SYNTHESIS METHODS AND TEMPERATURE EFFECTS ON TITANITE MICROSTRUCTURE

ABSTRACT

In this study, this titanosilicate ceramic product (Sphene, CaTiSiO₅) was prepared using co-precipitation, sol-gel and solution incineration methods. All of these processes produced amorphous powders in the initial phase, which were found to crystallize on further calcination and form co-crystals with perovskite, titania, and cristobalite. The obtained powder with more moisture, calcined at 1000°C for 2 hours, was found to be the optimal condition for the formation of single-phase sphene. Powders obtained using sol-gel-based and incineration-based methods were confirmed to require high temperature (2 h 1200°C) to form single-phase sphene. When sphene powder obtained by co-precipitation method was sintered at 1250 C for 2 hours, a theoretical density of 98% was achieved, and samples with a uniform volume microstructure of the sintered body microstructure up to 1 mm were obtained.

АННОТАЦИЯ

В этом исследовании этот титаносиликатный керамический продукт (сфен, CaTiSiO₅) был приготовлен с использованием методов соосаждения, золь-гель и обжига в растворе. В результате всех этих процессов на начальном этапе образуются аморфные порошки, которые, как было обнаружено, при дальнейшем прокаливании кристаллизуются и образуют сокристаллы с перовскитом, титания и кристобалитом. Полученный более влажный порошок, прокаленный при 1000°С в течение 2 часов, оказался оптимальным условием для образования однофазного сфена. Подтверждено, что порошки, полученные золь-гель-основе методами и методами сжигания-основе, требуют высокой температуры (2 часа 1200°С) для образования однофазного сфена. При спекании порошка сфена, полученного методом соосаждения, при 1250 С в течение 2 часов была достигнута теоретическая плотность 98 % и получены образцы с однородной объемной микроструктурой с микроструктурой спеченного тела до 1 мм.

Keywords: sphene, sol-gel, silico fume, sintering, calcination.

Ключевые слова: сфен, золь-гель, кремниевая пыль, спекание, прокаливание.

Introduction. Sphene (CaTiSiO₅), which is the main crystalline phase of titanosilicate glass-ceramics, is considered as the main material for immobilization of radioactive waste generated in nuclear reactors[1]. Sphene crystals are thermodynamically stable in granite and groundwater, contain many fission products and actinides, and form a solid solution[2]. Many studies have been carried out on the synthesis of oxide materials for immobilization of radioactive waste by combustion method, and in continuation of them, in this study, sphene is synthesized by co-precipitation, sol-gel and combustion methods. In the synthesis of sphene, the production methods are of great importance and they have a great influence on the powder properties, crystallization sequence, sintering and microstructure of sphene[3-5]. Surface area and particle size measurements, thermogravimetric and differential thermal analysis (TG-DTA), X-ray powder diffraction and scanning electron microscopy are particularly important in this research.

Experimental and analytical techniques.

Various methods are used for the production and synthesis of nanomaterials, such as the sol-gel method (solution method), steam phase compression method, mechanical alloy method or collision with high-energy granules, plasma and electrochemical methods. Although all of the mentioned methods allow the production of large-scale nanomaterials, the sol-gel method is more efficient than other existing methods and has great practical importance on an industrial scale. Due to its unique properties, this method allows obtaining high-quality nanoparticles on an industrial scale and preparing composites with high purity (99.99%). Another advantage of this method compared to traditional methods is that the process takes place at a lower temperature, so the production of glass and glass-ceramic nanomaterials by this method allows saving energy at temperatures from 80°C to 300°C [6-7].

In the co-precipitation method in the study, calcium nitrate was precipitated as hydroxide using ammonium hydroxide in an aqueous solution containing titanyl chloride and hexafluorosilicic acid. Titanyl chloride (TiOCl₂) was prepared by diluting TiCl₄ in HCl under ice-cold conditions. Hexafluorosilicic acid was prepared by dissolving silica in a medium of at least 5% aqueous HF. This reaction can be reversed in an alkaline medium to provide more silicon hydroxide than is needed to neutralize the hexafluorosilicic acid.

Under normal conditions, an aqueous solution containing equimolar amounts of calcium nitrate, titanyl chloride, and hexafluorosilicic acid (0.2 M for each element) was precipitated by slow addition of dilute NH₄OH. The precipitate was washed with distilled water to remove excess ammonium hydroxide and dried in a drying oven at 105°C for 12 hours. The dried hydroxide powder (sphene-I) was precalcined at 700°C for 2 h to remove the adsorbed water and hydroxides and recalcined at higher temperatures to study the thermal phase evolution.

In the sol-gel synthesis of sphene, calcium nitrate, tetraethyl orthosilicate Si(OC₂H₅)₄ and n-butyl titanate Ti(OC₄H₉) are used [8]. An ethanolic solution of tetraethyl orthosilicate was mixed with a stoichiometric amount of an ethanolic solution of n-butyl titanate, and the clear solution was thoroughly stirred for 1/2 hour. A solution of HNO₃ (0.2 M) is added to each mole of n-butyl titanate before mixing with tetraethyl orthosilicate to prevent the formation of a cloudy titanium-rich precipitate and to keep the ethanolic solution of n-butyl titanate clear[9-11]. The alkoxide/alcohol molar ratio was maintained at approximately 1:100. Hydrolysis of this alkoxide mixture was carried out by adding an aqueous solution of calcium nitrate in sphene stoichiometry, and the alkoxide/water molar ratio was maintained at 1:20. The resulting clear solution was stirred for 1 hour and then gelled in a water bath. The resulting gel was dried in an air oven at a temperature of 105°C for 12 hours. The dried gel (sphene-II) was calcined at 700°C for 2 h to remove organic matter and nitrates.

Sphene-III was prepared by burning an aqueous reducing mixture containing stoichiometric amounts of calcium nitrate, titanyl nitrate, silicon dioxide. Titanyl nitrate is made from titanium tetrachloride. Silicon (ⅳ) oxide dust (surface area '200 m/g) was used as a silicon source. Accordingly, the stoichiometry of the oxidation-reduction mixture was calculated. Incineration processes were carried out in a muffle furnace heated to 500°C.

Simultaneous TG-DTA (Polymer Laboratories, model STA 1500) of the sphene powders (sphene-I, -II, and -III) was performed. Room temperature powder XRD patterns recorded on a Philips 3710 diffractometer (T=20 ̊C; CuKα radiation; 2θ range 9̊–145̊, ∆2θ step 0.02̊ per 4 s) was used to identify the various phases. Samples were calcined in alumina boats at various temperatures in air with a soaking period of 2 h to study the sphene phase evolution. The powder properties were evaluated by BET surface area measurement (Micrometrics Instrument Corp., model 2100E Accusorb, Norcross, GA), particle size analysis (Seishin, model SKC 2000 micron photosizer), and density measurements. Sintering of compacted powders was carried out at 1100–1375°C in air at a heating rate of 10°C/min and a soaking period of 2 h. The sintered pellets were polished and etched with 5% HF for 20 s for the microstructure investigation using a scanning electron microscope (Jeol, model JSM-840 A).

The co-precipitation product (sphene-III) had a high density (98% theoretical) and was found to have a uniform microstructure in its electron microscope image[Picture 1.]

Picture 1. Scanning electron micrographs: sphene-I (a) 1200 and (b) 1300, sphene-ⅠI (e) 1250 and (d) 1300

Results and discussion.

The simultaneous TG-DTA for sphene-I showed a weight loss of about 7% in the temperature range 60–300°C and an endotherm around 60°C. This could be attributed to the removal of adsorbed moisture and ammonia. Sphen crystallization occurred at 800°C, as observed by an exotherm.

Figure. 2 TG-DTA of (a) sphene-I, (b) sphene-II, and (c) sphene-III

TG-DTA of sphene-II is shown in Picture 1(b). The initial weight loss of about 8% in the temperature range 60–140°C and the corresponding endothermic peak results from the evaporation of alcohol and adsorbed moisture. The second-step weight loss in the temperature range of 350–550°C can be attributed to the pyrolysis of organic compounds such as ethyl and isobutyl alcohols or species formed during the hydrolysis reaction and the gelation process. The total weight loss observed was 45%. The exotherm at 840°C may be due to the crystallization of sphene. Sphene-III shows a weight loss of approximately 5% around 500°C, and there is no weight loss above this temperature (Pic. 1c). DTA trace shows one endothermic peak at 470°C and two exothermic peaks at 780°C and 875°C. The endotherm may be due to the removal of impurities such as trapped nitrate ions and adsorbed water.

The sphene crystallization temperature for sphene-III was 20–60°C, lower than that for sphene-I and sphene-II. However, the sphene-I powder crystallized at a faster rate above 900°C, and single phase sphene was formed at 1000°C, which is about 200°C lower than the temperature required for sphene-II and sphene-III powders. This could be attributed to the mineralizing effect of some adsorbed fluoride ions, which favors the formation of sphene phase at lower temperature in sphene-I. A similar mineralizing effect has been observed in the preparation of zircon (ZrSiO₄)-based pigments [11], where NaF is deliberately added as a mineralizer to reduce the zircon formation temperature.

Compacted sphene, derived from the sol-gel process (sphene-II), was sintered at 1200– 1300°C for 2 h. Sphene-II achieved 85% theoretical density at 1200°C, and the density increased to 93% on sintering at 1300°C. The micrograph of the surface of a pellet sintered at 1200°C shows poor densification and the presence of pores (Pic. 3c). The micrograph shown in Picture 3d reveals the dense nature of the compact sintered at 1300°C. The grains are nearly diamond shaped with almost uniform grain size ranging from 2–4 mm. The smaller grain size and higher densification of sol-gel-derived sphene could be attributed to the lesser agglomeration in the powders. The scanning electron microscopy microstructure of sphene-III (1250°C) shows poor densification (83% theoretical density) and the presence of open pores with the grain sizes ranging from 2–4 mm. When the sintering temperature was increased to 1375°C, the bulk density increased to 96% theoretical value.

Summary. Sphene (CaTiSiO₅) was prepared by co-precipitation, sol-gel and combustion processes. The crystallization and sintering of sphene is greatly influenced by the preparation method. Single-phase sphene formation occurs at a lower temperature (1000°C) in sphene-I compared to sphene-II and sphene-III ('1250°C).

Small amounts of CaTiO₃ and cristobalite SiO₂ appeared as impurities even after calcination at 1200°C due to homocondensation of alkoxides in sphene-II. The co-precipitation product (sphene-III) had a high density (98% theoretical) and was found to have a uniform microstructure in its electron microscope image[Picture 1.]

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