to the high-velocity region is quite smooth and gradual as expected of porous materials. The effective thermal conductivity of the PMH compacts is quite well described by an empirical formula (4) in which the effect of porosity is expressed by the factor 1 - et. In contrary to the increase of the permeabil-
18
ity cu’ with the open porosity, the thermal conductivity lZeif decreases with increasing porosity (open or total). Major requirements from PMH compacts are high hydrogen and thermal yields, which are contingent upon high heat and high mass transfer rates prevailing simultaneously. As the heat-and mass transfer rates depend on keri and (Y’ respectively, these two quantities must be optimized in the range 0.16 < f < 0.20 in which neither heff nor o! are too small. Acknowledgments This research was sponsored by N.C.R.D., Israel and the Kernforschungsanlage Jiilich, F.R.G. The authors are indebted to Mr. E. Levy for his technical assistance. References 1 M. Ron and M. Elemelech, Proc. Znt. Symp. on Hydrides for Hydrogen Energy Storage, Norway, August 77, Pergamon, Oxford, 1978, pp. 417 - 430. 2 S. Suda, N. Kobayashi, K. Yoshida, Y. Iahido and S. Ono, J. Less-Common Met., 74 (1980) 127. 3 M. Ron, 11 th Zntersociety Energy Conversion Engineering Conf., State Line, Nevada, 1976;p. 954. 4 M. Ron, D. M. Gruen, M. H. Mendelsohn and I. Sheft, J. Less-Common Met., 74 (1980) 445. 5 M. Ron, D. M. Gruen, M. H. Mendeisohn and I. Sheft, U.S. Patent 4,292,265 (1981). 6 M. Ron, U.S. Patent 4,507,263 (1985). 7 Y. Josephy, Y. Eisenberg, S. Perez, A. Ben-David and M. Ron, J. Less-Common Met., 104 (1984) 297. 8 E. Suissa, I. Jacob and Z. Hadari, J. Less-Common Met., 104 (1984) 287. 9 E. E. Eton, C. E. Olsen, M. Sheinberg and W. A. Steyert, Znt. J. Hydrogen Energy, 6 (1981) 609. 10 M. Groll and Nonnenmacher, Proc. 17th Zntersociety Energy Conversion Engineering Conf., Los Angeles, CA, August 8 - 12,1982, p. 1185. 11 E. Bershadsky, Y. Josephy and M. Ron, 2. Phys. Chem. N.F., to be published. 12 K. H. Moer, Znt. J. Pow. Metal2 and Pow. Tech. 15 (1979) 33. 13 L. Green Jr. and P. Duwez, J. Appl. Mech, March 1951, p. 39. 14 W. Neumann, High Temp.-High Press, 13 (1981) 687. 15 E. Bershadsky, Y. Josephy and M. Ron, to be published. 16 P. P. Tye, ASME publication 73-HT-47 American Society of Mechanical Engineers, New York, 1973.
Metals, 153 (1989)
PERMEABILITY AND THERMAL CONDUCTIVITY METALLIC MATRIX HYDRIDE COMPACTS E. BERSHADSKY,
Y. JOSEPHY
August
6, 1988;
OF POROUS
and M. RON
Department of Materials Engineering, Haifa (Israel) (Received
65
65 - 78
Technion-Israel Institute of Technology,
in revised form December
6, 1988)
Summary PMH compacts with various contents of aluminium matrix were prepared by a method including a final step of sintering under high hydrogen pressure. The gas permeability was determined as a function of the weight fraction of the aluminium matrix, f. A correlation between the microstructure, morphology and porosity of the PMH compacts and the coefficient of permeability (x’ was found. A tridimensional aluminium matrix was found to form in compacts having fractional aluminium contents in the range 0.15 - 0.22, and to be followed by a steep decrease of c!. On the other hand, the effective thermal conductivity k eff of the PMH compacts is known to increase with the increasing aluminium content. Major requirements from PMH compacts are high hydrogen and thermal yields, which are contingent upon high heat and mass transfer rates prevailing simultaneously. As the heat and mass transfer rates depend on keff and (Y’ respectively, it is concluded that these quantities must be optimized in a range where keff as well a’ are sufficiently high. The permeability results were interpreted also in terms of dimensionless parameters, such as the friction factor Cf and Reynolds’ number Re. From the results it is seen that the transition from the D’Arcy region to the highvelocity region is quite smooth and gradual as expected of porous materials.
1. Introduction Many applications of metal hydrides for hydrogen energy storage require high hydrogen flow rates. These are contingent upon rapid kinetics, high heat and high Hz mass transfer rates. It has been concluded that the poor heat transfer of a container filled with a hydride powder bed is mainly due to the low effective thermal conductivity keff of the bed [l, 21. Likewise, the H, mass transfer can be substantially hindered by selfdensification and clogging of fine powder metal hydride beds. In order to relieve the heat and mass transfer problems, we have suggested converting powder metal hydrides into composite compacts 0022-5088/89/$3.50
@ Elsevier
Sequoia/Printed
in The Netherlands
66
dubbed PMH (porous metallic-matrix hydrides) [3 - 61. The PMH compacts have a number of advantages in comparison to powder metal hydride beds. (i) The compacts have an effective thermal conductivity keff increased by a factor of 10 - 50 in comparison to the fine powder of the same metal hydride [ 71. (ii) They form a stable structure of constant porosity. (iii) The handling of large amounts of compacts is easier than that of equivalent amounts of fine powder. The microstructure of the PMH compacts consists of a sufficiently porous, metallic tridimensional matrix with incorporated metal hydride particles. The PMH compacts were produced by admixing fine powders of a metal and of a metal hydride and consolidating the mixture by methods of powder compaction. The process of preparation of PMH compacts is accomplished by a step of sintering at an elevated temperature T, under a pressure of hydrogen exceeding P,,(T,) - the equilibrium pressure at the sintering temperature [ 6,7]. By this process a porous structure is produced possessing considerable strength and stability upon cyclic hydrogen sorption. The thermal conductivity keff of powder metal hydride beds, as well as that of PMH compacts, has been the subject of several studies [ 2, 7 - 91. The dependence of k eff on the porosity is commonly expressed by a factor 1 - E, where E is the fractional porosity [7]. Only a few studies have been devoted to the Hz mass transfer of a metal hydride bed and its dependence on the porosity, density or morphology of the bed [9 - 111. Groll and Nonnenmacher [lo] have studied the dependence of the H, flow on the packing density of a bed of LaNisH, in a tubular shape reactor. Eaton et al. have studied various compacts for their mass transfer [ 91. The hydrogen and thermal yields of PMH compacts with an aluminium matrix have been studied by Josephy et al. [7]. They found that increasing weight fraction of the aluminium matrix up to f = 0.18 caused the hydrogen flow rates to increase, whereas further increasing of f resulted in a decrease of the flow rate. It was suggested that raising the amount of aluminium matrix increases the thermal conductivity keff but concurrently lowers the porosity of the compacts, which results in increased resistance to the hydrogen flow. In the present investigation, the permeability was determined for a series of LaNi,H, PMH compacts, with various contents of aluminium matrix. A correlation between the microstructure, morphology and porosity of the PMH compacts and the coefficient of permeability CX’was found. Also the viscous and inertial resistance coefficients were determined as functions of the open porosity fraction. The behaviour of the friction factor Ct us. Reynolds’ number is shown to be smooth in the transition range between low and high flow velocities. 2. Experimental
procedures
The starting materials used were aluminium powder and LaNis alloy. The alloy in the’form of chunks was supplied by Ergenics Co., U.S.A. The
67
aluminium powder was supplied by Alcoa Co. under the trade mark Alcoa # 1401, and a nominal size of 6 - 9 pm. The powder was produced by atomizing and was of 99.5% purity. The preparation of the PMH samples consisted of the following main steps. (a) grinding the LaNi, to a fine powder; (b) mixing of LaNi, and aluminium powders; (c) activating the metal hydride powder; (d) cold compaction; (e) high-hydrogen-pressure sintering. During the grinding and mixing the powders were immersed in a protective liquid. The final product was kept in a protective liquid. The LaNi, was ground in a mortar-type grinder to fine powder. A particle size distribution histogram for the LaN& powder is shown along with one for the aluminium powder in Fig. 1. An SEM micrograph of the two powders is shown in Figs. 2(a) and 2(b). The aluminium particles are seen to have a rounded shape whereas the LaNi, particles have rather sharp edges and faces characteristic of brittle crystalline materials. The two powders were mixed together within the mortar grinder. The LaNi, was activated by carrying out of about 30 hydrogen sorption cycles on the powder mixture under hydrogen pressures from sub-atmospheric pressure up to 30 atm and at temperatures from 25 to 70 “C. The preparation process has been described elsewhere [6,7]. A PMH compact of LaNi, with aluminium in the form of a cylindrical pellet is shown in Fig. 3. Testing of PMH compacts for mechanical stability was performed in specially designed reactors with transparent windows. The reactors were connected to a high-pressure vacuum apparatus and
1
9
12 15 18 21 24
PARTICLE SIZE RANGE (microns1
Fig. 1. Particle volume distribution of the LaNi5 and Al powders utilized for the preparation of PMH compacts.
(4
(b)
Fig. 2. Scanning electron micrograph of particles of (a) LaNi5 and (b) Al.
Fig. 3. Photograph
of (a) “green” and (b) sintered PMH compacts.
periodically charged and discharged with hydrogen. The formation of cracks or disintegration of the compacts was followed through the transparent windows. PMH compacts have been tested for over 33 000 cycles and found stable [ 111. Specimens of the series prepared in this investigation were tested for 20 000 cycles and only a few transverse cracks were detected. The durability of the PMH compacts, described above, is remarkable in view of the fact that the metal hydride component expands and contracts by ~20% in volume upon each sorption cycle. The measured density pmeas was determined by a gravimetric method of high precision [ 123. The PMH total porosity et was evaluated by means of the density difference E = Pcalc - Pmeas t Pcalc
(1)
where pCalC is the PMH density calculated from the known bulk densities of LaNi, and aluminium under the assumption of zero porosity. by immersing the comThe PMH open porosity E,, was determined pacts
in benzoyl
alcohol
and
evacuating
the
container.
The
low-surface-
69
OUT
Sample
Fig. 4. Schematic presentation of PMH compacts.
Flowmeter
of the apparatus for the measurement
of permeability
BakeWe,,? P.M.H. Compact
Fig. 5. Sample for permeability diameter = 11 mm.
measurement
embedded
in bakelite holder. Sample
tension and low-viscosity liquid penetrated into the open pores after air and other gases have bubbled out. Then the change in the weight of the sample was precisely determined. The determination of the coefficient of permeability CX’was carried out by using an apparatus schematically shown in Fig. 4 and specially prepared specimens as the one shown in Fig. 5. The specimens were prepared by placing PMH compacts into cylinders filled with a bakelite powder, which was subsequently polymerized, and the two parallel surfaces were machined. Steady state flow conditions were preserved by keeping Pi, and Pout constant. Helium gas was used for most of the permeability measurements. The flow velocities used in this investigation did not exceed 0.8 1 min-‘. A parabolic equation appropriate to describe the gas flow through the porous structure in the longitudinal direction was used [ 131 Pin2
-
Pout 2 = Ml&
+
M2Q2
(2)
with
and
where Pi, and Pout are the upstream and downstream pressures; cx is the viscous resistance constant; /3 is the inertial resistance constant; Q is the volume flow rate; L and A are the specimen length and cross-sectional area;
70
p is the gas viscosity; M is the molecular weight of the gas; pS is the gas density; and g, is the units conversion factor. The parameter known as permeability (a’) is determined in the range of low flow rates by D’Arcy’s equation and corresponds to (Y’= l/a. The values of (Y’ are average values obtained from measurement of 3 - 4 samples with standard deviation from 5 X lo-l2 cm2 for samples with low aluminium content to 0.7 X lo-l2 cm2 for samples with high aluminium content. The thermal conductivity measurements were carried out by using the comparative method in a vacuum chamber [ 14,161. The apparatus was designed and built in our laboratory. The standard deviations obtained for . . heff are marked m Fig. 17.
3. Results and discussion 3.1. Morphology, porosity and microstructure Compacts made of aluminium powder of 6 - 9 pm average particle sizes were prepared by cold compaction in order to examine its sinterability. The density of the aluminium compacts pcomp is shown in Fig. 6 us. cold compaction pressure PC,. The density of the aluminium compacts is seen to change from 0.975 to 0.985 of the bulk density, PA1= 2.7 g cmp3, with P,, changing in the range 5 ton crne2
I
lb
COLD COM?ACTION PREsSuRe. Pee [ton/cm21
Fig. 6. Density of compacted ium powder diameter 6 - 9 pm.
aluminium us. pressure of compaction.
Initial alumin-
71
Fig. 7. Effective density vs. pressure of cold compaction for PMH compacts in “green product” condition of various Al contents: *, 12 wt.% Al; A, 15 wt.% Al; q,18 wt.% Al; n, 25 wt.% Al. Fig. 8. Porosity vs. weight fraction of matrix of LaNiS with Al matrix PMH compacts; n, cold compacted; 0, sintered.
product condition) is shown in Fig. 7. The dependence of the density on PC, of these composite compacts is substantially different from that of the single component compacted aluminium. For a given weight fraction of aluminium f the measured density, pcomp, increases quite linearly with PC,, while for a given P,,, pcomp decreases with increasing f. The measured density pcomp depends not only on the relative fractions of the constituents (hydride and metal) but also on the fraction of porosity. For compacts cold-compacted at P,, = 10 ton cm-’ and for subsequently sintered compacts the total porosity fraction et is shown in Fig. 8. A substantial increase in porosity is seen to result from the high hydrogen pressure sintering process. The porosity increment, upon the high-hydrogenpressure sintering process, was found to be approximately 0.2V,/(l + 0.2VJ, where VA is the volume fraction of alloy LaNi,. Measured values of the total porosity of a sintered PMH and values calculated as a sum of et for the green product and the increment upon sintering are given in Table 1. As seen from Table 1, the measured values compare well with the calculated ones. The open porosity E,~ and the total porosity et were determined (see experimental procedures) and plotted US. PC, in Fig. 9 for compacts containing 15 wt.% of aluminium matrix, in the “green” product condition. The total (et), open (e,,) and closed (e,r) porosities plotted us. the weight fraction of the aluminium matrix f are given in Fig. 10, for a sintered PMH compact compacted with PC, = 10 ton cmp2. The open porosity, which is instrumental in the H, mass transfer, is seen to be at an almost constant
72 TABLE 1 Measured and calculated with Al matrix Volume fraction of LaNiS VA
values of the total porosity et for PMH compacts
Measured Et for green product
of LaNi,
Sintered compacts Calculated increment to total porosity et o.zv,
Total porosity Et Calculated
Me~ured
0.275 0.28 0.249 0.239 0.214 0.195
0.324 0.255 0.251 0.283 0.180 0.175
1. + 0.2VA 0.60 0.57 0.563 0.521 0.433 0.382
0.175 0.185 0.155 0.152 0.142 0.131
0.1 0.095 0.094 0.087 0.072 0.064
Fig. 9. The dependence of the total and open porosity of “green” PMH compacts LaNis with a 15 wt.% aluminium matrix on the compaction pressure.
of
Fig. 10. The dependence of the total (A) and open porosity (A) of a sintered PMH compact of LaNis and aluminium matrix on the Al content.
value of aO.22 for aluminium weight fractions of f < 0.18 and to fall off to a value of ~0.15 for f > 0.24. The particle volume distributions in Fig. 1 are seen to differ to some extent from each other. A micrograph of a compact of La& with 18 wt.%, in the “green” product condition is shown in Fig. 11. The microstructure is seen to be quite non-uniform. The non-uniformity of the microstructure seems to be related to imperfect admixing of the two powders rather than to the difference in their particle volume distributions.
73
Fig. 11. Metallographic cross-section of a compact of LaNis with 18 wt.% of the aluminium matrix. The light phase with a smooth surface is the LaNis, the light phase with pits on its surface is aluminium, while the dark phase represents the pores.
An SEM micrograph of a fracture surface of a PMH compact of LaNi, with 18 wt.% Al matrix, in sintered condition is shown in Fig. 12(a). An electron backscattering micrograph of the same surface is shown in Fig. 12(b). The two pictures show an area of a consolidated aluminium matrix in which indentations produced by hydride particles, which fell out upon the fracture of the compacts are clearly seen. All in all, it is evident that a tridimensional aluminium matrix forms in the range of aluminium contents of 0.15 < f< 0.22, followed by a decrease in the open porosity. 3.2. Permeability and thermal conductivity The permeability of the PMH compacts was determined as described in the previous section (experimental results) and a plot of (Pin2 - Pout*)/2 vs.
Fig. 12. (a) Scanning electron micrograph of fracture surface of a LaNis PMH compact with 18 wt.% aluminium matrix: (b) electron backscattering micrograph of the same surface: aluminium, dark phase; LaNis, light phase.
FLOW VEELOCITY.
”
~om,rsc,
a
Fig. 13. Graphs of (Pin* -P ,,ut*)/2 us. the gas flow velocity LJthrough PMH compacts of various Al contents: *, 12 wt.% Al; A, 13.5 wt.% Al; 0, 15 wt.% Al; n, 18 wt.% Al; 0, 25 wt.% Al. Fig. 14. The variation of the permeability coefficient (Y’ of sintered PMH compacts with the fraction of Al matrix.
the flow velocity is shown in Fig. 13 for a number of sintered PMH compacts. The corresponding permeability coefficient a’ is plotted us. the weight fraction of aluminium in Fig. 14. The permeability ~11’is seen to decrease steeply in the range of 0.15 < f < 0.22 in which the aluminium matrix forms. A correlation was found between the permeability coefficient CY’and the fraction of open porosity, E,~ as shown in Fig. 15, holding c! = AE,,,~
(3)
where A is a constant. The difference (Pin* -Pout 2)/2 is plotted us. the flow velocity in Fig. 16, for a number of 15 wt.% aluminium matrix compacts in various conditions: (i) a “green product” condition, measured with He gas; (ii) a sintered condition, measured with He gas; (iii) a sintered condition, measured with H2 gas. The corresponding permeabihties are marked in Fig. 16. The permeability measured with H, gas is lower than the one measured with He gas. The reason for this is that the LaNis particles absorb hydrogen, transform to hydrides and increase their volume, thereby reducing the porosity of the compact. The dependence of the thermal conductivity ketf of porous compacts on the thermal conductivities of its constituents is quite complex. It has been found that, for a series of PMH compacts, keff can be fitted to an
Fig. 15. The variation of the permeability porosity Cop.
coefficient
Q! with the fraction
of open
Fig. 16. Graph of (Pin2 -P 0ut2)/2 us. the gas flow velocity u for “green” (A), sintered (o), and sintered and hydrided (0) PMH compacts of La& with 15 wt.% of Al matrix.
empirical [16] :
formula,
which
is a modification
+ k2 1+
v-2 B,V*2 1
[15]
of the one used by Tye
(4)
where k, and k, are Et = 0 extrapolated values of the thermal conductivity of Al and LaNis compacts respectively; V, and I$, are the fractional volume of the Al and LaNiS respectively, with V, + V, = 1; and B, and B, are constants appropriately adjusted for each series of experimental measurements. For the curve shown in Fig, 17, B, = 60 and B, = 9. The constants B, and B2 represent the effect of the nature of the interfacial contact areas, which cannot be explicitly accounted for. Measured values of keff and (Y’are plotted us. the aluminium content in Fig. 17 for a series of LaNi, H,-aluminium matrix PMH compacts. As seen in Fig. 17, for obtaining concurrently high heat and H, mass transfer rates, values of cy’ and keff must be optimized and lie in the transition region of ~15-22 wt.% aluminium. Figure 18 shows the variation of the viscous and inertial resistance coefficients (Yand /3, with the fraction of open porosity, E,,. The corresponding relationships are ar = A1~,p-3
(5)
and fi = A2~,p-4.2
(6)
76
Fig. 17. The variation of the permeability and the thermal conductivity coefficients with the volume fraction of AI matrix for PMH compacts of LaNiS. Fig. 18. The variation of the viscous and inertial resistance coefficients a! and p with the fraction of open porosity.
where A, and A2 axe constants. It may be of use to describe the Pin2 - Pout2 vs. gas flow velocity by means of dimensionless parameters as follows [ 131 Cf, the friction factor, is given by
where u is the flow velocity (u = Q/A); Re, the Reynolds’ number, is given by Re=& o/J Cf is then given by 2 Cr= -+2 Re
(8)
(9)
The first term (Z/Re) corresponds to the D’Arcy regime of low flow velocities, while the second represents the regime of inertial high flow velocities. A plot of the friction factor Cf us. Re is shown in Fig. 19. The fit of the experimentally determined data points, to the curve, calculated co~esponding to eqn. (9), is seen to be quite good. The gradual transition from the low-velocity D’Arcy region, to the highvelocity inertial region, is typical of flow through porous structures.
77
Fig. 19. The friction factor Cf us. the Reynolds’ numbers for PMH compacts of various Al contents: A, 12 wt.% Al;., 13.5 wt.% Al; 0, 15 wt.% Al; m, X8 wt.% Al; 0, 25 wt.% Al.
4. Summary
and conclusions
A series of PMH compacts of LaNis with aluminium matrices were produced_with varying fractional contents of aluminium. The compacts were tested and found to be mechanically stable upon about 20 000 hydrogen sorption cycles in spite of the expansion in volume of about 20% of the LaNi, H, component with each cycle. The sintering process at high hydrogen pressure produces additional porosity, which renders the compact able to accommodate the volume expansion of the LaNisH, component and to be permeable to hydrogen gas flow. The total and open porosities, et and E,,~ respectively, were determined as a function of the ~um~ium matrix fractional content f_ Both et and E,,~ decrease with increasing f. The permeability factor or was determined as a function of f. cr’ is a gas mass transfer characteristic of a porous material, and its value was found to fall off quite steeply in the range of 0.16 < fG 0.20. Metallographic studies show that in that range of f a trid~ensional aluminium matrix forms upon the compaction process. A correlation between E,~ and QI’was found in the form of: o’ Q: eop3, The permeability results were interpreted also in terms of dimensionless parameters, such as the friction factor Cf and the Reynolds’ number Re. From the results plotted in Fig. 19, it is seen that the transition from the D’Arcy region to the high-velocity region is quite smooth and gradual as expected of porous materials. The effective thermal conductivity of the PMH compacts is quite well described by an empirical formula (4) in which the effect of porosity is expressed by the factor 1 - et. In contrary to the increase of the permeabil-
18
ity cu’ with the open porosity, the thermal conductivity lZeif decreases with increasing porosity (open or total). Major requirements from PMH compacts are high hydrogen and thermal yields, which are contingent upon high heat and high mass transfer rates prevailing simultaneously. As the heat-and mass transfer rates depend on keri and (Y’ respectively, these two quantities must be optimized in the range 0.16 < f < 0.20 in which neither heff nor o! are too small. Acknowledgments This research was sponsored by N.C.R.D., Israel and the Kernforschungsanlage Jiilich, F.R.G. The authors are indebted to Mr. E. Levy for his technical assistance. References 1 M. Ron and M. Elemelech, Proc. Znt. Symp. on Hydrides for Hydrogen Energy Storage, Norway, August 77, Pergamon, Oxford, 1978, pp. 417 - 430. 2 S. Suda, N. Kobayashi, K. Yoshida, Y. Iahido and S. Ono, J. Less-Common Met., 74 (1980) 127. 3 M. Ron, 11 th Zntersociety Energy Conversion Engineering Conf., State Line, Nevada, 1976;p. 954. 4 M. Ron, D. M. Gruen, M. H. Mendelsohn and I. Sheft, J. Less-Common Met., 74 (1980) 445. 5 M. Ron, D. M. Gruen, M. H. Mendeisohn and I. Sheft, U.S. Patent 4,292,265 (1981). 6 M. Ron, U.S. Patent 4,507,263 (1985). 7 Y. Josephy, Y. Eisenberg, S. Perez, A. Ben-David and M. Ron, J. Less-Common Met., 104 (1984) 297. 8 E. Suissa, I. Jacob and Z. Hadari, J. Less-Common Met., 104 (1984) 287. 9 E. E. Eton, C. E. Olsen, M. Sheinberg and W. A. Steyert, Znt. J. Hydrogen Energy, 6 (1981) 609. 10 M. Groll and Nonnenmacher, Proc. 17th Zntersociety Energy Conversion Engineering Conf., Los Angeles, CA, August 8 - 12,1982, p. 1185. 11 E. Bershadsky, Y. Josephy and M. Ron, 2. Phys. Chem. N.F., to be published. 12 K. H. Moer, Znt. J. Pow. Metal2 and Pow. Tech. 15 (1979) 33. 13 L. Green Jr. and P. Duwez, J. Appl. Mech, March 1951, p. 39. 14 W. Neumann, High Temp.-High Press, 13 (1981) 687. 15 E. Bershadsky, Y. Josephy and M. Ron, to be published. 16 P. P. Tye, ASME publication 73-HT-47 American Society of Mechanical Engineers, New York, 1973.