Studying the influence of properties of initial ZrO2-(7 %)Y2O3 powders on properties of ceramic products

Introduction

Zirconium oxide-based ceramic products have a number of unique properties such as low thermal conductivity, high thermal expansion coefficient, high thermal stability, high strength, crack resistance, thermal shock resistance [1] and are used as thermal protection coatings, biocompatible materials, corrosion-resistant materials, and conductive ceramics [2]. The slip casing method is widely used to form ceramic products based on zirconium oxide [3]. The process of slip making includes a powder grinding stage which takes a lot of energy and effort and leads to powder contamination by grinding media and to the loss of powder. To make slip according to a standard production technology, prepared powders shall be ground by bead mills to obtain a grain size of 3–5 µm. That is why it is important to study the synthesis of zirconium oxide-based submicron powders for making ceramic products by using the slip casing method without a mechanical power grinding stage. Sol-gel technology is widely used for making submicron powders [4][5], but synthesis results largely vary, that is why we had to turn to the relevant patents and study the effect of hydrolysis processes on the properties of powders. It is difficult to study this problem because there are many parameters that affect the particle properties as well as small size of seeds being developed at the initial stage of synthesis [6][7]. The distinction of the research paper is associated with the study
of the effect of parameters such as the pH value for precipitation and addition of sulphate ions. The research objective is to study the effect of synthesis conditions on the properties of prepared powders and ceramic samples.

Experimental part

Zirconium carbonate ZrOCO₃ and yttrium oxide Y₂O₃. were used as initial compounds. The compounds were dissolved in a stoichiometric amount of nitric acid solution, then the prepared solutions were mixed at a proportion required for forming the end product containing 93 % of ZrO₂ and 7 % Y₂O₃ by weight. The best suitable concentration of the combined solution was determined after dissolving 150 g/l expressed as the sum of oxides.

The next stage was intended to make slip of the prepared powders, skipping the mechanical grinding stage. To make slip, we took 20 g of powder, 2 g of fibre (Al₂O₃ - 50-56 %, ZrO₂ - 14-17 %, SiO₂ - 27-36 %, with the average fibre diameter of 2–4 µm), 40 g of 5 % paraffin solution in petroleum, then stirred all the reagents in a glass jar and distilled petroleum off at a temperature of 130 °С. A mould heated up to 60 °С was filled with the dried mixture and pressed at 5 MPa. Semi-finished products were removed from a mould and sintered at 1200 °С for 1 h at a heating rate of 4 °С per minute. Samples were cut from the prepared semi-finished products and passed bend tests using a pull test machine [8].

The X-ray diffraction analysis of samples was carried out using the XPertPro MPD diffractometer with a solid-state pixel detector in Сu_Кα radiation using the secondary beam β filter. Coherent scattering regions (CSR) were determined using the Scherrer method by reflections at small scattering angles (form factor K = 0.9).

Electronic pictures of material samples were obtained with the help of the Carl Zeiss SIGMA VP electronic scanning microscope in the high vacuum mode using the InLens detector. The accelerating voltage was 2 kV.

The specific surface area and porosity characteristics of the samples were determined with the help of the low-temperature nitrogen adsorption/desorption method using the Quantachrome NOVA 1200Е analyser. Adsorption/desorption isotherms for all the samples under study were determined in the following range of relative pressure values: 0.1; 0.2; 0.3; 0.4; 0.45; 0.5; 0.55; 0.6; 0.65; 0.7; 0.75; 0.8; 0.85; 0.9; 0.95; 0.98, and 0.99. The preliminary sample gas removal time was 1 h.

The powder particle size distribution was determined with the help of the laser diffraction method using the ANALYSETTE 22 NanoTec plus particle sizer manufactured by FRITSCH.
Measurements were conducted in the water medium with an ultrasonic generator used for additional particle dispersion during measurement.

The sample density was determined by measuring their weight and dimensions. To determine open porosity, water weighing of samples was carried out. 

Results and discussion

According to the X-ray diffraction analysis, all synthesized powders are substitutional solid solutions of tetragonal structure with the empirical formula similar to Y_0,08Zr_0,92O₂. The only difference is the sizes of coherent scattering regions (CSR). For samples ZrY–8 и ZrY–8go, we found traces of ~7 % monoclinic phase. Tetragonal phase CSR sizes for all samples are given in Table 1. Sample ZrY–8go has the smallest CSR size. Hydrothermal treatment leads to reduction of CSR sizes, probably, due to a higher defect rate and porosity on the grain boundary.

The powder particle size distribution is shown in Fig. 1. The average particle size (D_cр) is given in Table 1. All samples feature bimodal particle size distribution with the first peak in the area of 1 µm and the second peak in the area of 10–18 µm. The samples that precipitated at рН = 5 have the smallest average diameter, which may be caused by the proximity of the zirconium oxide’s isoelectric point. It has been found out that hydrothermal treatment has no significant effect on the samples’ particle size distribution. Sample ZrY–8go has the largest average diameter of particles.

Fig. 2 shows nitrogen adsorption and desorption isotherms for all samples. The shape of hysteresis loops of isotherms allows to determine the pore shape. Generally, the resulted samples have cylindrical pores, but sample ZrY–5 has slot-shaped pores [9]. Sample surface and porosity parameters are given in Table 1. Sample ZrY–8go has the largest specific surface area, porosity, and pore diameter. The pH value for precipitation is likely to have the strongest effect on CSR sizes and porosity parameters: precipitation at a higher рН leads to a high degree of supersaturation in the place of droplet fallout, as well as high specific adsorption of hydroxyl ions on the surface of particles being formed, thus preventing the processes
of recrystallization of hydrated oxide particles in the course of precipitation and hydrothermal treatment, and leads to formation of particles with a small CSR size and with well-developed porosity. Precipitation at рН = 5 makes conditions for reducing the supersaturation degree in the place of droplet fallout as well as for reducing the specific adsorption of hydroxyl ions. This leads to more intense processes of particle growth during recrystallization in the course of precipitation and evidently to an increase in CSR sizes. 

Fig. 3 shows electronic pictures of samples ZrY–5go and ZrY–5. The pictures show that both samples are weakly coupled aggregates consisting of particles, the shape of which is similar to the spherical shape with the size of about 25 µm. These characteristics correspond to CSR sizes based on X-ray diffraction analysis data. Sample ZrY–5go has a lower density of initial particle aggregation, thus corresponding to a larger value of the surface area and porosity of the sample compared to sample ZrY–5. Implementation of the hydrothermal treatment stage probably leads to reduction of excess surface energy due to the processes of gradual dehydration and recrystallization.

Fig. 4 shows electronic pictures of samples ZrY–8go and ZrY–8. Sample ZrY–8go demonstrates fragmented aggregates, which comprise initial particles with close-packed arrangement and sizes that are far smaller than those of the previous samples. This corresponds to the size of CSR 120 Å based on X-ray diffraction analysis data. This sample has the largest specific surface area and the smallest CSR size, thus triggering intense aggregation of particles. Fig. 4 shows that the sample ZrY–8 has initial particles of larger size than the sample with hydrothermal treatment. This corresponds to the X-ray diffraction analysis data.

Parameters of composites prepared from synthesized powders are given in Table 2. A ceramic product made from sample ZrY–8 fell apart, and we failed to measure its properties, presumably, due to large particle size, relatively low surface area and rather high porosity. During the sintering process, a ceramic sample, which did not have a developed surface, failed to build consistent internal bonds, was severely shrunk and fell apart – probably due to the shrinkage. All ceramic samples under study demonstrate low bending strength values in comparison to samples based on a traditional technology. Probably, the grinding process involves not only mechanical grinding, but also mechanochemical activation of the sample surfaces, which ensures better sintering. Thus, composite ZrY–8go has the highest density and bending strength at the level of 18 MPa only. Apparently, due to high porosity and a large surface area of powder, particles are able to sinter efficiently, even without additional mechanochemical activation.

All mechanical characteristics of samples ZrY–5 and ZrY–5go are lower than those of composite ZrY–8go, while there is an upward trend in density and bending strength of composites as the specific surface area of initial powder materials grows and the CSR size reduces. Implementation of the preliminary dispersion procedure for yttriumstabilized zirconium dioxide powders using a bead or colloid mill will possibly lead to significant improvement of mechanical properties of composites produced with the help of the slip casing technology.

Conclusion

The paper describes the synthesis of ZrO₂–(7 %) Y₂O₃ powders in various conditions. We have managed to synthesize high-dispersive powders without extra grinding, reaching a peak
value of distribution for particles with the size less than 1 µm. Using the slip casing method, ceramic composites have been synthesized from produced powders, while properties of powders and sintered products have been investigated. We have found out that the tendency to sintering basically depends on the specific surface area of powders rather than on the precursor particle size. In order to improve mechanical properties of composites, the stage of additional mechanochemical activation of powder surface shall be introduced.