Influence of dysprosium oxide doping on thermophysical properties of LaMgAl11O19 ceramics

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<ul><li><p>tchn</p><p>inl11Oat 1s harmaith9 ce</p><p>levels. The measured thermal conductivity of La1xDyxMgAl11O19 ceramics is located in the range of 2.52</p><p> 2010 Elsevier Ltd. All rights reserved.</p><p>minatbite str coatts oftalysts</p><p>melting point and sintering resistance at elevated temperaturesare needed as TBCs materials [10]. The current state-of-the-artthermal barrier coating material is 78 wt.% yttria stabilized zir-conia (78YSZ), which starts rapid degradation at operating tem-peratures above 1200 C due to phase transitions and serioussintering phenomenon [8,10,11]. LHA has high operating temper-ature (&gt;1300 C [1]), good thermochemical stability (&gt;1400 C[12]), low thermal conductivity (1.22.6 Wm1 K1 [12]), and</p><p>in the latest 10 years, there are many reports on doping with oneor multi-trivalent rare-earth elements at the A sites in A2Zr2O7pyrochlore, such as (SmxGd1x)2Zr2O7 [17], La2xNdxZr2O7 [18],La1.7Gd0.3Zr2O7, La1.7Yb0.3Zr2O7, La1.7Gd0.15Yb0.15Zr2O7 [19], to im-prove their thermophysical properties.</p><p>In the present study, Dy3+ cation was introduced into the struc-ture of LaMgAl11O19 at La3+ site. La1xDyxMgAl11O19 (x = 0, 0.1,0.2, 0.3) ceramics with a magnetoplumbite structure wereproduced by pressureless-sintering process at 1700 C for 10 h inair. The inuence of dysprosium oxide doping on thermophysicalproperties of LaMgAl11O19 ceramics was investigated.</p><p>* Corresponding author. Tel./fax: +86 451 86414291.</p><p>Materials and Design 31 (2010) 33533357</p><p>Contents lists availab</p><p>an</p><p>elsE-mail address: (J.-H. Ouyang).plumbite structure, one rare-earth oxide layer as a crystallographicmirror plane is followed by four spinel layers [1]. The rare-earthatom in the mirror plane located in an oxygen position suppressesthe diffusion processes and leads to crystallize in the habit plane oflayered structure [2].</p><p>Thermal barrier coatings produced by plasma-spraying (PS) orelectron-beam physical vapor deposition (EB-PVD) methods havebeen widely used to protect hot-section metallic components inadvanced gas-turbine and diesel engines [8,9]. Generally, lowthermal conductivity, high thermal expansion coefcient, high</p><p>citric acid solgel method and examined the effect of rare-earthoxide doping on thermal properties. Thermal conductivity ofGd0.7Yb0.3MgAl11O19 composition was lower than that of undopedGdMgAl11O19, however, thermal expansion coefcient was inde-pendent of the composition. In the comparison on ambient-tem-perature mechanical properties, magnetoplumbite oxides ofLaMnAl11O19, LaMgAl11O19 and LaZnAl11O19 exhibited a higherstrength (&gt;300 MPa) and a higher fracture toughness(&gt;3 MPa m1/2) than the pyrochlore oxides of Ln2Zr2O7 (Ln = La,Nd, Gd) [16]. In the development of the TBC materials, especially1. Introduction</p><p>Lanthanum magnesium hexaalucomplex oxides with magnetoplummuch attention as thermal barriedielectric resonators, active elemencatalytic combustion supports or ca0261-3069/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.01.058e (LnMgAl11O19, LHA)ructure have attractedings (TBCs) materials,solid-state lasers, and[17]. In the magneto-</p><p>high thermal expansion coefcient (9.510.7 106 K1 [12]) asa potential TBC candidate. The results of water quenching testsindicated that thermal shock resistance of LnMgAl11O19 (Ln = La,Nd, Sm, Gd) is superior to that of 8YSZ [13]. LaLiAl11O18.5 mayform segmentation cracks during heat treatment leading to a con-siderable expansion tolerance [14]. Bansal and Zhu [15] synthe-sized LnMgAl11O19 (Ln = La, Gd, Sm) and Gd0.7Yb0.3MgAl11O19 byThermal conductivityThermal expansion 2.89 Wm</p><p>1 K1 at 1200 C. Thermal expansion coefcient of La0.8Dy0.2MgAl11O19 ceramic is slightlyhigher than that of undoped LaMgAl11O19 ceramic at identical temperature levels.Inuence of dysprosium oxide doping onof LaMgAl11O19 ceramics</p><p>Yuan-Hong Wang, Jia-Hu Ouyang *, Zhan-Guo LiuInstitute for Advanced Ceramics, Department of Materials Science, Harbin Institute of Te</p><p>a r t i c l e i n f o</p><p>Article history:Received 5 December 2009Accepted 30 January 2010Available online 6 February 2010</p><p>Keywords:LaMgAl11O19Dy2O3 doping</p><p>a b s t r a c t</p><p>This paper deals with thegAl11O19 ceramics. LaMgAwere pressureless-sintered(x = 0, 0.1, 0.2, 0.3) ceramictoplumbite structure. Theceramics were measured wsivity of La1xDyxMgAl11O1</p><p>Materials</p><p>journal homepage: www.ll rights reserved.hermophysical properties</p><p>ology, No. 92, West Da-Zhi Street, Harbin 150001, PR China</p><p>uence of dysprosium oxide doping on thermophysical properties of LaM-19 ceramic powders doped with different contents of dysprosium oxide700 C for 10 h in air to fabricate dense bulk ceramics. La1xDyxMgAl11O19ve a relative density of 90.796.0%, and exhibit a single phase of magne-l diffusivity and thermal expansion coefcients of La1xDyxMgAl11O19a laser ash method and a high-temperature dilatometer. Thermal diffu-ramics decreases with increasing Dy2O3 content at identical temperature</p><p>le at ScienceDirect</p><p>d Design</p><p>evier .com/locate /matdes</p></li><li><p>As the sintered specimens were not fully dense, the measuredthermal conductivity data were corrected for the residual porosityu of the samples, using the equation [21],</p><p>kk0</p><p> 1 43u 2</p><p>where k0 is the corrected thermal conductivity for fully densematerials.</p><p>The linear thermal expansion coefcients of sintered ceramicsusing the specimens with the dimensions of 2.5 mm 3.5 mm 14.0 mm were determined by a high-temperature dilatometer(Netzsch DIL 402C, Germany) in an argon atmosphere. Specimenswere measured in the temperature range of 501200 C at a heat-ing rate of 4 C/min during heating, and were corrected by theknown thermal expansion of a certied standard alumina. Thethermal expansion coefcient is given by</p><p>density of La1xDyxMgAl11O19 ceramics sintered at 1700 C for</p><p>nd Design 31 (2010) 335333572. Experimental</p><p>2.1. Materials preparation and characterization</p><p>In the present study, lanthanum oxide and dysprosium oxidepowders (Grirem Advanced Materials Co. Ltd., Beijing, China; pur-ity P99.9%), aluminum nitrate (Al(NO3)39H2O) and magnesiumnitrate (Mg(NO3)26H2O) (Tianjin Guangfu Fine Chemical ResearchInstitute Ltd., Analytical pure) were used as the reactants. La1x-DyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) powders were synthesized bychemical-coprecipitation and calcination method. La2O3 andDy2O3 powders were calcined at 900 C for 2 h in air to removeabsorptive water and carbon dioxide before weighing. La2O3 andDy2O3 were dissolved in dilute hydrogen nitrate, while Al(-NO3)39H2O and Mg(NO3)26H2O were dissolved in de-ionizedwater. Above two solutions were mixed in appropriate mole ratiosof La1xDyxMgAl11O19 (where x = 0, 0.1, 0.2, 0.3) under continuousstirring. The precursor solution was slowly added under stirringinto excessive ammonium hydrate solution with a pH value of 12to obtain gel-like precipitates. After that, these gels were washedwith de-ionized water several times until pH = 7, and were thenwashed twice with absolute alcohol. The remains were dried at80 C for 24 h and calcined at 800 C for 5 h in air for crystalliza-tion. The La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) ceramic powderswere preformed at 20 MPa for 2 min, cold isostatically pressed at280 MPa for 5 min, and then pressureless-sintered at 1700 C for10 h in air to obtain densied samples for thermophysical proper-ties measurements.</p><p>Crystal structure of sintered La1xDyxMgAl11O19 ceramics wasidentied by X-ray diffractometer (XRD, Rigaku D/Max 2200VPC,Japan) with Cu Ka radiation at a scan rate of 4 deg/min. The mor-phologies of La1xDyxMgAl11O19 ceramics were characterized byscanning electron microscope (SEM, CamScan MX 2600FE, UK).The sintered specimens were carefully ground, polished and ther-mally etched at 1550 C for 1 h in air for SEM observations. Thedensities of La1xDyxMgAl11O19 ceramics were measured by theArchimedes method with an immersion medium of de-ionizedwater. The theoretical densities of all compositions were calculatedusing lattice parameters acquired from XRD results and relativemass of one unit cell.</p><p>2.2. Thermophysical properties measurements</p><p>Thermal diffusivity of La1xDyxMgAl11O19 ceramics was mea-sured using the laser-ash diffusivity technique (Netzsch LFA427, Germany) from room temperature to 1200 C in an argonatmosphere. The dimensions of the disc-shaped samples were12.7 mm in diameter and 2.0 mm in thickness. Prior to thermal dif-fusivity measurement, the surfaces of the specimens were coatedwith a thin layer of sprayed colloidal graphite. This coating wasdone to minimize the radiative transport of the thermal ashthrough the samples, and to prevent direct transmission of the la-ser beam through the translucent specimens at high temperatures.Appropriate corrections were made in the thermal diffusivity cal-culations to account for the presence of these layers. The thermaldiffusivity measurement of all specimens was carried out threetimes at each temperature. The specic heat capacities were calcu-lated from the chemical compositions of La1xDyxMgAl11O19ceramics with the NeumannKopp rule as a function of tempera-ture. The heat capacity data of the component oxides (La2O3,Dy2O3, MgO, Al2O3) were obtained from the literature [20]. Thethermal conductivity k was determined from the heat capacityCp, density q and thermal diffusivity k, and using the following</p><p>3354 Y.-H. Wang et al. /Materials aequation:</p><p>k Cp q k 110 h. The relative densities of La1xDyxMgAl11O19 ceramics are inthe range of 90.796.0%, and La0.8Dy0.2MgAl11O19 has the highestrelative density of 96.0%. Fig. 2 shows the microstructure of La1x-DyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) ceramics sintered at 1700 C for10 h. From the SEM observations, the grains in La1xDyxMgAl11O19</p><p>10 20 30 40 50 60 70 80 90</p><p>(1116</p><p>)</p><p>(2014</p><p>)(220)</p><p>(2011</p><p>)(21</p><p>7)(20</p><p>9)(10</p><p>11)(2</p><p>06)(20</p><p>5)(00</p><p>10)</p><p>(203)</p><p>(114)</p><p>(112)(</p><p>008)(</p><p>110)</p><p>(107)</p><p>(105)(0</p><p>06)</p><p>(102)</p><p>(101)</p><p>Inte</p><p>nsity</p><p> (a.u.</p><p>)</p><p>2Theta (deg.)</p><p>x = 0</p><p>x = 0.2</p><p>x = 0.1</p><p>x = 0.3aT1T2 DL=L0T2 DL=L0T1</p><p>T2 T1 3</p><p>where aT1T2 represents the average change of length for unitlength specimen between the range of T1 and T2.</p><p>3. Results and discussion</p><p>3.1. Characterization of La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3)ceramics</p><p>X-ray diffraction patterns of La1xDyxMgAl11O19 ceramics sin-tered at 1700 C for 10 h in air are shown in Fig. 1. All peaks inthe XRD patterns of La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) ceram-ics match well with the magnetoplumbite crystal structure (JCPDSNo. 77-1429). This indicates that Dy3+ cations successfully occupypartial sites of La3+ cations in the crystal structure and form substi-tution solid solutions. The theoretical densities were calculatedaccording to the XRD results. The bulk densities of La1xDyxMgA-l11O19 ceramics were measured by Archimedes method with animmersion medium of de-ionized water. Table 1 shows the relativeFig. 1. XRD patterns of La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) ceramics sintered at1700 C for 10 h.</p></li><li><p>are inhomogeneous, and the platelets distribute randomly with asize of 120 lm.</p><p>3.2. Thermal conductivity of La1xDyxMgAl11O19 ceramics</p><p>The calculated specic heat capacities of La1xDyxMgAl11O19ceramics based on NeumannKopp rule at different temperatureswere shown in Table 2. The thermal diffusivity of La1xDyxMgA-l11O19 (x = 0, 0.1, 0.2, 0.3) ceramics as a function of temperature isshown in Fig. 3. The thermal diffusivity values in Fig. 3 are thearithmetic means of every three measurements of identical cera-mic materials. The error derived from the mean standard deviationfor three measurements of each specimen is less than 0.9%, and the</p><p>error bars in Fig. 3 are smaller than the symbols. Clearly, the ther-mal diffusivity of La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) decreasesrapidly with increasing temperature from room temperature to800 C, which suggests a dominant phonon conduction behaviorin most inorganic non-metallic materials [22]. The measured ther-mal diffusivity of La1xDyxMgAl11O19 ceramics keeps almost un-changed above 800 C due to the radiative contribution at hightemperatures [19]. Thermal diffusivity of La1xDyxMgAl11O19ceramics decreases with increasing Dy2O3 content at identical tem-perature levels. The decrease in thermal diffusivity at identicaltemperature levels is assigned to the lattice distortion due toDy3+ cations substitution for La3+ cations in the structure. Cationsubstitution causes lattice distortion, increases phonon scatteringand decreases phonon mean free path. On the other hand, the pho-non mean free path is proportional to the square of atomic weightdifference between the solute and host cations, according to thefollowing Eq. (4) [23]:</p><p>ll a</p><p>3</p><p>4pm4w4c</p><p>DMM</p><p> 24</p><p>where a3 is the volume per atom, m is the transverse wave speed, xis the phonon frequency, c is the concentration per atom, M is the</p><p>Table 1The relative density of La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) ceramics sintered at1700 C for 10 h.</p><p>Ceramic materials Relative density (%)</p><p>LaMgAl11O19 90.7La0.9Dy0.1MgAl11O19 93.0La0.8Dy0.2MgAl11O19 96.0La0.7Dy0.3MgAl11O19 95.6</p><p>Table 2Specic heat capacities of La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) bulk ceramics calculated with the NeumannKopp rule at different temperatures.</p><p>Ceramic bulk materials Specic heat capacities (J g1 K1)</p><p>25 C 200 C 400 C 600 C 800 C 1000 C 1200 C</p><p>LaMgAl11O19 0.6757 0.9003 0.9837 1.0273 1.0577 1.0826 1.1046La0.9Dy0.1MgAl11O19 0.6741 0.8978 0.9809 1.0243 1.0546 1.0794 1.1014La0.8Dy0.2MgAl11O19 0.6726 0.8954 0.9781 1.0213 1.0516 1.0763 1.0982La0.7Dy0.3MgAl11O19 0.6710 0.8929 0.9753 1.0184 1.0485 1.0731 1.0950</p><p>Y.-H. Wang et al. /Materials and Design 31 (2010) 33533357 3355Fig. 2. Microstructure of La1xDyxMgAl11O19 ceramics sintered at 1700 C for 10 h: (a) x = 0, (b) x = 0.1, (c) x = 0.2, and (d) x = 0.3.</p></li><li><p>1.0 m</p><p> La0.8Dy0.2MgAl11O19 La Dy MgAl O</p><p>nd D0 200 400 600 800 1000 12000.2</p><p>0.4</p><p>0.6</p><p>0.8</p><p>Ther</p><p>mal</p><p> diff</p><p>usiv</p><p>ity</p><p>Temprature oC</p><p>0.7 0.3 11 19</p><p>Fig. 3. Thermal diffusivity of La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) ceramics as afunction of temperature.1.2</p><p>1.4</p><p>m2 s</p><p>-1</p><p> LaMgAl11O19 La0.9Dy0.1MgAl11O19</p><p>3356 Y.-H. Wang et al. /Materials aaverage mass of the host atom, DM is the weight difference be-tween the solute and host cations. Because the atomic weights ofLa and Dy are 138.9 and 162.5, respectively, the effective phononscattering by Dy3+ is signicantly higher than that of La3+, whichcontributes to the lower thermal conductivity.</p><p>Thermal conductivity of La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3)ceramics is plotted in Fig. 4 according to Eq. (1) and the thermaldiffusivity, specic heat capacity and density of different samples.The values in Fig. 4 are corrected to a 100% theoretical densityaccording to Eq. (2) and Table 1. The error bars are omitted as theyare smaller than the symbols. From Fig. 5, the thermal conductivityof La1xDyxMgAl11O19 (x = 0, 0.1, 0.2, 0.3) ceramics is located in therange of 2.522.89 Wm1 K1 at 1200 C, and decreases withincreasing Dy2O3 content.</p><p>3.3. Thermal expansion of La1xDyxMgAl11O19 ceramics</p><p>The thermal expansion of sintered LaMgAl11O19 and La0.8-Dy0.2MgAl11O19 ceramics as a function of temperature are pre-sented in Fig. 5a....</p></li></ul>


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