Radical and oxidative pathways in the pyrolysis of a barium propionate-acetate salt
Introduction
Barium propionate (Ba(CH3CH2CO2)2, BaProp2) and barium acetate (Ba(CH3CO2)2, BaAc2) find application in the field of ceramic film synthesis through chemical methods, along with similar short chain carboxylate salts of other metals. In particular, they are some of the precursors to YBa2Cu3O7-∂ (YBCO) [1,2], a high temperature superconductor, obtained from fluorine-free (FF) [[3], [4], [5], [6]] and low fluorine [7] chemical methods. In fact, there are different precursor solutions for the synthesis of YBCO, which, classified with respect to the fluorine content in the metalorganic salt [8], can either be FF (fluorine-free), low fluorine [7] or all fluorine (TFA, trifluoroacetate route) [[9], [10], [11]]. In particular, the latter is a well-known route which permitted to overcome [12] the problem of BaCO3 formation as an undesired intermediate coming from pyrolysis of FF precursors like acetates and propionates [[3], [4], [5], [6]].
In fact, BaCO3 is a challenge for the epitaxial growth of YBCO due to the fact that its decomposition overlaps with the YBCO crystallization process, making it hard to optimize its decomposition and the growth conditions to obtain epitaxial YBCO films [3,13]. On the other hand, the TFA route presents two drawbacks: (i) being not environmentally friendly due to HF (hydrofluoric acid) formation, which in turn requires difficult furnace designs [14] and (ii) low yields, especially for thick films, due to the slow HF out-diffusion during BaF2 decomposition. Conversely, the acknowledgement that a Ba-Cu-O liquid phase is formed from BaCO3 decomposition [5] has opened up possibilities for FF-YBCO as a cost-effective chemical solution deposition route (CSD) [15] to replace the more expensive industrial physical methods. Nevertheless, to open up chemical methods to the industry, an optimization of the CSD thermal treatments (pyrolysis and growth) through the study of the thermal behavior of metalorganic precursors is of fundamental interest [[16], [17], [18], [19], [20]].
Regarding the thermal decomposition of BaProp2, It has already been demonstrated through evolved gas analysis (EGA) that carboxylate salts of M(II) and M(III) tend to decompose in inert atmosphere releasing a symmetrical ketone as major product, following a radical path of decomposition [[20], [21], [22], [23], [24]]. This however has been found to be affected by the metal center redox behavior, and therefore it does not hold in the case of those carboxylates whose metal easily undergoes redox reactions, like Cu or Ag [[25], [26], [27]], for which the main volatile consists of the corresponding acid. According to [28], the salt obtained from the acetate precursor in propionic acid and methanol results in a mixed acetate-propionate complex. By coupling mass spectrometry to thermogravimetry (TGMS) it is shown that during decomposition in air it releases CO2 in a first small (≈3%) mass loss, followed by 3-pentanone (m/z = 57), CO2 (m/z = 44) and acetone (m/z = 43 and 58) to yield BaCO3. Previously, it was also reported that barium propionate synthesized from the corresponding carbonate in excess of propionic acid, decomposes in inert atmosphere in two steps [29] of similar mass loss, yielding 3-pentanone and traces of acetone, but no CO2, in accordance with the stoichiometry of the reaction mechanism. Similarly, BaAc2 was shown to decompose to BaCO3, passing through an intermediate barium oxalate stable until 330 °C [30].
However, so far, the thermal decomposition of barium propionate and barium acetate has been studied only for samples in the form of powder and the volatiles observed only through EGA-MS; in fact, only a few studies for FF and fluorine precursors [12,31] can be found for films [32,33]. What happens during the actual pyrolysis of thin films of BaProp2 or BaAc2 has never been seen yet, due to the limiting amount of sample used for films. Additionally, the acetate-propionate equilibrium of barium precursor solution has never been explored in the context of YBCO pyrolysis. We will show that thermal analysis of BaProp2/Ba-Prop-Ac film samples is possible to achieve, and that the Ba carboxylate salt formed in solution depends on the solution history; complementary techniques (TG-FTIR and EGA-MS) have been used to confirm decomposition reactions and volatiles. Solid phases have been characterized by means of Elemental Analysis (EA), X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). We will also show that decomposition reactions depend on the atmosphere and sample geometry (film versus powder) [34] resulting in different decomposition paths, showing some similarities with YProp3 [35].
Section snippets
Materials and methods
The initial solution was obtained dissolving barium acetate (BaAc2, Sigma Aldrich) in propionic acid (Merck, ≥99%), kept under sonication until complete dissolution of the salt. Then methanol (VWR, ≥99.8%) was added in order to obtain a mixture of 1:1 in solvent composition and a [Ba2+] = 0.5 M. Film samples were obtained depositing the initial solution on LaAlO3 (LAO) substrates and drying them at 95 °C for a few minutes. The film thickness (H) was estimated with the following equation:
Characterization of the initial product
The elemental analysis results of the powder obtained from the BaAc2 precursor solution and from an acetate-free (BaCO3) precursor solution are shown in Table 1. For the latter, the values are in agreement with BaProp2 formation. Conversely, the product obtained from the BaAc2 solution shows a C and H% inferior to the theoretical value for the full replacement of acetates by propionates, indicating that some acetate ligands remain in the structure. In fact, the FTIR spectrum of the dry film in
Discussion
BaProp2 and Ba-Prop-Ac decomposition is diffusion-controlled: in an oxidizing atmosphere, films decompose at a temperature lower than powders due to the faster gas exchange [32,33,35], which helps the low-temperature decomposition mechanism triggered by oxygen. Unlike the YProp3 case, a humid atmosphere does not clearly accelerate decomposition through the hydrolysis of the salt (and propionic acid release), since this reaction path would require formation of the oxide and not the
Conclusions
The thermal decomposition of BaProp2 was studied as a function of sample geometry (films and powders) and the atmosphere (inert or oxidizing). It has been found that decomposition is enhanced in oxidizing with respect to inert conditions, but not much in a humid atmosphere. Like many carboxylates, the radical mechanism with 3-pentanone formation prevails at high temperatures and inert conditions but, unlike other carboxylates where the oxide is formed, the low temperature mechanism is favored
Acknowledgments
This work was funded by Ministerio de Ciencia, Innovación y Universidades (grant numbers RTI2018-095853-B-C21 and RTI2018-095853-B-C22); it was also supported by the Center of Excellence Severo Ochoa (SEV-2015-0496) and the Generalitat de Catalunya (2017-SGR-1519). SR wishes to thank the University of Girona for the IF-UdG PhD grant; all authors wish to thank the UdG and ICMAB (Institut de Ciència de Materials de Barcelona) for their scientific services.
References (50)
- et al.
Crystal structure and electrical conductivity of YBa4Cu3O8.5+∂
Phys. C
(1989) - et al.
Fluorine-free propionate route for the chemical solution deposition of YBa2Cu3O7−x superconducting films
Ceram. Int.
(2015) - et al.
Influence of sintering conditions in the preparation of acetate-based fluorine-free CSD YBCO films using a direct sintering method
Mater. Res. Bull.
(2012) - et al.
Conversion behavior comparison of TFA-MOD and non-fluorine solution-deposited YBCO films
Phys. C Supercond. Appl.
(2009) - et al.
Thermal decomposition of barium trifluoroacetate thin films
Thermochim. Acta
(2013) - et al.
Influence of sintering conditions in the preparation of acetate-based fluorine-free CSD YBCO films using a direct sintering method
Mater. Res. Bull.
(2012) - et al.
Reactions of oxyfluoride precursors for the preparation of barium yttrium cuprate films
Phys. C
(2004) Thermal decomposition of yttrium(III) propionate and butyrate
J. Anal. Appl. Pyrolysis
(2013)- et al.
Synthesis, crystal structure and thermal decomposition kinetics of yttrium propionate
J. Anal. Appl. Pyrolysis
(2014) Thermal decomposition of lutetium propionate
J. Anal. Appl. Pyrolysis
(2010)
Synthesis, crystal structure modeling and thermal decomposition of yttrium propionate [Y2(CH3CH2COO)6·H2O]·3.5H2O
J. Anal. Appl. Pyrolysis
The thermal behaviour of divalent and higher valent metal soaps: a review
Thermochim. Acta
Product analysis, reaction mechanism and kinetics of the thermal decomposition of some even chain-length mercury(II) carboxylates
Termochimica Acta
Synthesis, crystal structure and thermal decomposition study of a new barium acetato-propionate complex
J. Anal. Appl. Pyrolysis
Thermal analysis of metal organic precursors for functional oxide preparation: thin films versus powders
Thermochim. Acta
Thermal decomposition of yttrium propionate: film and powder
J. Anal. Appl. Pyrolysis
FT-IR spectroscopic investigations on sol—gel-derived coatings from acid-modified titanium alkoxides
J. Non-Cyrstalline Solids
Hydrolysis of titanium alkoxides: modification of the molecular precursor by acetic acid
J. Non-Cyrstalline Solids
Correlation of infrared spectra of zinc(II) carboxylates with their structures
Spectrochim. Acta Part A Mol. Biomol. Spectrosc.
Relationships between the carbon-oxygen stretching frequencies of carboxylato complexes and the type of carboxylate coordination
Coord. Chem. Rev.
On the interpretation of multiple melting peaks in poly (ether ether ketone)
Polymer (Guildf.)
Thermal decomposition of barium valerate in argon
J. Anal. Appl. Pyrolysis
Thermal decomposition of CuProp2: In-situ analysis of film and powder pyrolysis
J. Anal. Appl. Pyrolysis
Thermal decomposition of barium oxalate hemihydrate BaC2O4•0.5H2O
Thermochim. Acta
The thermal decomposition of oxalates. A review
Thermochim. Acta
Cited by (13)
ICTAC Kinetics Committee recommendations for analysis of thermal decomposition kinetics
2023, Thermochimica ActaEffect of glycerol on the thermal decomposition behavior of nickel propionate-based precursor
2021, Journal of Analytical and Applied PyrolysisElucidation of the decomposition reactions of low-fluorine YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7-x</inf> precursors during film pyrolysis
2020, Journal of Analytical and Applied PyrolysisCitation Excerpt :Regarding the fluorinated precursors, a few studies were carried out on the single-salt precursors (YTFA -yttrium trifluoroacetate, and BaTFA -barium trifluoroacetate) in the form of films [19,20]. Conversely, thermal analysis of fluorine-free precursors based on acetates and propionates can be found on both film [14–16] and powder form [21–23]. The aforementioned studies of the single-salt precursors, both propionate and trifluoroacetate based, will serve as reference to understand the complex behaviour of our low-fluorine solutions, where both fluorinated and fluorine-free species of Cu, Ba and Y can be present in the initial mixture, depending on the amount and characteristics of ligand exchange.
Effect of triethanolamine on the pyrolysis of metal-propionate-based solutions
2019, Journal of Analytical and Applied PyrolysisCitation Excerpt :Both effects observed before 300 °C by XRD and FTIR analysis were also noticed for the barium propionate-acetate salt alone, and attributed to dehydration, recrystallization and melting of the salt [26]. The main TEA absorptions and the ester peak disappear after this stage, leaving an FTIR spectrum at the beginning of the third stage (280 °C) that is identical to that of the barium carboxylate salt at this temperature [26]. The third TG step in Fig. 2 corresponds to the decomposition of Ba-Prop-Ac to BaCO3, with a characteristic DSC exothermic peak [26] and with a m500°C/m300°C ratio of 71.2% for this step (theoretical of 72.7% starting from Ba7Prop8Ac6).
Thermal decomposition of calcium propionate: films and powders
2023, Journal of Thermal Analysis and CalorimetryMeasuring the viscosity of films by thermomechanical analysis: application to metal organic precursor films of functional oxides
2023, Journal of Thermal Analysis and Calorimetry