1013
Langmuir 1990,6, 1013-1016
Genesis of a Solid Foam: Iron(II1) Metaphosphate Transformation in Sol-Gel Crystallization Processes Pompeu Pereira de Abreu Filho and Fernando Galembeck* Institute of Chemistry, Universidade Estadual de Campinas, Czmpinas SP 13081, Brazil
Fltivio Cesar Guimaraes Gandra, Mauro Lucian0 Baesso, Edson Correa da Silva, and Helion Vargas Institute of Physics, Universidade Estadual de Campinas, Campinas SP 13081, Brazil Received July 10, 1989. In Final Form: January 5, 1990 Poorly crystalline iron(II1) metaphosphate (IMP) is obtained by admixture of iron nitrate and sodium metaphosphate aqueous solutions. Under heating, this solid undergoes various transformations, which were followed by X-ray diffraction, IR spectrophotometry,ESR spectroscopy, surface area, and density and weight loss determinations. At least three different Crystalline phases are formed, at different stages; crystalline and noncrystalline phases coexist, in many situations. IMP yields a solid, rigid foam under heating to t > 620 "C and a glass at t N 1200 "C. Foam formation is explained, by use of the experimental data, as a result of concurrent water release, gaseous cell formation, and crystallization of the solid mass, without simultaneous fragmentation.
Introduction Colloidal dispersions of amorphous,hydrous metal oxides and hydroxide salts can be used in preparation of poorly crystalline solids which may, in turn, be transformed by further physicochemical processing in other, desirable materials. This is one (but not the major) route into the so-called sol-gel processing for making ceramic and glass materia1s.l We have found that iron(II1) hydroxide acetate2Jundergoes particle coalescence and crystallization at temperatures below 300 "C, much lower than normal sintering temperatures for iron oxides. Moreover, we found that iron(II1) hydroxide acetate can be used to generate synthetic magnetite, by thermal decomposition under mild condition^.^ These results prompted us to examine the behavior for other noncrystalline iron(II1) salts. This is a report on iron(II1) metaphosphate (IMP) preparation and thermal transformations. Upon heating, this solid undergoes various changes, which include the formation of (i) a solid, rigid foam and (ii) of a glass; these are new, interesting, and perhaps useful materials, which genesis is discussed in this paper. Metaphosphates, as well as other phosphates, form glasses very easily with transition and other metals.5 Iron phosphate glasses have spin glass behavior and may present speromagnetism.6 Reactions and crystallization of iron phosphate glasses prepared by melting together P205, Fez03, and dextrose yield Fe2+/Fe3+phosphate glasses' which crystallize at 500-700 "C to yield compounds of (1) Hench, L. L.; Ulrich, D. R. Science of Ceramic Chemical Processing; Wiley-Interscience: New York, 1986. (2) Jafelicci, M., Jr.; Conforto, E.; Galembeck, F. Colloids Surf. 1987, 23, 69. (3) Pinheiro, E. A.; Abreu Filho, P. P.; Galembeck, F.; Silva, E. C.; Vargas, H. Langmuir 1987,3,445. (4) Abreu Filho, P. P.; Pinheiro, E. A,; Galembeck, F.React. Solids 1987,3,241-50. (5)Tanaka, K.; Soga, N.; Ota,R.; Hirao, K. Bull. Chem. SOC.Jpn. 1986,59, 1075. (6) Coey, J. M. D. J . Appl. Phys. 1978, 49, 1646. (7) Doupovec, J.; Sitek, J.; Khkos, J. J . Thermal Anal. 1981,22, 213.
0743-7463/90/2406-lO13$02.50/0
formula Fe3(P04)2aH20 and FePOCxHPO. Iron metaphosphate glass formation was also observed when Fe(H2P04)3was heated a t 230-290 OC.s Further heating (480-800 "C) gives crystalline Fe(PO&, the cyclic metaphosphate; at 850 "C, crystalline pyrophosphate is obtained. Beyond that, ortho-, meta-, and pyrophosphates have been widely used in ceramic refractories, glazes, and g l a s ~ e s . Notwithstanding ~ their technical applications, it would be desirable to have a clearer picture of their behavior, sorting out the relevant basic physicochemical events: chemical reactions, changes in mobility, surface phenomena. The system chosen in this work, amorphous iron(II1) metaphosphate, proved suitable for this objective.
Experimental Section Preparation of amorphous iron(1II)metaphosphate (IMP) consisted of adding 250 mL of aqueous 1.5 M sodium metaphosphate (Reagen)solution quickly to 250 mL of 1M iron(II1)nitrate (Merck),under strong stirring. A white, voluminous precipitate was obtained. This was centrifuged (800g, 5 min), collected, rinsed with 500 mL of water, and centrifuged again. After a new rinsing and centrifugation, the solid was collected and dried in an oven at 150 "C for 8 h. The dried product was fragmented within a polyethylene bag, in a two-roll mill, and screened to 200 mesh. Reagents used were analytical grade. Fe was determinedby titration with potassium dichromate.10 P was determined spectrophotometrically, using the molybdenum blue method.11 X-ray diffractograms were obtained at room temperature by using a PW-1050/25 Phillips diffractometer, from the Instituto Agronbmico de Campinas: Co KCYradiation was generated at 36 kV, 20 mA. (8) Yvoire, F.Bull. SOC.Chim. Fr. 1962, 2277. (9) Westman, A. E. R. Phosphate Ceramics. In Topics in Phosphorus Chemistry; Wiley-Interscience: New York, 1977; Vol 9, p 231. (10)Basset, J.; Denney, R. C.; Jefferey, G. H.; Mendham, J. Vogel's Textbook of Quantitative Inorganic Analysis, 4th ed.; Longman: London, 1978; p 399. (11) Dee, F. S.; Ettre, L. S. Encyclopedia of Industrial Chemical Analysis; Interscience: New York, 1973; Vol. 17, p 82.
0 1990 American Chemical Society
1014 Langmuir, Vol. 6, No. 5, 1990
Abreu Filho et al.
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Density (skeleton, bulk) was determined by using a helium picnometer, Micromeritics Model 1305, from the Instituto de Quimica da UNESP, Araraquara. Infrared spectra, in the 4000-200-~m-~ range, were obtained by using a 1430 Perkin-Elmer IR spectrophotometer. CsI pellets were used, at a 1:200 mass ratio. Surface are? measurementswere done in a Instrumentos Cientificos CG (Sa0 Paulo) Model 2000 instrument. Prior to these measurements, IMP samples were heated for 2 h under a Nz stream to the desired temperatures. Sample weight losses were determined by weighing before and after surface area measurements. Scanningelectron micrographs (SEM)were obtained in a JEOL T300 instrument. Samples were prepared as follows: 1 mg of each sample was dispersed in 1-butanol and sonicated. Droplets of the dispersion were applied over a strip of adhesive aluminium tape (Balzers) placed on the sample holder. Butanol was allowed to evaporate, and the samples were coated with evaporated carbon and gold. Larger specimens were just glued with conductive paint over the holder and carbon/gold coated. Electron paramagnetic resonance (EPR) spectra were taken in a Varian E-12 spectrometer operating at 9.5 GHz (X-band), at room temperature (25 2 O C ) . Cavity coupling was done by using a DPPH sample; 0.6-0.8-mg samples (weighed to the nearest 0.01 mg) were placed within thin PTFE tubes. These were plugged with silicone vacuum grease and placed within silica tubes. Microwave power was 2 mW. Spectra were stored in an IBM PC compatible microcomputer, for deconvolution.
*
Results Addition of metaphosphate to iron salt solution yields a white, gel-like precipitate. After this precipitate is washed and dried a t 150 "C, solid iron(II1) metaphosphate (IMP) is obtained, with the following characteristics: it is poorly crystalline, according to the X-ray diffractograms and infrared spectra (Figure 1); it has a low surface area (Figure 2); and its density corresponds to that of amorphous iron hydroxide salts (Figure 3). Its infrared spectrum displays bands which can be assigned to water (3600 and 1650 cm-1),12 P-0 (1100 cm-I),l3 and P-0-P (930 and 750 cm-l),14groups, as well as to Fe-0 bands (510-320 cm-l)l6 (Figure 4). (12) Schwertman U.; Fisher, W. R. Geoderma 1973,10, 237. (13) Almeida, R. M.; Mackenzie, J. D. J. Non-Cryst. SoEids 1980,40, 535. (14) Corbridge, D. E. C.; Lowe,E.J . Chem. SOC.1954,493. (15) Hayashi, S.; Kanamori, H. J. Phys. C 1980, 13, 1529.
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Figure 3. Bulk (skeleton)density of IMP as a function of heating temperature (2 h, under Nz):
Fe content is 21.8%; P content is 22.6%. The Fe:P ratio corresponds to the formula Fe2P4O13aH20. Under heating, this solid undergoes many changes. As the temperature rises, the intensity of the diffraction peaks increases (up to 253 "C)and then decreases from this temperature to about 466 "C. In the 540-620 "C range, discrete peaks cannot be observed, and the diffractogram consists only of two amorphous halos. At 680 "C, new diffraction peaks are observed, but these do not agree with those observed in the 157-460 "C temperature range. This new phase coexists with another crystalline phase, from 720 to 850 "C, as evidenced by changes in the relative intensities of the diffraction peaks, in this temperature range. Representative diffractograms are in Figure 1 (other curves, which are mentioned above, were omitted for the sake of clarity). The observed diffractograms did not match those of known crystalline iron metaphosphates, for which reason the crystalline phases could not be identified. Under further heating (to 1200 "C), the solid loses gas, darkens, and melts to yield a black, brilliant noncrystalline solid, which can be poured and drawn as a low-viscosity glass. Infrared spectra were also taken from these same samples. Spectra of noncrystalline samples show a poorer resolution than the others, which is a characteristic found in other analogous systems. Beyond that, it can be observed that the intensities of the bands assigned to P-0 and P-0-P groups do not undergo major changes, from room temperature to 1200 "C. This indicates that there are no major changes in the degree of phosphate condensation in this system. On the other hand, water loss is clearly discerned by the decrease in the intensity of the bands assigned to OH groups. However, it is remarkable that these bands can still be discerned a t 540 "C, what shows that this solid has a remarkable ability to retain water a t higher temperatures. This is an essential feature for the foaming ability described ahead. Surface area changes are in Figure 2. Even though considerable variation is detected, surface area always remains
Genesis of a Solid Foam
Langmuir, Vol. 6, No. 5. I990 1015
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above this temperature). IOm
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Figure 1. I nning electron micrgympn .I a Irncture surfacc of i h v rigid iuam obtained by heating 1\11' (11, 1 0 760 "C. f~ 2 h iindrr n r i . (Bottom, A piece n i nonioamed IMP for comparisun.
lower than 10 m2 gl. Solid density changes follow a curious pattern, as described in Figure 3. It shows a decrease in the 20S403 O C range, going to values beneath 2 g em3. This is concurrent with weight loss (Figure 2) and loss of water, as seen in the infrared spectra (Figure 1). Low densities resulting from the loss of water can be understood assuming t hat closed pores or cells are formed within the iron metaphosphate particles. Above 680 " C , there is a loss of water vapor, and the solid undergoes rapid expansion, to give a rigid foam of apparent density 0.5 g cm-?. This foam is slightly gray and strongly resistant to compression (a full characterization of this material is under way, in this laboratory). Figure 4 gives a scanning micrograph of a fracture surface of the foam, showing the cells formed during expansion; a micrograph of the original, nonexpanded solid is given for comparison, in the same figure. ESR Spectra. ESR spectra were taken from samples previously heated at various temperatures, the game from which X-ray diffractograms are given. Relevant spectra are in Figure 5 and show some interesting characteristics. First, an apparently single-line spectrum is obtained for the IMP starting sample: a more complex pattern arises at intermediate temperatures, and a narrow line is obtained again, in the samples subjected to the higher temperatures. Second, the observed spectra can be deconvoluted into two or three component spectra, for which band intensities and linewidths are given in Figures 6 and 7. I n the lower temperature end, we notice two compo-
TEMPERATURE P C
Figure 6. Normalized intensities of component lines of deconvoluted ESR spectra, as a function of sample heating temperature:).( component phase A; (A)phase B (A)phase C; ( 0 ) phase D.
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Figure 7. Line widths of component lines of deconvoluted ESR spectra, as a function of sample heating temperature: (m) phase A; (A)phase B (A)phase C; ( 0 )phase D. nents with the same g values, but one of these (A) is broader than the other (B). As temperature rises, the width of component A band increases, as well as its intensity. On the other hand, the width of component B is nearly constant; its intensity increases to ca. 300 "C and decreases from there. These results are consistent with the X-ray diffractograms, provided we assume that the narrow band is due to crystalline material and the broad band to amorphous solid. Moreover, band broadening in the amorphous solid could be assigned to an increase
1016 Langmuir, Vol. 6, No. 5, 1990
in Fe-Fe coupling, as Fe(II1) ions are brought to shorter distances by dehydration. This means Fe-O-(HzO)-Fe clusters are changed into Fe-O-Fe clusters. Above 680 "C, spectra are best fitted by using three components having g values very close to each other (2.026, 2.009, and 2.006) but having different bandwidths. Two of these (C, D) have slow-changing bandwidths, but the third shows a steep decrease. Again, these results can be correlated with those of X-ray diffraction assuming that C and D are crystalline species, formed above 680 "C. The third band is assigned to amorphous material. The decrease in this bandwidth at higher temperatures is assigned to Fe dilution in the solid due to its transfer to crystalline phases, during crystallization. In this case, Fe-O-Fe clusters (to which we have previously assigned band broadening, in the amorphous solid) found in the amorphous material would be dissipated. Discussion The formation of an inorganic foam, such as the one described in this work, requires expansion of gaseous cells within the solid (due to volatilization of one or more its constituents) under conditions such that the nonvolatile fraction is sufficiently fluid to remain coalesced but sufficiently viscous to retain bubbles and to resist collapse, under gravity.16 The volatile component, in the present case, is water. In the iron(II1) metaphosphate, water is retained over a broad range of temperatures due to its binding to phosphate. Regarding the fluidity of the solid, we have demonstrated in previous works that amorphous solid particles may coalesce at rather low temperatures2provided a given temperature threshold is o ~ e r c o m e . ~ To look at this situation in another way, we may compare it to the many cases in which heating of a hydrated, noncrystalline solid yields porous powders, followed (at higher temperatures) by low surface area crystalline powders. When water vapor leaves a solid, it creates free volume, and this causes an increase in the mobility of ions and molecules. This mobility allows a reaccommodation of ions, with a decrease in free volume; contraction causes pore formation and fracture, both of which (16) Forster, E. Schaumkunstatoffe. In Polymere Werkstoffe;Batzer, H. H., Ed.; Georg Thieme Verlag: Stuttgart, 1984; Vol. 111, p 285.
Abreu Filho et al. are factors against cohesion. On the other side, foam formation requires that pore formation and growth do not cause fracture. That means cracks and fissures have to be healed fast, and this can only happen if mass diffusivity is kept high in the solid, during gas cell expansion. On the other side, too high mass diffusivity and fluidity may cause collapse. We believe that collapse is prevented, in the present case, by crystallization being concurrent with gas cell expansion. From what happens with synthetic organic homo- and copolymers, we know that interspersion of crystals and rubbery amorphous material yields useful viscoelastic properties to a solid b0dy.l' These different morphologies coexist in iron(II1) metaphosphate during foaming; we believe that they are responsible for the conversion of hydrous, poorly crystalline iron(II1) metaphosphate into a rigid foam instead of a powder, under heating. A definite identification of the phases involved in the transformations described in this work is not possible, at this moment, for the relevant pure phases have not yet been described, in the literature. On the other hand, we believe that the facts described in this work may help in finding routes for the fabrication of solids containing closed, gas-filled cells. These solids are desirable in many cases, for instance, in hightemperature insulators and in making white pigments to replace titanium dioxide. Titanium oxide is widely used as a pigment; a major factor of this use is its high refractive index and the concurrent intense light backscattering. Particles containing closed pores in the l-pmdiameter size range should be good scatterers, but the major events which could be used in their fabrication are not well-known, at this moment. We plan to extend the present work, in this direction. Conclusion The transformation of IMP, a poorly crystalline solid, into a rigid foam is the result of gas (HzO) release within a fluid mass, which hardens due to crystallization. Acknowledgment. P.P.A.F. was a FAPESP predoctoral fellow. This work was supported by PADCT Grant 701846187-9. Registry No. IMP, 27875-33-8. (17) Rodriguez, F. Principles of Polymer Systems, 2nd ed.; McGrawHill: New York, 1982; p 222.
