Thermodynamic properties of lanthanum, neodymium, gadolinium hafnates (Ln 2 Hf 2 O 7 ): Calorimetric and KEMS studies
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FOCUS ISSUE
THERMODYNAMICS OF COMPLEX SOLIDS
Thermodynamic properties of lanthanum, neodymium, gadolinium hafnates (Ln2Hf2O7): Calorimetric and KEMS studies Viktor A. Vorozhtcov1, Valentina L. Stolyarova1,a) Elizaveta P. Simonenko2, Nikolay P. Simonenko2
, Mikhail V. Chislov1, Irina A. Zvereva1,
1
Institute of Chemistry, Saint Petersburg State University, Saint Petersburg 199034, Russia Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Moscow 119991, Russia a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 22 March 2019; accepted: 29 May 2019
Using the data obtained by Knudsen effusion mass spectrometry, the standard formation thermodynamic properties of La2Hf2O7, Nd2Hf2O7, and Gd2Hf2O7 were calculated in the present study at high temperatures. Based on the results obtained, it was shown that the standard formation Gibbs energies of La2Hf2O7, Nd2Hf2O7, and Gd2Hf2O7 from the elements at the temperature 2445 K were consistent with the empirical rule concerning decrease of stability of pyrochlore hafnate phase with decrease in lanthanoid ionic radius. The La2Hf2O7 and Gd2Hf2O7 heat capacities were obtained in the present study by differential scanning calorimetry. These data were used along with those found earlier to evaluate the standard formation Gibbs energies of La2Hf2O7 and Gd2Hf2O7 from the elements at the temperature 298 K, which equal (−3937 ± 10) kJ/mol and (−3895 ± 10) kJ/ mol, respectively. The thermodynamic properties of La2Hf2O7, Nd2Hf2O7, and Gd2Hf2O7 estimated in a wide temperature range allowed consideration of reliability of data available in the literature.
Introduction Ceramics based on rare earth hafnates are known to be characterized with low volatility of components and thermal conductivity as well as high chemical stability and melting temperatures [1, 2]. It has been repeatedly shown that rare earth hafnates are promising for development of a wide range of new high temperature materials, such as thermal barrier coatings [3, 4] and casting molds for gas turbine engine blades [5, 6]. Nowadays high temperature materials are mostly based on zirconia stabilized with rare earth oxides [7]. However, its main drawback is limited range of working temperatures less than 1473 K. Consequently, as working temperatures of gas turbine engines increase, a need arises for new refractory materials, which possess lower thermal conductivity and larger temperature range of phase stability than stabilized zirconia. A promising approach to improve the characteristics of high temperature ceramics is substitution of zirconia to hafnia [7, 8]. Hafnia based ceramics outperform materials based on zirconia in several aspects including high temperature phase stability due to the higher temperature of the HfO2 transition from monoclinic to
ª Materials Research Society 2019
tetragonal structure at 1923 K as compared to the ZrO2 corresponding phase transition temperature as 1475 K [9]. Moreover, the higher temperature of the HfO2 phase tran
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