Hierarchically Structured Aluminium Oxide As a Functional Additive to Fire-Fighting Powder Compositions: Synthesis and Properties

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HIERARCHICALLY STRUCTURED ALUMINIUM OXIDE AS A FUNCTIONAL ADDITIVE TO FIRE-FIGHTING POWDER COMPOSITIONS: SYNTHESIS AND PROPERTIES

I.I. Lebedeva*, A.S. Starostin, V.A. Valtsifer, A.I. Nechaev and V. N. Strelnikov

Institute of Technical Chemistry, UB RAS, Academician Korolev St., 3, 614013 Perm, Russian Federation *[email protected]

ABSTRACT Hierarchically structured aluminium oxide was hydrothermally synthesized with carbamide as a precipitator. Raman spectroscopy, SEM spectroscopy, and X-ray phase analysis were used to investigate morphology and parameters of porously structured aluminium oxide as influenced upon by concentration of reactive solution, excessive portions of precipitator, and the counterion type. Differing mechanisms to form hierarchical structures with variable phase content of the hydrothermally synthesized product were evinced. The conditions under which spherulite-type aluminium oxide was formed were ascertained. Stable super-hydrophobic state of the surface of aluminium oxide spherulites was demonstrated as their characteristic feature obtained after hydrophobization with chlorosilanes. This feature pre-determines prospects of aluminium oxide spherulites as a functional additive for fire-fighting powder compositions to provide their stability against moisture and rheological properties improved.

INTRODUCTION Over last years, technical progress in the coal-mining industry due to the use of upgraded high-performance equipments, as well as to more complicated mining and geological conditions had resulted in excessive dust- and gas-pollution of mines and, consequently, to enhanced risk of explosions [1]. To prevent propagation of dust-gas-air mixes exploded, shale/water barriers and automatic systems to localize explosions are usable. Various substances, starting with simple phlegmatizers reducing ambient temperature down to the level at which combustion is suppressed, and ending with fire-fighting powder compositions (here and throughout as FFPCs), are usable. Extinguishing action of the FFPCs is based on the following phenomena: (a) inhibition of chemical reaction of combustion, (b) dilution of combustible media with gaseous products of degraded powders, (c) heterogeneous chain rupture on the surface of either particles or solid degradation products, (d) homogeneous inhibition on interaction with active centers of gaseous particles formable during evaporation and degradation of powders [1]. To solve the task aimed at suppression of mighty explosions, as well as at more high overall efficiency of automatic systems to suppress explosions, the mentioned systems upgraded both constructively and designedly appear to be insufficient. A specialized FFPC featured by improved rheological properties (mostly, by enhanced flowability) needs to be designed. Taking into account that FFPCs are multiple-size powder mixes, an enhanced flowability can be attained at the expense of reduced interaction forces between ingredients. Analysis of model powder systems has evinced an enhanced flowability to be attained at the expense of functional fillers based on micro-dispersive hydrophobized aluminium oxide

added. Besides, flowability of FFPCs is markedly influenced upon by the shape and particle size distribution of fillers [3]. To design fillers providing reduced caking and enhanced flowability of FFPCs, the authors had offered to use micro-dispersive hydrophobized and hierarchically structured aluminium oxide that would provide formation of a pre-set superficial texture preventing liquids from penetration into micro-relief of a surface. Liquids would contact the surface on an appreciably lesser area from which the droplets would freely roll off, thus providing super- hydrophobicity of fillers [4, 5]. Formation of hierarchical structures is a self-assembly process in which constructional blocks such as nanoparticles, nanofibers, and nanosheets self-organize into 3D ordered micro-structures with different morphology [6]. Development of an efficient, easy-to- implement, environmentally friendly, and low-cost method to synthesize hierarchically structured aluminium oxide is of current importance. Low-temperature hydrothermal synthesis (here and throughout as HTS) appears to be one of such methods. The process can flexibly be controlled by numerous variable parameters, namely: temperature, pressure, duration of synthesis, chemical composition of hydrothermal solutions. However, the use of the given method is restricted. The reason is that its stages include formation of intermediate solid-phase products of hydrolysis that can markedly influence morphology and structural- sensitive properties of aluminium oxide [7]. A peculiarity in formation of aluminium oxide texture as a crystallizing structure is related to phase transformations commonly possible in any stage including ageing of sol/gel (sediment), drying and baking. Besides, aluminium hydroxide is low-soluble. Consequently, on its sedimentation, loose coarse-dispersive sediment composed of incompletely hydrolyzed products and entrapped solution components is formed. Inside such sediment, when aged, hydrolysis reactions and structural ordering with formation of crystal nuclei continue proceeding throughout the volume of particles. These phenomena are accompanied by emission of water, particles of entrapped impurities, and by increasing density of the phase that crystallizes throughout the volume of initially amorphous particles. Crystals are formed not via the classical crystallization mechanism through dissolution, but via the directional accretion mechanism. In accord with the latter, the crystal structure originates not through dissolution of primary particle substance, but within primary particles through rearrangement of their polymeric structure. Ageing of sediment is accompanied by additional phase transfers governing re-dispersion (spontaneous peptization) of starting particles into the more or less crystallized particles that then directionally accrete [7]. The rate and depth of these processes are controlled by the temperature-time regime of synthesis. Earlier [9, 10], the authors had ascertained the boehmite spherulites to be formed under the HTS conditions from aluminium sulfate and carbamide at 130°C. The sorption investigations had shown the structure of spherulites as needle-shaped, with thin self-locking pores. After dehydration at 500-650°C, spherulites are characterized by multimodal pore size distribution explicable by formation of microblocks inside the sheets during dehydration of boehmite. Microblocks are spaced by means of micropores, needles – by means of mesopores. The phase composition of spherulites thermally treated at 650°C corresponds to that of γ-Al2O3. Homogeneous sedimentation provides gradual augmentation of pH value and enables controlling crystal structure, morphology, specific surface area, pore volume/diameter, and particle sizes of the finished product [11]. These properties are dependent on the type of precipitator, the H2O/Al2(SO4)3 ratio, pH value, temperature and duration of ageing.

Excessive urea results in increased pH values and contributes to formation of aluminium oxyhydroxide at pH above 9. Basic aluminium sulfates are formed in an acidic medium. A possibility for anhydrous aluminium sulfate salt to exist and to be degraded at high temperatures creates conditions for formation of a loose framework macrostructure of 2- aluminium oxide sulfate [12]. The CO3 anions formed from hydrolyzed urea govern carbonization of the system, with needle-shaped ammonium aluminium hydroxocarbonate formed at pH 6.5 attained [13, 14]. This work was aimed at exploration of the formation process of hierarchically structured aluminium oxide and at a possibility of using it as a functional additive providing stability of FFPCs to moisture and improving their rheological properties.

Experimental part

Hierarchically structured aluminium oxide was synthesized from either Al2(SO4)3∙18H2O, AlCl3∙6H2O, Al(NO3)3∙9H2O, or an equimolar mix thereof by the HTS method (homogeneous precipitation regime) isothermally at 130°C for 3-65 h. Carbamide CO(NH2)2 as precipitator was used to provide homogeneity of the process. Aluminium oxide was prepared by baking the HTS products at 650°C for 5 h. Table 1 summarizes compositions of the reaction solutions and conditions of synthesis. Table 1. Compositions of reaction solutions 3+ 3+ Specimen Al : CO(NH2)2: Duration of Salt Al H2O: i-PrOH synthesis, h 0.5S-2-65 0.5: 1: 100: 5 65 Al2(SO4)3∙18H2O 1S-2-3 1: 2: 100: 5 3 Al2(SO4)3∙18H2O 1S-2-8 1: 2: 100: 5 8 Al2(SO4)3∙18H2O 1S-2-24 1: 2: 100: 5 24 Al2(SO4)3∙18H2O 1S-2-65 1: 2: 100: 5 65 Al2(SO4)3∙18H2O 2S-2-65 2: 4: 100: 5 65 Al2(SO4)3∙18H2O 1S-3-65 1: 3: 100: 5 65 Al2(SO4)3∙18H2O 1S-4-65 1: 4: 100: 5 65 Al2(SO4)3∙18H2O 1S-5-65 1: 5: 100: 5 65 Al2(SO4)3∙18H2O

1Cl-2-65 1: 5: 100: 5 65 AlCl3∙6H2O 1SCl-2-65 1: 2: 100: 5 65 Al2(SO4)3∙18H2O, AlCl3∙6H2O

1N-2-65 1: 2: 100: 5 65 Al(NO3)3∙9H2O

1SN-2-65 1: 2: 100: 5 65 Al2(SO4)3∙18H2O, Al(NO3)3∙9H2O Aluminium oxide was hydrophobized by means of a mix composed of methyltrichlorosilane (MTCS), dimethyldichlorosilane (DDCS), and trimethylchlorosilane (TMCS) and taken at the equimolar ratio, with the mass ratio of silanes to the aluminium oxide specimen under treatment equaling 1:20. The hydrophobized specimens were the dried at 100°C for 60 min. Low-temperature nitrogen adsorption isotherms were registered at -196°C by means of the ASAP 2020 device (Micrometrics, USA), after degassing the material under vacuum at 350 °C for 3 h. Specific surface area (S) of specimens was calculated in accord with the BET method in 0.05-0.25 p/p0 interval of relative pressure values. Total pore volume (Vtot) was calculated from the quantity of nitrogen adsorbed at relative pressure p/p0 ≈0.99. Pore size

distribution and average pore diameter values were determined from desorption isotherms using the BJH method. Diffractograms were registered on the automatic XRD-7000 diffractometer (Shimadzu, Japan) using the CuKa-radiations in the angle interval 2Θ=10-80°. Microphotographs of specimens were taken on the scanning electron microscope, model XR-3000 (Evex, USA). The Raman spectra were registered with use of the multifunctional Senterra spectrometer (Bruker, Germany) in 400-1200 cm-1 diapason. Preparatory to registration, specimens were dried under vacuum at 105 °C. Thermogravimetric analysis (TGA) and different scanning calorimetry (DSC) were carried out on the TGA/DSC 1 analyzer (Mettler Toledo, Switzerland) at ramp temperature 25˚C – 100˚C. The specimens were heated with the air flow at the rate 10 K∙min-1. The limiting wetting angle was determined on the laboratory Ramé-Hart Goniometer by the sitting drop method on the surface of a hydrophobized aluminium oxide specimen preliminary compacted. Also, the laboratory goniometer was helpful in appraisal of the angle of slope at which a droplet of water spontaneously rolled off the surface of the specimen.

Results and discussions

Figure 1 displays microphotographs of aluminium oxide specimens synthesized at 3+ 3+ variable ratios of Al : CO(NH2)2: H2O. With the Al : H2O ratio varying from 1:50 to 1:200 (Fig.1b), the spherulites gained in their dimensions. The 10 µm sized spherulites were formed 3+ 3+ at Al : H2O = 1:100 (Fig.1c). The CO(NH2)2: Al ratio increased from 2 to 5 resulted in the aluminium oxide structure varying from spherulites (Fig.1c) to accretions of needle-shaped particles (Fig.1f).

Fig. 1. SEM images of aluminium oxide specimens: 0.5S-2-65 (a), 2S-2-65 (b), 1S-2-65 (c), 1S-3-65 (d), 1S-4-65 (e), 1S-5-65 (f) The analyzed Raman spectra of the HT-synthesized products had evinced an increased content of CO(NH2)2 in the reaction solution to result in a changed phase composition of 3+ powders. At CO(NH2)2: Al = 4:1, the Raman spectra, apart from the bands typical of crystal lattice of boehmite at 360, 450, 496, and 675 cm-1, contain the bands assigned to valence 2- -1 -1 vibrations in carbonate groups СО3 at 725 and 760 cm (ν4), and 1100 cm (ν1), to lattice vibrations (librations) at 199 и 286 cm-1, and to strain vibrations of hydroxyl groups at 559 -1 cm [16] characteristic of ammonium aluminium hydroxocarbonate NH4Al(OH)2CO3.

Fig. 2. Raman spectra of HT-synthesized products: 1S-2-65 (1), 1S-3-65 (2), 1S-4-65 (3), 1S- 5-65 (4), + - γ-AlOOH, * - NH4Al3(SO4)2(OH)6

Aluminium oxide formed as accretions of needle-shaped particles observable under formation conditions of NH4Al(OH)2CO3 can be described in the framework of the mechanism proposed in [7] wherein the authors, in accord with the von Weimarn theory, explain formation of hierarchically structured NH4Al(OH)2CO3 from aluminium nitrate at the 3+ CO(NH2)2: Al ratio increased up to 10:1. With increasing content of urea in solution, as the authors report, micro-tubes start dividing in smaller sheets followed by their consequent aggregation as urchin-like structures. Whereas, the formation process of boehmite spherulites can be explained in the framework of the directional accretion mechanism [8], as was confirmed by us in the X-ray phase analysis and SEM-assisted study of the HTS products 3+ obtained from aluminium sulfate, at CO(NH2)2: Al = 2:1 and isothermally treated for variable durations. As is apparent from Fig.3 and 4, micro-spherical agglomerates as a mix of amorphous hydrogenated aluminium oxide and ammonium aluminium hydroxocarbonate are formed during HT-synthesis for 3 h. Fig.3b and 4a show how, during 8 h hydrothermal synthesis, boehmite particles originate inside primary spherical particles by rearranging their polymeric structure in the crystallization process. Ageing of the sediment is accompanied by the phase 2- transfer (NH4)2Al2(OH)n(SO4) 3-n/2 → γ-AlOOH. This phenomenon, apparently, causes re- dispersion of starting agglomerates into accretions of needle-shaped boehmite particles crystallized that, in turn, directionally aggregate into spherulites, as is observable after 24 h HT-synthesis (Fig.3c, 4a). The phase composition of spherulites thermally treated at 650°C corresponds to that of γ-Al2O3 (Fig.4b).

Fig. 3. SEM microphotographs of aluminium oxide specimens: 1S-2-3 (а), 1S-2-8 (b), 1S-2- 24 (c)

Fig. 4. Diffractograms of HT-synthesized products (a) and aluminium oxide specimens (b): 1S-2-3 (1), 1S-2-8 (2), 1S-2-24 (3), * - γ-AlOOH, o - NH4Al3(SO4)2(OH)6 + - γ-Al2O3

As is apparent from microphotographs in Figure 5, the HT-synthesis under conditions 3+ γ-AlOOH, CO(NH2)2: Al = 2:1 results in formation of spindle-shaped somatoids as aggregated needle-shaped particles. The HT-synthesized mix of sulfate and aluminium chloride/nitrate results in formation of γ-AlOOH aggregates as urchin-like structures.

Fig.5. SEM images of aluminium oxide specimens: 1Cl-2-65 (a), 1SCl-2-65 (b), 1N-2-65 (c), 1SN-2-65 (d) Exploration of produced aluminium oxide specimens by the low-temperature adsorption method evinced specific surface area and total pore volume to be dependent on the aggregate structures (Table 2). The somatoid-like structures are characterized by larger total pore volumes and by lesser specific surface areas as compared with spherulites.

Table 2. Parameters of aluminium oxide porous structure

Specimen SBET, V, DBJH, 2 -1 3 -1 m g cm g nm 0.5S-2-65 168 0.344 3.3, 5.6 1S-2-3 40 0.055 3.8 1S-2-8 114 0.171 3.6 1S-2-24 163 0.217 4.0 1S-2-65 196 0.253 4.1 2S-2-65 225 0.281 3.7 1S-3-65 161 0.255 3.5, 5.1 1S-4-65 177 0.332 3.8, 6.8, 8.9 1S-5-65 221 0.320 3.7, 8.7 1Cl-2-65 164 0.361 3.6, 7.1 1SCl-2-65 164 0.404 3.3, 7.7 1N-2-65 138 0.402 3.5, 8.6 1SN-2-65 177 0.382 3.5, 8.6

Thus, aluminium oxide spherulites (specimen 1S-2-65) were opted to be hydrophobized. Aluminium oxide particles exhibit hydrophilic properties due to availability of superficial hydroxyl groups and sorption water. When hydrophobized with an equimolar mix of MTCS, DDCS, and TMCS, stable super-hydrophobic state of the surface with the following parameters: (a) limiting wetting angle θv over 150°, (b) angle of slope at which droplets of water roll off less than 8°, was confirmed for aluminium oxide spherulites (Fig.6).

Fig.6. Results of measurements of limiting wetting angle θv (a) and angle of slope at which a droplet of water rolls off (b) the surface composed of hydrophobized aluminium oxide particles compacted (specimen 1S-2-65)

When examined by means of electron microscopy, the hydrophobized aluminium oxide spherulites (specimen 1S-2-65) evinced no changes in their structure during hydrophobization process.

CONCLUSIONS

To produce aluminium oxide spherulites from aluminium oxide sulfate during HT- 3+ synthesis, the ratio CO(NH2)2 : Al should not exceed 3: 1. With excessive portions of precipitator, formation of needle-shaped crystals of NH4Al(OH)2CO3 occurs. With the ratio 3+ H2O: Al reduced from 100 :1 to 50: 1, spherulite particles lessen in size from 10 µm to 5 10 µm. 3+ Under formation conditions γ-AlOOH, CO(NH2)2: Al = 2: 1 and during HT- synthesis, spindle-shaped somatoids are formed. Somatoid structures are characterized by larger total pore volumes and by lesser specific surface areas as compared with spherulites. The HT-synthesized mix of aluminium sulfate and aluminium chloride/nitrate results in formation of γ-AlOOH aggregates as urchin-like structures. When hydrophobized with chlorosilanes, stable super-hydrophobic state of the surface with the following parameters: (a) limiting wetting angle over 150°, (b) angle of slope at which a water drop rolls off less than 8° is characteristic of aluminium oxide spherulites. These parameters pre-determine using the hydrophobized aluminium oxide spherulites as functional additives providing stability of FFPCs to moisture and improving their rheological properties.

Acknowledgements

This work was financially supported by the Ministry of Education and Science of the Russian Federation under Agreement Nr. 14.607.21.0160 (RFMEFI60716X0160).

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