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 57 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. 58 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