Microstructure, permeability and mechanical behaviour of ceramic foams

MATERIAlS SCIEMCE& ElIClMEERIMe ELSEVIER

Materials Science and Engineering A209 (1996) 149 155

A

Microstructure, permeability and mechanical behaviour of ceramic foams Rogerio A. Lopes, Ana M. Segadaes Unitusidade de AU'iro, Del!. Ellg. Cen/mica c do Vidro, P-381O Areiro, Porlllga!

Abstract

This work is part of a model study aimed at upgrading the technique of creating porosity via the incorporation of organic particles used in traditional ceramics. exploring at the same time colloidal processing from coagulated slurries, The method described in another work. based on the manipulation of short-range repulsive (lubricating) hydration force and long-range attractive van der Waals force. was used to pack to a high density a bimodal mixture of submicron ceramic particles (matrix) and much larger organic particles (inclusions) during consolidation by pressure filtration of dispersed suspensions coagulated by added electrolyte, Investigations carried out to produce strong porous ceramic bodies, with a tailored pore structure, are described, The liquid-like rheology of the saturated consolidated body produced from the coagulated suspension explains the characteristic features observed after burn-up of the organic inclusions and sintering. The porous ceramic body reflects its processing history, which can be observed in its microstructure. mechanical behavior and gas permeability. Kenmrds: Permeability: Mechanical behaviour: Ceramic Foams

1. Introduction Most of the research work carried out in the field of advanced ceramic materials has been aimed at investigating the mechanisms and variables that control and promote maximum densification of powder compacts in order to reach theoretical density and full elimination of porosity, Only then can the specific characteristics of ceramics. namely hardness, abrasion resistance, chemical inertia and mechanical strength, be fully established, However, there are many engineering applications in which very porous ceramics could compete with metals or polymers, provided that a processing route is found to retain, or create, porosity without sacrificing those special properties, Membranes for separation processes in the chemical industry and foamed filters for hot gas cleanup and molten metal processing are just a few examples of applications where ceramics are playing an important role. These are processes in which thermal stresses are incurred, with some, even if minimal, loadbearing requirements (to withstand the fluid pressure and high pressure drops across the membrane). Long term durability for some of these applications still needs to be established, and one way to improve it is by 0921-5093/96/$15.00 ~~! 1996 SSDI0921-5093(95)10146-2

Elsevier Science S.A. All rights reserved

developing an optimum processing method, namely the shape forming technique. Given the difficulties encountered when applying to ceramics the generally known methods of shaping materials, most engineering ceramics are still formed as powder compacts and densified by thermal treatment (sintered). Current techniques of powder consolidation are based on dry pressing which entails the presence of agglomerates and the contamination with foreign inclusions. These remain in the sintered body as microstructural inhomogeneities. potential stress concentrators and. as such, preferable sites for catastrophic failure. Other than going against the mechanisms that favour densification. the obvious alternative is to create "artificial" porosity, Whereas in traditional ceramics high levels of macro porosity are usually obtained via the incorporation of organic compounds, the majority of engineering (structural) foams, are produced by the replica technique, using a foamed polymer as a precursor. This technique consists in dipping the polymer foam into a slurry containing an appropriate binder and ceramic phases, followed by pressureless sintering at elevated temperatures. However, the cellular ceramics thus produced contain varied microstructural flaws,

150

R.A. Lopes, A.M. Segadiies! Materials Science and Engineering A209 (1996) 149-155

mostly owing to processing, and present comparatively low strength and fracture toughness [I]. The colloidal treatment of ceramic powders minimizes the occurrence of those inhomogeneities through the manipulation and control of interparticle forces. To take full advantage of the colloidal methods, the consolidated body should be formed directly from the suspension. The most promising techniques are based on the partitioning of the suspension, as is the case of pressure filtration [2]. Ideally, the suspension should be stable and easy to manipulate, with a high volume fraction of solids, viscous enough to prevent mass segregation and produce a high particle packing density in the consolidated body. It has been shown [3,4] that coagulation by the addition of suitable counter-ions produces suspensions which fulfil all the above requirements, due to the development of strong short-range repulsive (lubricating) hydration interparticle forces. This work is part of a model study aimed at upgrading the technique of creating porosity via the incorporation of organic particles used in traditional ceramics to produce insulating materials, exploring at the same time the colloidal processing from coagulated slurries. The ceramic material chosen was a submicron high-alumina composition [5], known to sinter at 1400 °C for 4 h, from dry-pressed powder compacts, reaching ~ 97% relative density and a cold Modulus of Rupture of ~ 280 MPa. As for the pore precursor, suspension PVC was used, readily available as roughly spherical particles of ~ 80.um average diameter.

2. Experimental procedure The selected ceramic composition [5] was prepared from Alcoa CT3000SG alumina (96 wt.%) and equal amounts of Merck titanium and manganese oxides. Consolidated bodies with the fixed ceramic composition and variable volume proportions of CIRESI suspension PVC, were produced from aqueous slurries, dispersed at pH 3 and coagulated with NH 4 Cl, by pressure filtration under ~ 0.5 MPa ( ~ 5 bar). Before and after drying at 40, 60 and 105°C for successive periods of 24 h, the solid bodies (cylinders:::::: 4.5 cm in diameter and:::::: 2.0 cm high) were weighed to access the particle packing density obtained. Calcination of the dried bodies, up to 1250 °C (heating rate I °C min - I), was carried out very carefully, following a previously determined heating schedule, based on DTA and TGA results, to account for the decomposition of both the salt and the PVC particles (48-72 h soaking periods at the critical temperatures). After calcination, sintering was carried out at 1400 °C for 4 h in a Super Kanthal I CIRES-Companhia Industrial de Resinas Sinteticas S.A., 3861 Estarreja, Portugal.

TermoLab 2 chamber furnace. The sintered bodies were then cooled down to ambient temperature at 5 °C min-I. Standard techniques were used to characterize the sintered bodies, namely X-ray powder diffraction (Philips PW 1840), Archimedes porosity and density measurements (ASTM C20), scanning electron microscopy (Jeol JSM-35C) of fracture surfaces, and fourpoint flexure determination of cold modulus of rupture (Shimadzu Autograph AG 25TA). Sample charging in SEM was prevented by gold coating. The modulus of rupture test-pieces were round edged, polished, 30 x 3 x 3 mm bars, precision cut (Struers Accuton) from bigger samples. The room temperature gas (nitrogen) permeability of selected sintered bodies (disks) was determined, using a home-made apparatus, by measuring the pressure drop across the particular disk as a function of the gas flow rate.

3. Results and discussion 3.1. Packing of particles and microstructure

Previous work with the model ceramic composition [6] showed: that dispersed slurries with 20 vol.°;;) solids can be coagulated through the addition of NH 4 Cl; that there is a critical salt concentration ( ~ 2 molar) above which no significant increase in the slurry's viscosity can be observed; that the volume fraction of salt needed to reach the optimum coagulation depends only on the volume fraction of ceramic particles in the suspension; and that, since the addition of the larger PVC particles causes the viscosity of the suspension to decrease (owing to the increase in the average particle size in the mixture of solids), stable coagulated slurries can be formulated with higher solids concentration (~50 vol.%) and adequate viscosity for further processing by pressure filtration. Regardless of the volume fraction of solids in suspension, it was found [6] that there is no density gradient within the consolidated body produced by pressure filtration and that the final packing density of the bimodal mixture of particles is independent of the length of the filtration. This can undoubtedly be attributed to the liquid-like rheology of the saturated consolidated body produced from the coagulated suspension. Particle packing densities of bimodal mixtures of fine (matrix) and coarse (inclusions) particles, were first studied by Furnas [7], who established the basis of all subsequent work. There is a critical inclusion volume fraction f?, at which the inclusions reach their maximum packing density. Below f?, the large inclusions 2

TermoLab-Fornos Elcctricos, Lda, 3752 Agueda, Portugal.

R.A. Lopes, A.M. Sel',adlies

Maleria!.\ Science and Enl',ineerinl', A209 (1996) 149 155

151

(B)

(A)

Fig. 1. Schematic diagram of Zok et al.'s model [9] to represent the disruption of fine particle packing due to the presence of larger inclusions: (a) the wall effect and (b) the contact effect.

replace the matrix powder and its associated void space to produce a relative composite density p~. of o

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f
.

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I

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F

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Thus, the variation in the relative density of the composite mixture with the inclusion content is characterized by two regimes, below and above the critical inclusion volume fraction!7. Furnas' model ignores the powder packing disruption at the surface of the inclusions (i.e. extra void space within the composite), overestimating the composite relative density. Furnas' work was followed up by Messing and Onada [8] and complemented by Zok et a!. [9]. The latter authors realized that the distruption of fine particle packing owing to the presence of larger inclusions has a strong effect on the packing efficiency of composite mixtures. This disruption can be of two types, the wall effect and the contact effect, as illustrated in Fig. I, each of which has an associated excluded volume, V" and Vn respectively. Therefore, the composite relative density P(, should be given by Pc=p~-V,,-V(

Through geometrical assumptions, the excluded volume corresponding to each type of disruption can be calculated as a function of the relative sizes of the matrix and inclusion particles and their undisturbed packing efficiencies. In Zok et al.'s model [9], the excluded volumes are somewhat overestimated since they are independently calculated as if they did not overlap.

In other words, for each contact between inclusions, creating a contact-effect excluded volume, a fraction Se of the inclusion surface is not available to participate in the wall effect and the corresponding excluded volume. Lam [10] included the two together, taking into account the number N of contacts between inclusions, dependent on their volume fraction, and the packing efficiencies of the matrix particles at the surface and the contacts between inclusions, 'I" and 'Ie respectively. The total excluded volume Vexc is then given by Vexc = NVc (1 - 'I,)

+ V,,(1

- '1,,·)(1 - SiN)

and the composite relative density will be Pc' = p~.

-

Vc\1

If the model correctly describes the experimental bimodal particle packing in this work, it is to be expected that for low PVC contents, i.e. below the percolation threshold, the wall effect should dominate the particle packing, whereas, once percolation sets in, the contribution from the contact effect should be more significant. As in Zok et a!. 's work, the packing density of both the matrix (ceramic particles) and the inclusions (PVC particles) by themselves was assumed to be that of random dense packing (p 7" = P 7 ~ 0.64), and the coordination number, and thus the maximum number of inclusion contacts, typically between 6 and 9, was selected to be 7. Fig. 2 compares the calculated values, according to Lam's model, for the relative packing density of the composite mixture (the size ratio between matrix and inclusion particles was taken to be 0.0125) with the experimental densities measured from dried green bodies produced as described earlier, both as a function of the inclusion volume fraction. Very good agreement can be observed. Evidence was found that the matrix packing disruption caused by the much larger PVC particles does not extend beyond the first layer of ceramic particles around each PVC inclusion. This can be observed after

152

R.A. Lopes, A.M. Segadiies / Materials Science and Engineering A209 (1996) 149-/55

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sintering, when the PVC particles have been burned out leaving behind the corresponding void space (Fig. 3). Owing to sintering shrinkage, the decreased matrix packing efficiency caused by the wall effect translates into circumferencial cracks and, roughly, each PVC particle will produce, after sintering, one hollow ceramic shell, i.e. one closed pore. Fig. 3 (a) shows, if the PVC content in the original dried body was low (below percolation threshold), how isolated shells can be found throughout the microstructure (only wall effect and no contact effect). As the PVC volume fraction increases, so does the number of contacts between inclusions. Each of these contacts and its corresponding excluded volume originate, after sintering, one opening to a pore. If the PVC

(a)

(b)

Fig. 3. (a) Typical sintered microstructure produced from a dried body with a low PVC/ceramics volume fraction, showing the hollow ceramic shells; (b) Typical sintered microstructure produced from a dried body with a 0.60 PVC/ceramics volume fraction, showing the development of the open cell structure, with shared solid single-layered walls (bar = 10 ,am).

content in the original dried body was above percolation, individual shells in the sintered body start interacting, and an open cell structure, with shared solid single-layered walls, is established. The resulting microstructure is shown in Fig. 3 (b), in which the open cell structure, with essentially flawless solid walls, can be clearly seen. As the PVC content increases, because of the contact effect, a communicating pore network will develop, and the closed porosity will tend to be made up only from small pores trapped between ceramic grains. If Lam's model is again used to calculate the fraction of the total excluded volume that corresponds to the wall effect, VI\'/ Vcw as a function of the inclusions volume fraction (Fig. 4 (a)), a clear indication can be found that the contact effect becomes dominant once the inclusion percolation threshold is passed. A similar behaviour can be observed in the variation of the open porosity fraction with the volume fraction of PVC in the mixture of solids in the original dried body (Fig. 4 (b)), with a clear indication that the dependency changes once percolation sets in, i.e. one regime for PVC contents below the percolation threshold (isolated PVC particles) and another for contents above the percolation threshold (network of contacting PVC particles). These results indicate that the critical PVC volume fraction (inclusions) in the solids for percolation if", 0.32, corresponding to a PVC fraction in the composite green body of", 0.22, meaning that all inclusions belong to the same percolative network [9]. 3.2. Gas permeability and mechanical behaviour

In all the technological applications of very porous ceramics mentioned above, fluid flow through porous media is important. For pore sizes below the millimeter range, the contribution of fluid inertia to the energy

R.A. Lopes, A.M. Segadiies

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R.A. Lopes, A.M. Segadiies I Materials Science and Engineering A209 (1996) 149-155

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dissipation (i.e. the pressure drop) in the porous media is negligible, and the dependence between the flow rate and the applied pressure is governed by Darcy's law:

!1P

fl

Y=k u where !1P is the total pressure drop across a medium of length L and specific permeability k. The superficial velocity u of the fluid (with viscosity fl) is assumed to be the same throughout the medium. For a gas this implies isothermal flow at a low pressure drop. Room temperature nitrogen permeabilities of sintered disks produced from selected, wide apart, volume ratios PVC/ceramic in the green body were determined for various flow rates and the corresponding pressure drops. The experimental data so obtained were found to be in good agreement with Darcy's law, as shown in Fig. 5. The specific permeability, shown in Fig. 5 next to each data set, together with the PVC/ceramic ratio, is clearly dependent on the open porosity of the porous body and comparable to the values obtained for ceramic foams produced by the replica technique [11]. Any porous medium, particularly those with open cell structures like foamed materials, can be readily described by its relative density p /p" where p is the foam density and Ps is the density of the solid that the foam is made of. All other properties can be directly or indirectly related to the relative density through equations derived for geometrical representations of the particular cell structure. Ashby [12] has shown that, for open-cell foams, a linear dependence of relative Young's modulus E/E\. on the square of the relative density is a good description for the linear elastic behaviour of a large range of polymeric (both openand closed-cell) and ceramic foams with varied densities. The large scatter of available data can be best fitted when the proportionality constant is unity.

In this work, preliminary results for Young's Modulus were obtained from cold Modulus of Rupture four-point flexural tests. Those results show comparable scatter to the data gathered by Ashby [12] and satisfy the above description in a comparable manner. The best fit of Ashby's model in this case, however, suggests a proportionality constant of 0.13, as shown in Fig. 6.

4. Conclusions

The work described shows that the traditional technique of creating artificial porosity via the addition of organic inclusions, coupled with the colloidal processing of coagulated slurries, can be successfully used to produce very porous engineering ceramic bodies. The processing variables can be closely controlled and the properties of the porous ceramic body reflect its processing history in a reliable and reproducible manner.

Acknowledgements

This work was partially funded by JNICT-Portugal, in the form of a Ph.D. grant, and by CNPq-Brazil, in the form of a post-doctoral fellowship. The contribution given by M.e. Greca and N.O. Moutinho, from INT-Brazil, with the permeability measurements is gratefully acknowledged.

References [I] R. Brezny and DJ. Green, Fracture behavior of open-cell ceramics, J. Am. Ceram. Soc. 72 (7) (1989) 1145-1152. [2] F.F. Lange, Powder processing science and technology for increased reliability, J. Am. Ceram. Soc., 72 (I) (1989) 3~ 15.

R.A. Lopes, A.M. Segadiies

Malerials Science and Engineering A209 (1996) 149 155

[3] F.F. Lange and K.T. Miller, Pressure filtration: consolidation kinetics and mechanics, Am. Ceram. Soc. Bull., 66 (10) (1987) 1498-1504. [4] B.V. Velamakani, J.e. Chang, F.F. Lange and D.S. Pearson, New method for efficient colloidal particle packing via modulation of repulsive lubricating hydration forces, Langmuir, 6 (7) (1990) 1323 1325. [5] M.e. Moreira and A.M. Segadaes, Phase equilibrium relationships in the system AI 20,' TiOl-MnO, relevant to the low-temperature sintering of alumina, 1. Eur. Ceram. Soc.. in press. [6] R,A. Lopes and A.M. Segadaes, Effect of the green body processing history on the properties of very porous ceramics in P. Duran and J.F. Fernandez (eds.) Proc. ECerS'93, 3rd Euro-ceramics Vol. I: Processing oj' Ceramics, Faenza Editrice Iberica, S.L., Madrid, 1993, pp. 579-584. [7] e.e. Furnas, Grading aggregates: I, mathematical relations for

[8]

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beds of broken solids of maximum density, Ind. Eng. Chem., 23 (9) (1931) 10521058. G.L. Messing and G.Y. Onada Jr., Inhomogeneity-packing density relations in binary powders, 1. Am. Ceram. Soc., 61 (1-2) (1978) I 5. F. Zok, F.F. Lange and J.R. Porter, Packing density of composite powder mixtures, J. Am. Ceram. Soc., 74 (8) (1991) 1880-1885. D.e.e. Lam, Processing control and mechanical properties in porous ceramics, Ph.D. Thesis, University of California, Santa Barbara, 1991. A.P. Phil ipse and H.L. Schram, Non-darcian airflow through ceramic foams, 1. Am. Ceram. Soc., 74 (4) (1991) 728- 732. M.F. Ashby, The mechanical properties of cellular solids, Mewll. Trans., 14A (1983) 1755-1769.