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Production of zirconia-based ceramic fibers using viscose material

Abstract

The paper focuses on the technology of producing ZrO2-Y2O3 ceramic fibers by impregnating twisted viscose yarns with zirconyl nitrate solutions with the addition of yttrium nitrate and subsequent heat treatment. We determined the effect of the impregnating solution concentration on the strength characteristics of the obtained ceramic fibers. As a result, we proposed a method for determining the tensile strength of discrete ceramic fibers

For citation:


Titova S.M., Obabkov N.V., Zakirova A.F., Dokuchaev V.S., Zakirov I.F. Production of zirconia-based ceramic fibers using viscose material. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2019;(1):85-90.

Introduction

The oxide ceramic materials have high corrosive and chemical resistance, radiation tolerance, and low thermal conductivity, which enables longterm operation of ceramic products under exposure to aggressive media and high temperatures [1][2]. Oxide ceramic materials with high porosity find wide use in state-of-the-art technologies, serving, for example, for ensuring thermal protection of assemblies and parts of aviation and rocket equipment, manufacturing catalyst elements, filters, partially permeable membranes, electrodes, fuel and electrolyte elements, as well as components of bone prosthetics and implants [3][4][5][6]. A significant disadvantage of porous ceramics are the low values of their strength characteristics, which can be improved through reinforcement with high-strength fibres and fibrous crystals [7][8][9][10]. For ceramic materials operating under high temperatures in oxidising media, the most efficient way is the use of oxide fibres [2]. However, as of today the technologies for obtaining fibres with specified composition and properties are developed insufficiently, which makes the problem of creating novel ceramic fibres a relevant one.

There exist several methods for synthesising ceramic oxide fibres, among them are extraction, extrusion, sol-gel method [11][12][13][14]. One of the most technologically advanced and readily implemented is the method of adsorption, or impregnation, based on the capability of synthetic polymers to absorb inorganic compounds [15].

The paper presented here is an applied research project aimed at development of a technology for manufacturing ceramic fibres of specified chemical and phase composition for reinforcement of composites. Obtaining ceramic fibres of different diameter is possible using appropriate viscose yarns. When setting up production of such fibres, a continuous process can be arranged, which includes impregnation of continuousyarn tows, followed by drying and firing in feedthrough machines. A technology has been developed within the framework of this paper for obtaining oxide fibres of ZrO2–Y2O3 composition by precipitating them from saline solutions onto organic fibres, the latter being represented by twisted viscose yarns having periodic structure as required for reinforcement, which comprises the novelty of this approach.

Experimental part and discussion of results

The process of ceramic fibre formation is based on precipitation of saline mixture of zirconium and yttrium nitrates onto the surface of viscose fibres. For that purpose the fibres are immersed in a saline solution of appropriate composition and cured in it for a certain period of time. After withdrawal from the solution, the impregnated fibres are subjected to drying and firing. Firing is accompanied by the process of formation of ceramic fibre structure due to thermal destruction of viscose and decomposition of saline mixture, with formation of ZrO2–Y2O3 ceramics [15]. In so doing, used as the carrier fibre is viscose, since this material absorbs several times as much water and solutions as other fibres, e. g. cotton ones. It allows to obtain ceramic fibres with lower porosity and higher strength [16].

The working solutions of zirconyl nitrate and yttrium nitrate used in impregnation were prepared by dissolving quantities of the basic zirconium carbonate ZrO(CO3)2 and yttrium oxide
Y2O3 in concentrated nitric acid.

The solution for impregnating viscose material was prepared by mixing the obtained nitrate solutions so as to obtain the oxide phase with composition ZrO2 – 7 mass % and Y2O3 – 93 mass %. Such a composition of zirconia solid solution is partially stabilised in tetragonal modification, with a small addition of monoclinic phase, and has a higher thermal resistance [1][7][17]. Additionally introduced in the impregnating solution as a plasticiser and thickener was polyvinyl alcohol in the form of a 10 % aqueous solution, taken in amount of 10 % of the impregnating solution total mass [18].

Used in this paper were viscose yarns of the 1st grade, glossy, dia. 0.55 mm, which corresponds to yarn linear density of 200 tex. The yarns for saturation were immersed in the prepared solution and vacuum-treated for 40 minutes; the total concentration of zirconium and yttrium was varied within a range of 200 to 500 kg/m3 (in terms of the equivalent amount of oxides). The рН of the prepared solution was 0.1–0.2. The total viscose fibre curing time in the solution was 24 hours. After impregnation the yarns were withdrawn from the solution and dried at a temperature of 80 °С for 2 hours, then the temperature was raised to 400 °С and the yarns were cured in those conditions for another 2 hours. The fibres formed after drying had an increased through porosity, thus enabling their re-impregnation for strengthening the fibres, which was done applying solution of the same composition and under the same conditions as in the first impregnation. The dried fibres were further subjected to firing at a temperature of 1400 °С for 2 hours. In so doing, considerable shrinkage was observed in the process of ceramic fibre sintering, so the diameter of finally sintered fibre, with the content of ZrO2 – 7 mass % and Y2O3 – 93 mass %, amounted to 0.3–0.4 mm. After firing of long fibres, their fragmentation into pieces 5–20 mm long (Fig. 1, а) occurred because of their high fragility.

Fig. 1. Ceramic fibres ZrO2–Y2O3 synthesised by the method of double impregnation of twisted viscose yarn with solution of zirconyl nitrate and yttrium nitrate with total concentration of 500 kg/m3 (in terms of the equivalent amount of oxides): а – external appearance; b, c – microphotographs

Analysis of the microstructure, performed using optical microscope Olympus (Figs. 1, b, c) and scanning electron microscope JSM-6390LA (Fig. 2), showed that fibre cross-section featured fairly large longitudinal pores.

Fig. 2. Ceramic fibre ZrO2–Y2O3 synthesised by the method of double impregnation of twisted viscose yarn with solution of zirconyl and yttrium nitrates: а – external appearance; b – fracture

An X-ray diffraction analysis of the samples of synthesised ceramic fibres was performed on instrument XPertPro MPD (Malvern Panalytical, the Netherlands) in CuK emission. The content of phases in a fibre sample was determined by the Rietveld method. The analytical data were processed by means of XPert High Score Plus software. Based on the X-ray diffraction analysis data, three phases were identified in the fibre samples: cubic and tetragonal (Zr–Y)O2 (the content in sample 45.3 and 28.2 %, respectively), as well as a baddeleyite phase (26.5 %). An X-ray image of ceramic fibre sample is given in Fig. 3.

Fig. 3. X-ray image of ZrO2-Y2O3 ceramic fibre sample synthesised by the method of double impregnation of twisted viscose yarn with solutions of zirconyl and yttrium nitrates: O – baddeleyite phase; Δ – cubic (Zr–Y)O2 phase; I – tetragonal (Zr–Y)O2 phase

To study strength properties of the obtained ceramic fibres, the authors of the paper developed a method based on determining tensile strength of composite “ceramic fibre – epoxy resin”. Samples of the composite material were prepared by casting multi-purpose epoxy glue into a mould of special shape (Fig. 4). The studied fibres in amount of up to 35 pieces were entered into resin in the neck area of the sample. After hardening, the samples were subjected to thermal treatment at a temperature of 50 °С for their strengthening. Samples without fibres, intended for testing strength properties of resin as a matrix material in the composite, were prepared under the same conditions.

Fig. 4. Sample for measuring ceramic fibre tensile strength: 1 – composite material matrix – epoxy resin; 2 – ceramic fibre

The tensile tests were run on a tensile testing machine at a deformation rate of 1.0 mm/min. The process would continue until complete destruction of the sample, then the tensile strength would be determined separately for the “resin – fibre” composite and the resin. Fig. 5 features a fractographic image of composite workpiece neck fracture. The blurred fibre fracture indicates irregularity of its surface. The matrix fracture is represented by smooth glass-like resin facets. The tensile strength of ceramic fibres was calculated by formula (1) proceeding from the additivity concept, having first determined cross-section areas of the sample neck and all the fibres [19]:

where σкм, σвол, σм - tensile strength of the composite material, fibre, matrix, respectively (МPа);

Sвол - cross-section of fibres in relative units (sample neck cross-section).

Fig. 5. Fractographic image of composite material “ceramic fibre – epoxy resin” sample neck fracture

The effect of impregnating solution concentration on the strength properties of the obtained fibres under single-impregnation conditions was studied. It was experimentally determined that the obtained fibre strength almost linearly depends on the impregnating solution concentration (Fig. 6). The maximum strength of the fibres is 190 МPа with the impregnating solution concentration of 500 kg/m3 (as per the sum of oxides). Viscose yarn reimpregnation facilitates fibre tensile strength improvement up to 213 MPa.

Fig. 6. Dependence of tensile strength of ZrO2-Y2O3 ceramic fibres synthesised by the method of viscose yarn single impregnation on total zirconium and yttrium concentration in solution

A concentration of the impregnating solutions exceeding 500 kg/m3 is counter-productive, since viscosity of the working solution increases considerably and, accordingly, impregnability of the viscose yarn deteriorates.

Conclusion

A technology has been developed for synthesising ZrO2–Y2O3 ceramic fibres by the method of twisted viscose yarns impregnation with zirconium oxynitrate solution with the addition of yttrium nitrate. It has been established that the strongest fibres with tensile strength up to 190 MPa are obtained under solution concentration of 500 kg/m3 (in terms of the equivalent amount of oxides). The diameter of the obtained fibres is 0.3–0.4 mm, length – 5–20 mm. At a second impregnation of fibres after their annealing at 400 °С, applying solution of the same composition, followed by high-temperature firing (at 1400 °С), the tensile strength of the fibres increases up to 213 МPа.

About the Authors

S. M. Titova
Ural Federal University named after the First President of Russia B.N. Yeltsin
Russian Federation


N. V. Obabkov
Ural Federal University named after the First President of Russia B.N. Yeltsin
Russian Federation


A. F. Zakirova
Ural Federal University named after the First President of Russia B.N. Yeltsin
Russian Federation


V. S. Dokuchaev
Ural Federal University named after the First President of Russia B.N. Yeltsin
Russian Federation


I. F. Zakirov
Ural Federal University named after the First President of Russia B.N. Yeltsin
Russian Federation


For citation:


Titova S.M., Obabkov N.V., Zakirova A.F., Dokuchaev V.S., Zakirov I.F. Production of zirconia-based ceramic fibers using viscose material. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2019;(1):85-90.

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ISSN 2542-0542 (Print)