L. LIU et al.: EFFECTS OF Dy 3+ -DOPING ON THE THERMOPHYSICAL PROPERTIES OF Ba2YbAlO5 CERAMICS 615–618 EFFECTS OF Dy 3+ -DOPING ON THE THERMOPHYSICAL PROPERTIES OF Ba 2 YbAlO 5 CERAMICS VPLIV DOPIRANJA Ba 2 YbAlO 5 KERAMIKE Z Dy 3+ NA NJENE TERMOFIZIKALNE LASTNOSTI Ling Liu 1,2 , Wei Zheng 1,2 , Zhouwei Zhang 1,2 , Zhuang Ma 1,2 1 Beijing Institute of Technology, School of Materials Science and Engineering, no. 5 Zhongguancun South Street, Beijing 100081, China 2 Beijing Institute and Technology, National Key Laboratory of Science and Technology on Materials under Shock and Impact, no. 5 Zhongguancun South Street, Beijing 100081, China richard@bit.edu.cn Prejem rokopisa – received: 2017-12-27; sprejem za objavo – accepted for publication: 2018-04-26 doi:10.17222/mit.2017.223 Ba2(DyxYb1–x)AlO5 ceramics were prepared by solid-state sintering in air for 10 h at 1560 °C.The influence of Dy 3+ doping on the thermal physical properties of Ba2YbAlO5 was studied. The phase structure of these ceramics were identified by X-ray dif- fraction, which indicates that all of these ceramics have a perovskite structure. The thermal physical properties of these ceramics measurement show that the thermal expansion coefficient of them by Dy 3+ doped significantly increased, and the maximum value of about 12.2 × 10 –6 K –1 was reached at x = 0.3. The thermal conductivity of Ba2(DyxYb1–x)AlO5 decreased firstly and then increased with the increasing of x. For samples with the same compound point x = 0.2, the minimum value (0.998 W·m –1 ·K –1 ) was realized. The excellent thermal physical properties mean that these solid solutions are potential materials for ceramic layers in thermal barrier coatings. Keywords: perovskite-like ceramics, thermal expansion coefficient, thermal conductivity, thermal barrier coatings Avtorji prispevka so pripravili Ba2(DyxYb1–x)AlO5 keramiko s postopkom sintranja v trdnem stanju in raziskovali vpliv razli~ne koli~ine dodatka Dy 3+ na njene termofizikalne lastnosti. Sintranje je potekalo na zraku 10 ur pri 1560 °C. Fazno strukturo te keramike so identificirali z rentgensko difrakcijo. Ta je pokazala, da ima keramika perovskitno strukturo. Meritve termofizikalnih lastnosti te keramike so pokazale, da toplotni razteznostni koeficient z dodatkom Dy 3+ znatno naraste in dose`e maksimalno vrednost okoli 12,2 × 10 –6 K –1 pri x = 0,3. Toplotna prevodnost Ba2(DyxYb1–x)AlO5 najprej pada in nato nara{~a z dodatkom Dy 3+ . Tako je naprimer pri vzorcih z enako sestavo in x = 0,2 dose`ena minimalna vrednost 0,998 W·m –1 ·K –1 . Odli~ne termofizikalne lastnosti pomenijo, da so te trdne raztopine materiali, potencialno uporabni kot termi~ne pregrade v kerami~nih prevlekah. Keywords: perovskitna keramika, toplotni razteznostni koeficient, toplotna prevodnost, prevleke, uporabne kot termi~ne pregrade 1 INTRODUCTION Thermal barrier coatings (TBCs) are widely used to protect the hot-section components of gas turbines from hot gases. 1 With the development of aeroengines to high thrust-weight ratios and high inlet temperatures, the re- quirements for thermal insulation performance of TBCs will also be improved. The most advanced TBC material is yttria stabilized zirconia (YSZ), but YSZ still has some disadvantages when exposed to high temperatures (>1200 °C) for extended intervals, because the phase transition and shrinkage can damage the coatings. 2 Therefore, it is very urgent to develop alternatives to YSZ for advanced TBC applications. The excellent ce- ramic candidates for TBCs must possess a few important performances, such as low thermal conductivity, appro- priate thermal expansion, good phase stability at high temperature and so on. 3 Among the thousands of possi- ble candidates, anionic defect aluminates Ba 2 RAlO 5 (R represents rare-earth element) such as Ba 2 RAlO 5 show a lower thermal conductivity and high thermal expansion coefficient (TEC), which gives them great potential for applications in the field of TBCs. 4 The main purpose of the present study is to find ways to further improve its heat insulation ability. But there are few reports on the thermal physical performance data of Ba 2 RAlO 5 , which is doped with rare-earth oxides. In this paper, the Ba 2 YbAlO 5 was doped in In sites by Dy, in order to im- prove the thermal physical properties of Ba 2 RAlO 5 . The influence of the Dy 3+ -doping concentration on the ther- mal conductivity and TECs of Ba 2 RAlO 5 was also stud- ied. 2 EXPERIMENTAL PART Ba 2 (Dy x Yb 1–x )AlO 5 (x =0 ~0.3) samples were syn- thesized by solid-state reaction using BaCO 3 ,A l 2 O 3 (purity 99.0 %, Beijing Chemical Co. Ltd.), Yb 2 O 3 and Dy 2 O 3 (purity 99.99 %, Rare-Chem Hi-Tech Co. Ltd.), the powders of Ba 2 (Dy x Yb 1–x )AlO 5 (x=0~0.3) were sufficiently mixed with ethanol for 6 h and dried at 120 °C overnight and then calcined at 1200 °C for 8 h. The obtained powders were subjected to cold isostatic Materiali in tehnologije / Materials and technology 52 (2018) 5, 615–618 615 UDK 67.017:666.3/.7:546.664:543.58:536.2 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(5)615(2018) pressing at a pressure of 100 MPa and sintered in air at 1560 °C for 10 h. The microstructures of the pellets were characterized by scanning electron microscopy (SEM, Philips S-4800, Hitachi Ltd., Yoshida-Cho, Totsuka-Ku, Yokohama, Japan). The phase compositions of the sintered ceramics was determined by X-ray diffraction (XRD, RIGAKU D/Max-rB, Rigaku International Corp., Sendagaya, Shibuya-Ku, Tokyo, Japan). The thermal conductivity of the samples with the size of 12.7 mm × 2.0 mm were processed. The thermal diffusivity ( ) of each sintered sample was measured using a laser-flash method (Model NETZSCH LFA 427, Netzsch Co., Ltd., Selb, Germany) and the densities ( ) of the bulks were measured accord- ing to Archimedes’ principle. The thermal conductivity k is calculated in ref. 5 . The TECs between room temperature and 1300 °C of the sintered samples having a size of 25 mm×4mm×3mmwere obtained using a high-temperature expansion meter (NETZSCH DIL 402C, Netzsch Co. Ltd., Selb, Germany). 3 RESULTS AND DISCUSSION Figure 1 shows the phase compositions of the Ba 2 (Dy x Yb 1–x )AlO 5 (x =0 ~0.3) bulks synthesized at 1560 °C for 10 h. It can be seen that the X ray diffraction patterns of the samples agree with the standard spectrum (37-0292) of the perovskite structured Ba 2 YAlO 5 , and there is no other phase in each sample. A pure Ba 2 (Dy x Yb 1–x )AlO 5 (x =0 ~0.3) ceramic with the perovskite structure was synthesized. Figure 2 shows the microstructures of partial Ba 2 (Dy x Yb 1–x )AlO 5 (x = 0,0.3) ceramics. It can be seen that the surface morphology of the sample before the experiment is flat and dense (Figure 2a). Ba 2 (Dy x Yb 1–x )AlO 5 (x = 0, 0.3) ceramics have a flat shape. From Figure 2a and 2b it can be seen that the microstructures of the synthesized products are dense, but there are some obvious pores. In Table 1 the volume density and relative density of Ba 2 (Dy x Yb 1–x )AlO 5 (x=0,0.3) with accurate measurements using Archime- des’ method are listed. Table 1: Densities and relative densities of Ba 2 (Dy x Yb 1–x )AlO 5 (x = 0~0.3) ceramics Samples Experimental density (g/cm 3 ) Relative density (%) Ba 2 YbAlO 5 6.333 99.70 Ba 2 (Dy 0.1 Yb 0.9 )AlO 5 6.321 99.86 Ba 2(Dy 0.2 Yb 0.8 )AlO 5 6.309 99.60 Ba 2(Dy 0.3 Yb 0.7 )AlO 5 6.297 98.80 Figure 3a shows the thermal diffusivities as function of temperature for different Ba 2 (Dy x Yb 1–x )AlO 5 ceram- ics. Obviously, the thermal diffusivities monotonically decrease with the increase of temperature, i.e., T –1 ,be - tween room temperature and 1200 °C. The T –1 depend- ence of diffusivity for Ba 2 (Dy x Yb 1–x )AlO 5 ceramics ex- hibits a major phonon-conduction behavior, which is very common in polycrystalline materials. 6 After the porosity correction the temperature dependence of the thermal conductivity of Ba 2 YbAlO 5 with different Dy 3+ contents is shown in Figure 3b.As can be seen from Figure 3b, as the doping content of Dy 3+ increases to x = 0.2, the thermal conductivity decreases gradually and then increases at x = 0.3. Obvi- L. LIU et al.: EFFECTS OF Dy 3+ -DOPING ON THE THERMOPHYSICAL PROPERTIES OF Ba2YbAlO5 CERAMICS 616 Materiali in tehnologije / Materials and technology 52 (2018) 5, 615–618 Figure 2:SEMofBa 2 (Yb x Dy 1–x )AlO 5 (x= 0, 0.3) ceramics: a) x=0, b) x = 0.3 Figure 1: X-ray diffractograms of Ba 2 (Dy x Yb 1–x )AlO 5 (x =0 ~0.3) bulks ously, the thermal conductivity of Ba 2 (Dy 0.2 Yb 0.8 )AlO 5 bulk ceramics at 1200 °C is the lowest at ~0.998 W·m –1 ·K –1 . In insulating materials, thermal conductivity is caused by changes in lattice vibrations, which are often described by phonon scattering theory. The deformation of the crystal structure and different ion radius will affect the phonon scattering. The composition of the atomic weight may lead to varying degrees of phonon scattering. Usually, the coefficient of thermal conductivity increases with the decrease of atomic mass. The low thermal conductivity of Ba 2 (Dy x Yb 1–x )AlO 5 (x = 0.1, 0.2) is attributed to the increase of additional phonon scattering by substitutional solute cation Dy 3+ replacing the host cation Yb 3+ and RO 6 octahedron tilting. The ion radius of Dy 3+ ions and Yb 3+ ions is 0.091 nm and 0.086 nm, respectively, according to the following Equation (1): 7 12 34 3 22 2 l ca J R R p = ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Δ (1) where, a 3 is the volume per atom, the transverse wave speed, the phonon frequency, c the concentration per atom, J the constant, the Grüneisen parameter, R the average ionic radius of the host atom, R the differ- ences of ionic radius between the substituted and the substituting atoms. It can be seen that for the normal polycrystalline ceramic oxide, the mean free path of the phonon is proportional to the square of the ion radius difference between the solute and the host cation. The radius difference between Dy 3+ and Yb 3+ results in an effective phonon scattering, This contributes to the lower thermal conductivity of Dy 3+ doped Ba 2 YbAlO 5 . At the same time, since the Ba 2 RAlO 5 compound is a perovskite variant, their structural stability can be de- scribed by a tolerance factor as Equation (2): 8 t rr rr = + + Ba R,Al 0 0 2( ) (2) where, r Ba is the radius of Ba 2+ , r O the radius of O 2– , and r (R,Al) the average radius of R 3+ and Al 3+ . For the monoclinic perovskite belonging to the Ba 2 RAlO 5 com- pound, a decrease in tolerance means that the BO 6 octa- hedral must be tilted to match the AO 12 polyhedron, which results in additional phonon scattering. Since Dy 3+ has a larger radius than Yb 3+ , the tolerance factor decreases as the Dy 3+ doping content increases. Thus additional phonon scattering increases due to the in- crease in the BO 6 octahedral tilt. Therefore, the thermal conductivity of Ba 2 (Dy x Yb 1–x )AlO 5 (x = 0, 0.1, 0.2) de- creases with an increase of the Dy 3+ content. It is well known that Dy (162.5) has a smaller atomic weight than Yb (173). The thermal conductivity of Ba 2 (Dy x Yb 1–x )AlO 5 samples increased significantly at x = 0.3 due to the decrease in atomic mass. With an increase of the Dy 3+ content, the decrease of atomic mass will weaken the phonon scattering, resulting in an increase in the thermal conductivity. Figure 4 shows the TECs of the dense Ba 2 (Dy x Yb 1–x )AlO 5 (x=0~0.3) ceramics. It can be seen that with the increase of Dy 3+ -doping content, the TEC of Ba 2 (Dy x Yb 1–x )AlO 5 (x=0~0.3) increases from 11.75×10 –6 /K to 12.2×10 –6 /K. The TEC ( ) is closely related to the lattice energy (E), which can be given by the following Equation (3): = + a Eb (3) L. LIU et al.: EFFECTS OF Dy 3+ -DOPING ON THE THERMOPHYSICAL PROPERTIES OF Ba2YbAlO5 CERAMICS Materiali in tehnologije / Materials and technology 52 (2018) 5, 615–618 617 Figure 3: a) Thermal diffusivity and b) thermal conductivity versus temperature for Ba 2 (Dy x Yb 1–x )AlO 5 (x=0 ~0.3) ceramics Figure 4: TECs of Ba 2 (Dy x Yb 1–x )AlO 5 (x=0 ~0.3) ceramics where a and b are both constants, and the lattice energy is given as Equation (4): EN zz R Ae n =− ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ +− 2 1 1 (4) where, N is the Avogadro constant, z + and z – the charge of positive and negative ions, respectively, R the dis- tance between positive and negative ions, A the Made- lung constant, e an electronic power, and n the constant. When the oxide ceramics have the same crystal struc- ture, the larger radii of the positive ions in the lattice leads to a large distance between the positive ions and the negative ions, which will lead to higher TEC. There- fore, the thermal expansion coefficient is improved due to the relatively large radius of the Dy 3+ . 4 CONCLUSIONS Ba 2 (Dy x Yb 1–x )AlO 5 (x=0~0.3) compounds with perovskite-like structure were synthesized by solid-state sintering. The effect of Dy 3+ Doping on the thermal conductivity and the TECs were investigated. The results show that the thermal properties are improved due to Dy 3+ doping in the range of x < 0.3. The lower thermal conductivity of Ba 2 (Dy x Yb 1–x )AlO 5 (x=0.1, 0.2) can be mainly attributed to the larger ionic radius of the substitutional cation in the materials. Compared with the thermal conductivity (1.14 Wm –1 K –1 ) and TEC (11.75 × 10 –6 K –1 ), the aluminates Ba 2 (Dy x Yb 1–x )AlO 5 (x=0.1, 0.2) ceramics are better than that of Ba 2 YbAlO 5 .Atx=0.2, the thermal conductivity reached a minimum value of about 0.998 W m –1 K –1 , the TEC reached a maximum value of about 12.2 × 10 –6 K –1 atx= 0.3. 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