Research Article |
Corresponding author: Nataliya Yu. Tabachkova ( ntabachkova@gmail.com ) © 2022 Dmitrii A. Agarkov, Mikhail A. Borik, Galina M. Korableva, Aleksej V. Kulebyakin, Elena E. Lomonova, Filipp O. Milovich, Valentina A. Myzina, Pavel A. Popov, Nataliya Yu. Tabachkova.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Agarkov DA, Borik MA, Korableva GM, Kulebyakin AV, Lomonova EE, Milovich FO, Myzina VA, Popov PA, Tabachkova NYu (2022) Thermal conductivity of single crystals zirconia stabilized by scandium, yttrium, gadolinium, and ytterbium oxides. Modern Electronic Materials 8(1): 1-6. https://doi.org/10.3897/j.moem.8.1.85242
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The phase composition and heat conductivity of (ZrO2)0.9(R2O3)0.1 solid solution single crystals have been studied, where R = (Gd, Yb, Sc, Y), (ZrO2)0.9(Sc2O3)0.09(Gd2O3)0.01 and (ZrO2)0.9(Sc2O3)0.09(Yb2O3)0.01. Single crystals have been grown by directional melt crystallization in a cold skull. The phase composition of the crystals has been studied using X-ray diffraction and Raman spectroscopy. The heat conductivity of the crystals has been studied using the absolute steady-state technique of longitudinal heat flow in the 50–300 K range. We show that at a total stabilizing oxide concentration of 10 mol.% the phase composition of the crystals depends on the ionic radius of the stabilizing cation. The (ZrO2)0.9(Sc2O3)0.1 crystals have the lowest heat conductivity in the 50–300 K range while the (ZrO2)0.9(Gd2O3)0.1 solid solutions have the lowest heat conductivity at 300 K.
Analysis of the experimental data suggests that the heat conductivity of the crystals depends mainly on the phase composition and ionic radius of the stabilizing cation. Phonon scattering caused by the difference in the weight of the co-doping oxide cation has a smaller effect on the heat conductivity.
zirconia, crystal growth, heat conductivity, phase analysis
Zirconia based materials are widely used in engineering nowadays [
At normal pressure zirconia have three polymorphic modifications: monoclinic, tetragonal and cubic which are stable in different temperature ranges. Stabilization of the high-temperatu-re tetragonal and cubic phases at room temperature is usually achieved by doping with alkaline-earth and rare-earth elements, yttrium or scandium [
ZrO2 based solid solutions are also widely used as heat-insulating protective coatings. These coatings can be operated at high temperatures and should have low heat conductivity and good mechanical properties for long-term operation [
Many types of crystals having a disordered structure including zirconia based solid solutions have low heat conductivity over a wide range of temperatures (0.1 < T < 300 K) which is typical of amorphous materials [
Typically the thermophysical properties of the zirconia based materials are studied for ceramic specimens [
The heat conductivity of cubic and tetragonal single crystal solid solutions of ZrO2–Y2O3 was studied earlier [
The aim of this work was to study the effect of the stabilizing cation (Gd, Yb, Sc, Y) on the heat conductivity of ZrO2 based solid solution single crystals. The stabilizing oxide concentration in the test solid solutions was 10 mol.%.
(ZrO2)0.9(R2O3)0.1 solid solution crystals where R = (Gd, Yb, Sc, Y), (ZrO2)0.9(Sc2O3)0.09(Gd2O3)0.01 and (ZrO2)0.9(Sc2O3)0.09(Yb2O3)0.01 were grown using directional melt crystallization in a cold skull (130 mm diam.) at a 10 mm/h rate [
The phase composition of the specimens was studied using X-ray diffraction on a Bruker D8 instrument and Raman scattering. The excitation source was a 633 nm laser.
The heat conductivity of the crystals was studied using the absolute steady-state technique of longitudinal heat flow in the 50–300 K range. The absolute heat conductivity determination error was within ± 6%. The 7×7×20 mm specimens were cut from the crystals along the growth axis and had an arbitrary crystallographic orientation.
Two series of crystals were grown: ZrO2 solid solutions stabilized with 10 mol.% Yb2O3, Y2O3, Gd2O3 or Sc2O3 hereinafter denoted as 10YbSZ, 10YSZ, 10GdSZ and 10ScSZ, respectively, and ZrO2 solid solutions co-stabilized with 9 mol.% Sc2O3 and 1 mol.% Gd2O3 or 1 mol.% Yb2O3 hereinafter denoted as 9Sc1GdSZ and 9Sc1YbSZ, respectively.
The 10ScSZ and 9Sc1GdSZ solid solution crystals were inhomogeneous and light-scattering but contained no pores. The other test specimens were homogeneous and transparent single crystals.
According to X-ray diffraction data the 10YbSZ, 10YSZ and 10GdSZ crystals had a cubic fluorite structure and were single-phase in the entire bulk. The 10ScSZ crystal was a mixture of two phases, i.e., the cubic and rhombohedral ZrO2 modifications. Figure
Figure
Heat conductivity of crystals as a function of temperature: (1) 10YSZ, (2) 10YbSZ, (3) 10GdSZ and (4) 10ScSZ
As can be seen from the data in Fig.
Despite the difference in the low temperature heat conductivities (50–150 K) for the 10YbSZ and 10ScSZ crystals due to their different phase compositions, an increase in temperature makes their heat conductivities almost equal at 300 K. The small ionic radii of the Sc3+ and Yb3+ cations entail the variety of possible defect structures and a higher disordering of the cation and anion sublattices [
The low-temperature heat conductivities of the 10GdSZ crystals are higher than those of the 10ScSZ crystals. However in the 150–300 K range the heat conductivities of the 10GdSZ crystals are the lowest for this test series of specimens. The size of the Gd3+ cations is greater than those of Y3+, Yb3+ and Sc3+, and this may cause greater stress and disorder in the anion sublattice and entail a lower heat conductivity of the 10GdSZ crystals as compared with those of other solid solutions at 300 K.
Zirconia is often stabilized with several oxides for the modification of the structure and physic-chemical properties of its solid solutions [
According to phase analysis the 9Sc1GdSZ crystals were a mixture of the tetragonal and cubic ZrO2 modifications. The 9Sc1YbSZ crystals had a cubic fluorite structure. Figure
Thus substitution of 1 mol.% Sc2O3 in the 10ScSZ crystals for 1 mol.% Gd2O3 or Yb2O3 produces crystals with different phase compositions.
Figure
Heat conductivity of crystals as a function of temperature k (T): (1) 9Sc1GdSZ, (2) 9Sc1YbSZ and (3) 10ScSZ
The heat conductivities of the 9Sc1GdSZ and 9Sc1YbSZ crystals are close and higher than those of the 10ScSZ crystals in the entire experimental temperature range.
Thus zirconia co-doping with two types of stabilizing oxide may change the pattern of the k (T) function and the heat conductivity in comparison with that of the crystals stabilized by sole scandia. Zirconia co-doping with two stabilizing oxides while retaining the total stabilizing oxide concentration (10 mol.%) should change the defect structure of the cation sublattice. Co-doping with oxides one of which has a small ionic radius (RSc3+ = 0.87) and the other one is a big cation (RGd3+ = 1.053) changes the stress pattern in the crystal lattice. Furthermore this entails a change in the formation of defect complexes: a statistical distribution of oxygen vacancies relative to the Zr4+ and Sc3+ cations changes to a distribution for which the oxygen vacancies are predominantly located in the vicinity of the bigger stabilizing oxide cation Gd3+. For co-doping with oxides both of which have small ionic radii (RSc3+ = 0.87 and RYb3+ = 0.985) these changes are far less intense. Furthermore the heat conductivity of the crystals also depends on its phase composition.
A change in the phase composition of the 9Sc1GdSZ and 9Sc1YbSZ crystals increases their heat conductivity in comparison with that of the 10ScSZ crystals. Phonon scattering caused by the difference in the radii and weights of the cations has a smaller effect on the heat conductivity.
(ZrO2)0.9(R2O3)0.1 solid solution crystals where R = (Gd, Yb, Sc, Y), (ZrO2)0.9(Sc2O3)0.09(Gd2O3)0.01 and (ZrO2)0.9(Sc2O3)0.09(Yb2O3)0.01 were grown using directional melt crystallization in a cold skull. The stabilizing oxide concentration in the test solid solutions was 10 mol.%.
The heat conductivity of the ZrO2 based solid solutions depends largely on the phase composition of the crystals. The presence of a mixture of the cubic and rhombohedral ZrO2 modifications in the (ZrO2)0.9(Sc2O3)0.1 crystals leads to the lowest heat conductivity of these crystals in the 50–100 K range compared with the other test crystal compositions. At 300 K the difference in the weights of the Sc3+ and Yb3+ cations has little if any effect on the heat conductivity. The ionic radius of the stabilizing cation has the greatest effect on the heat conductivity.
This work was carried out with financial support under RNF Grant 19-72-10113. The structure was studied at the Joint Use Center for Materials Science and Metallurgy of the National University of Science and Technology MISiS with financial support from the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2021-696).