Crystal Structure, Phase Analysis, and Thermal Conductivity of

Article pubs.acs.org/JPCC

Crystal Structure, Phase Analysis, and Thermal Conductivity of Nanocrystalline Gd2O3−HfO2 Coatings C. V. Ramana,*,† Jamie J. Gengler,‡,§ and John G. Jones§ †

Department of Mechanical Engineering, University of Texas at El Paso, El Paso, Texas 79968, United States Spectral Energies, LLC, 5100 Springfield Street, Suite 301, Dayton, Ohio 45431, United States § Air Force Research Laboratory, Materials and Manufacturing Directorate, Thermal Sciences and Materials Branch, 2941 Hobson Way, Wright-Patterson Air Force Base, Ohio 45433, United States ‡

ABSTRACT: Gadolinia (Gd2O3) doped hafnia (HfO2) nanocrystalline coatings were fabricated employing magnetron sputter-deposition. The coatings were grown onto Si substrates by varying the Gd2O3 composition in the matrix within the range of 0−38 mol % (balanced by HfO2) The crystal structure, phase, morphology, and thermal conductivity of the Gd2O3−HfO2 nanocrystalline coatings were evaluated employing X-ray diffraction (XRD), scanning electron microscopy (SEM), and timedomain thermoreflectance (TDTR) techniques. The combined XRD and SEM studies indicate that the effect of Gd2O3 is remarkable on the microstructure evolution of nanocrystalline Gd2O3−HfO2 coatings. The grown coatings exhibit stabilized, cubic HfO2 phase as the Gd2O3 content increased from 4 to 12 mol % at which point the lattice expansion is considerably higher. The morphology of the Gd2O3−HfO2 coatings is characterized by the triangular-shaped nanograins uniformly distributed on the surface. The thermal conductivity of all the Gd2O3-doped HfO2 samples is lower than pure HfO2, and an inverse relationship between Gd2O3 mol percent and heat transport was discovered.

I. INTRODUCTION Ceramic materials based on zirconia (ZrO2) and hafnia (HfO2) find widespread applications in mechanical, aerospace, and energy related applications. Coatings or thin and thick films of these ceramics are desired in a number of technological applications where protection against wear, heat, and harsh environments are required. Examples include aero-engines, gas turbines, coal-based advanced power generation systems, combustion and propulsion systems, boilers, fuel cells, and thermal and nuclear power plants. The most important to mention are their application as thermal barrier coatings (TBCs), which prevent damage of the engine components that are exposed to the very high temperature of hot gases.1−9 Widespread efforts have been directed to develop appropriate thermal barrier coating materials for high temperature application.10−27 TBCs allow the gas turbine to operate at higher temperature by reducing the heat transfer from hot gas to superalloy blades and thereby improving the efficiency. Yttria (Y2O3) stabilized zirconia (YSZ) is the current industry standard TBC due to its low thermal conductivity, phase stability at relatively high temperature, high thermal expansion coefficient compared to other ceramics, and good erosion resistance.3 However, at temperatures above 1200 °C, t′-tetragonal to tetragonal and then cubic to monoclinic phase transformation occurs resulting in a volume change, which leads to the formation of cracks in the coatings.4,8,10 This limits the application of YSZ at elevated temperature. Increasing demand to improve the © 2012 American Chemical Society

efficiency of the gas turbines leads to a search for alternate TBCs that will allow the operating temperature to increase further. In order to develop a successful TBC, it is very important to understand the behavior of the composite ceramic materials. In this context, the present work was performed on the gadolinia (Gd2O3) doped HfO2 system, where Gd2O3 was used as a stabilizer. It has been reported that thermal conductivity of the Gd2O3−ZrO2 system is lower compared to conventional Y2O3 stabilized ZrO2.23 Also 4 mol % Gd2O3 stabilized ZrO2 sintered more slowly than similar compositions of the Y2O3−ZrO2 system.23 The Gd2O3−HfO2 system has been proven to have lower thermal conductivity compared to YSZ TBCs. 6 Furthermore, recent studies have shown that the Gd-based pyrochlore structure type can be designed to function in highradiation environments by changing the composition, crystal size, and degree of inherent disordering.28 In addition, the temperature stability of HfO2 is higher than that of ZrO2. It transforms into the tetragonal form when heated to temperatures higher than 1700 °C. Further transformation into the cubic polymorphic form having the fluorite structure takes place at 2700 °C.29−31 Therefore, it is worth investigating the Gd2O3− HfO2 coatings coupled with nanoscale dimensionality based Received: January 31, 2012 Revised: October 31, 2012 Published: November 27, 2012 25178

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benefits to find better or further improve the high-temperature performance of these materials in the desired applications. Gd2O3−HfO2 nanocrystalline coatings with variable composition were prepared by varying the Gd2O3 composition in the range of 0−38 mol % (balanced by HfO2). The effect of Gd2O3doping on the crystal structure, phase formation, and thermal conductivity of Gd2O3−HfO2 coatings is reported in this paper. The challenge of measuring and understanding thermal transport at the nanoscale is greatly facilitated with the use of ultrafast laser metrology. The benefits of such measurements to the disciplines of physical chemistry, surface/interface chemistry, and nanoscience are well documented in the literature. For example, transient absorption spectroscopy is useful for studying interfacial thermal transport between functionalized metallic nanoparticles32 (or fullerene molecules33) with various solvents. Time-domain thermoreflectance (TDTR) has helped quantify flash-heating dynamics in thin metal films.34 This information was used in conjunction with time-resolved sum frequency generation measurements of heat flow in organic self-assembled monolayers.34 A final example employed time-resolved ellipsometry to study heat transfer at solid−gas interfaces.35The use of TDTR was chosen for the measurements of thin Gd2O3−HfO2 coatings in this report because of the advantage of nanometerscale depth resolution and thermal property evaluation of the solid surfaces and interfaces.

performed employing a Bruker D8 Advance X-ray diffractometer. All the measurements were made ex-situ as a function of the coating composition. XRD patterns were recorded using Cu Kα radiation (λ = 1.54056 Ǻ ) at room temperature. The coherently diffracting domain size (dhkl) was calculated from the integral width of the diffraction lines using the well-known Scherrer’s equation (after background subtraction and correction for instrumental broadening). Surface imaging analysis was performed using a high-performance and ultra-high resolution scanning electron microscope (Hitachi S-4800). The secondary electron imaging was performed on Gd2O3−HfO2 coatings. The grain detection, size-analysis, and statistical analysis was performed using the software provided with the scanning electron microscope. Time-Domain Thermoreflectance. Thermal conductivity measurements were performed with a two-color time-domain thermoreflectance (TDTR) lab. This experiment is a modified version of a previously reported system,36 and only details of the modifications will be given here. The output of a mode-locked Ti:sapphire laser (λ = 787 nm) is split into a pump and a probe beam. The pump beam is sent first through a pulse compressor (for correction of pulse-stretching effects) and then through an electro-optic modulator (EOM), which imposes a square-wave pulse train with a frequency of 9.8 MHz. The pump beam is then aligned along a mechanical translation stage to systematically alter the timing between the pump and the probe pulses. The probe beam is sent through an optical parametric oscillator for wavelength modification (λ = 700 nm). Both pump and probe beams also have half-waveplate/polarizer combinations for arbitrarily controlling the beam intensity. Powers of 75 (pump) and 20 mW (probe) were employed. Both beams are then focused to a spot size of ∼50 μm diameter at a 45° angle to the sample. The reflected probe beam is spatially filtered, recollimated, and sent through a 750-nm short-pass optical filter to reject scattered pump-beam light (polarization filtering is not required). Finally, the probe beam is passed through a neutraldensity filter (optical density = 1.0) and focused onto a silicon (Si) photodiode detector. The detector (New Focus model 2051FS) has built-in bandwidth filtering that is set to accept the 9.8 MHz modulation frequency while rejecting all higher odd harmonics. The output of the detector is first sent through an additional electronic filter (Mini-Circuits model BBP-10.7+) and then to the input of a dual-phase, radio frequency lock-in amplifier that has its reference channel connected to the same electronic signal that drives the EOM. The scans and data acquisition are computer controlled by means of a homemade LabVIEW program. The GSH samples were coated with an ∼80−100 nm thick layer of sputtered aluminum (Al) to produce a high reflectivity surface, which is a standard procedure for the TDTR technique. The Al film thickness for each sample was independently measured from picosecond acoustics displayed in the TDTR raw data. Sputtering was conducted in a vacuum chamber with a base pressure of 99.99% pure Al target was operated with a power of ∼50 W in a 0.2 Pa Ar background. Samples were insulated from ground during Al deposition and were not actively heated or cooled. In order to compare and contrast thermal conductivity of nanocrystalline, cubic GSH samples, a pure, nanocrystalline, monoclininc HfO2 sample with a comparable grain size was also examined following the same procedure and TDTR method.

II. EXPERIMENTAL SECTION A. Sputtering Target Synthesis. Ceramics with variable Gd2O3 content in the range of 0−38 mol % (balanced by HfO2) were prepared using Gd2O3 (99.999%) and HfO2 (99.99%) powders. Gd2O3 and HfO2 powders were properly ground and mixed in a mortar. Then the mixed powder was compressed in a die and punch to make 2 in. diameter pellets using a pressure of 7600 psi. The pellets were then sintered at 1400 °C for 24 h. The sintered pellets were then finally attached to a copper backing plate. These gadolinia stabilized hafnia (Gd2O3−HfO2 or GSH) ceramics attached to Cu plates were employed for sputterdeposition of the coatings. B. Coating Fabrication. The Gd2O3−HfO2 nanocrystalline coatings were deposited onto silicon (Si) (100) wafers by radio frequency magnetron sputtering. All the substrates were thoroughly cleaned and dried with nitrogen before introducing them into the vacuum chamber, which was initially evacuated to a base pressure of ∼10−6 Torr. GSH targets with variable Gd2O3 content were used for sputtering. The GSH target was placed on a sputter gun, which was correspondingly set at a distance of 8 cm from the substrate. A sputtering power of 30 W was initially applied to the target while introducing high purity argon (Ar) into the chamber causing plasma ignition. Once ignited, the power was increased to 60 W to deposit the films. The flow of the Ar was controlled using an MKS mass flow meter. Before each deposition, the GSH-target was presputtered for 10 min using Ar, keeping the shutter above the gun closed. Deposition was made to obtain a ∼0.3 μm thick coating. The samples were deposited at a substrate temperature (Ts) of 500 °C. The substrates were heated by halogen lamps, and the desired temperature was controlled by an Athena X25 controller. C. Coatings’ Characterization. Crystal Structure, Morphology, and Phase Analysis. The grown Gd2O3−HfO2 coatings were characterized by studying their crystal structure, surface/interface morphology, chemical composition, and thermal conductivity. Crystal structure determination of the coatings was made using X-ray diffraction (XRD) measurements 25179

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III. RESULTS AND DISCUSSION A. Crystal Structure and Phase Analysis. XRD patterns of GSH nanocrystalline coatings are shown in Figure 1. The curves

Figure 2. Expanded, high resolution scan of the (111) peak of the Gd2O3−HfO2 coatings. The peak shift with progressive addition of Gd2O3 is evident.

Figure 1. XRD patterns of Gd2O3−HfO2 coatings.

shown are as a function of the Gd2O3 content in the coatings. The peaks are sharp and well-defined compared to the patterns obtained at room temperature (not shown). A shift in the most intense (111) peak with increasing Gd2O3 is evident from the XRD patterns. The peak shifts to lower diffraction angle. Indexing of the patterns indicate that the coatings exhibit the crystal structure of the cubic hafnia phase. The cubic HfO2 phase formation can be attributed to the effect of Gd2O3 acting as a stabilizer. However, the effect of Gd2O3 as a stabilizer resulting in the cubic HfO2 phase is effective up to a concentration of 12 mol %, at which point the formation of pyrochlore Gd2Hf2O7 occurs. The grains are highly oriented along the (111) direction. The (111) peak position shifts from 2θ = 30.32° to 29.62° with increasing Gd2O3 content from 4 to 38 mol % (Figure 2). This is an indication of the expansion of the lattice by the expansion of the interplanar spacing (d) with Gd2O3 concentration. The lattice constant of the GSH coatings was calculated from the d value of the most intense (111) peak. The peak positions were obtained after fitting the peak with a Gaussian function. The error in measuring the (111) peak position (2θ) peak was ∼0.0015°. The lattice constant variation with Gd2O3 composition is shown in Figure 3. It is obvious from the plot that the lattice constant of the GSH coatings increases from 5.101 to 5.219 Å as the Gd2O3 concentration increases from 4 to 38 mol %. The XRD results and the physical mechanisms involved in the lattice parameter variation of Gd2O3−HfO2 coatings can be understood as follows. The lattice expansion and cubic-phase stabilization in Gd2O3−HfO2 coatings for lower concentrations of Gd is a result of oxygen vacancy creation. The Gd3+ ions having radii (0.94 Å) larger than that of Hf4+ imparts lattice distortion and enforces elongation of the bond with oxygen. The lattice parameter enhancement in Gd2O3−HfO2 coatings compared to pure HfO2 is, therefore, due to the Gd3+ ions and associated lattice distortion. The observed further increase in lattice parameter with progressive increase of Gd2O3 can be attributed to higher concentration of Gd3+ ions with sizes larger

Figure 3. Lattice parameter variation of Gd2O3−HfO2 coatings with Gd2O3 composition.

than that of Hf4+. The lattice distortion increases when the number of Gd4+ ions is more than the Hf4+ ions in the matrix leading to overall lattice expansion to accommodate the Gd3+ and Hf4+ ions. B. Morphology. The SEM images of GSH nanocrystalline coatings are shown in Figure 4. The GSH coatings grown at room temperature (not shown) are amorphous, which is in agreement with the XRD analysis. Coatings grown at 500 °C are highly crystalline for all the compositions. The grains are nanostructured and triangular in shape at a substrate temperature of 500 °C. On the basis of the results obtained and results reported for EB-PVD grown yttria-stabilized hafnia coatings and sputtering grown yttria stabilized hafnia-zirconia coatings, it appears that thin, elongated triangular shape morphology is the characteristic of stabilized hafnia. However, as seen in XRD, the deviation of such morphology with progressive addition of Gd2O3 is believed to be due to the distortion as a result Gd ions replacing the Hf ions. At low percentage (4%) of Gd2O3, the particles are smaller compared to that for higher amount of Gd2O3. Random void spaces are still visible all over the sample. The particles get attached together with the increase of Gd2O3 addition (8%). At 12% of Gd2O3, particles become bigger by the consumption of small particles. The void spaces decrease and pores are barely visible at this composition. The material looks 25180

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Figure 4. SEM images of Gd2O3−HfO2 ceramics as a function of Gd2O3 content.

very much packed at 20% Gd2O3 with no visible pores. The agglomerated big domains are attached to one another without maintaining any gap in between. This change in morphology is in good agreement with the crystal structure evaluated by XRD measurement. As evident in the XRD analysis, the lattice expansion occurs continuously with progressive addition of Gd2O3 which, perhaps, make the crystal domain bigger. Because of this favorable structure the material might have smaller surface energy and coalesce, making the compact surface morphology. This is not unexpected since the material will have excess Gd3+ at 38% Gd2O3, where the morphology is different compared to others. The most remarkable feature noted in this work is the columnar growth of the coatings. All the coatings exhibit columnar structure that is typically observed in hafnia and zirconia based ceramic coatings. A representative of the Gd2O3− HfO2 coatings (12 mol % Gd2O3) interface structure exhibiting the columnar growth is shown in Figure 5. C. Thermal Conductivity. Analysis of the data for extraction of thermal conductivities was accomplished with a frequency-

Figure 5. SEM crossectional image of Gd2O3−HfO2 coating for 12 mol % Gd2O3. The columnar structure is evident in the micrograph.

domain model37 in which the ratio of the in-phase and out-ofphase lock-in amplifier signals is calculated as a function of time: 25181

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sample (amorphous silicon dioxide at 1.3 W m−1 K−1) yielded approximately 95% accuracy. The value for pure HfO2 (0 mol % Gd2O3) is plotted in Figure 6 for comparison, illustrating that the most striking feature is a significant drop in thermal conductivity with addition of Gd2O3. The measured value at room temperature for the 4 mol % Gd2O3 sample (1.7 ± 0.1 W m−1 K−1) is lower than ZrO2 samples doped with 4 mol % of either Y2O3 (∼2.5 W m−1 K−1) or Gd2O3 (∼2.3 W m−1 K−1).23 Hence, the Gd2O3−HfO2 materials are better suited as thermal barrier coatings. Figure 6 also illustrates a trend of decreasing thermal conductivity with increasing Gd2O3 mol % (up to 12 mol %). Such an inverse trend of thermal conductivity with doping levels has previously been observed in Y2O3−ZrO2 systems.41 For the Gd2O3−HfO2 materials studied here, the decrease in thermal conductivity with larger doping contents is consistent with oxygen vacancies playing the dominant role in phonon scattering (as stated in ref 41). These are structural vacancies in the hafnia due to charge compensation of Gd3+ ions substituting for Hf4+ ions. Another factor known to influence thermal conductivity in thin films is grain size.41 The morphology analysis indicated that coalescence to form larger grains resulted as the mol % of Gd2O3 increased, which would increase the thermal conductivity. This is because larger grains result in fewer grain boundaries which can scatter phonons. However, this trend was not observed in our results, thus supporting our hypothesis that oxygen vacancy effects predominantly govern thermal transport. Figure 6 also reveals a discontinuity in the thermal conductivity trend for Gd2O3 content higher than 12 mol %, which is possibly due to substantial increase of Gd3+ concentration and oxygen vacancies formation.

Vin Vout m

=

∑−m (ΔT (m /τ + f ) + ΔT (m /τ − f ))exp(i 2πmt /τ ) m

i ∑−m (ΔT (m /τ + f ) − ΔT (m /τ − f ))exp(i 2πmt /τ ) (1)

Here m is an integer denoting summation over pump pulses, τ the time between unmodulated laser pulses (12.5 ns), f the modulation frequency (9.8 MHz), and t the time delay between pump and probe pulses. The function ΔT is calculated with the Feldman matrix algorithm as explained in ref 37 (eq 1 is also multiplied by a phase shift of ei2πf t). Data for pump advance times earlier than t = 100 ps were not taken into account since electron−phonon coupling and Al thermalization effects have not equilibrated. Picosecond acoustics also perturb this regime (which facilitates direct measurements of the Al thickness). For the model, a five-layer system was used comprised of two layers for the Al film (as explained in ref 37), an interfacial conductance, ∼300 nm for the sample layer, and a semi-infinite silicon substrate. The variation of the thickness of the sample layers was taken into account for TDTR modeling purposes. This five-layer system was used since the sample coatings were comparable to the thermal penetration depth of the experiment (D/2πf)1/2 ∼ 100 nm. Here D is the thermal diffusivity of the sample layer and f is the TDTR modulation frequency. Because of the large aspect ratio of laser spot size to thermal penetration depth, TDTR measures one-dimensional cross-plane thermal conductivity (perpendicular to the surface). In modeling the data, the volumetric heat capacities used for the sample layers were weighted averages (based upon the mol percent of Gd2O3) calculated from the known quantities of 2.73 J cm−3 K−1 for HfO238,39 and 2.14 J cm−3 K−1 for Gd2O3.40 The samples were assumed fully dense when considering heat capacity estimates as a function of Gd2O3 concentration. For the samples studied, TDTR data were acquired from five locations on each sample surface. The scans were individually modeled, and an average thermal conductivity ± standard deviation value was calculated for each sample. The results of the thermal conductivity measurements are shown in Figure 6, while examples of individual TDTR scans and data models are shown in the inset. The error bars from Figure 6 represent no more than 7% uncertainty. A TDTR calibration

IV. CONCLUSIONS Gd2O3−HfO2 nanocrystalline coatings of varying Gd2O3 content (0−38 mol %) were synthesized by magnetron sputtering. Sample characterization by XRD reveals the cubic phase formation with significant lattice expansion with increasing Gd2O3 content. SEM images show that the nanocrystalline grains increase in size with increasing Gd2O3 mol percent. The TDTR measurements show a significant drop in thermal conductivity for Gd2O3−HfO2 coatings as compared to pure HfO2, and all doped samples have lower thermal conductivity values than the more conventional TBC material yttria-stabilized zirconia. Oxygen vacancies appear to play the dominant role for hindering thermal transport in Gd2O3−HfO2 materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon the work supported by the Department of Energy (DOE) under Award Number DEFE0000765. The authors at the University of Texas at El Paso acknowledge with pleasure the support from DOE. The thermal conductivity measurement system acquisition and operation at the Air Force Research Laboratory was supported under Contract No. FA8650-07-D-5800.

Figure 6. Thermal conductivity results of Gd2O3−HfO2 coatings with Gd2O3 composition. The inset shows example TDTR raw data and models. 25182

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