Radical and oxidative pathways in the pyrolysis of a barium propionate-acetate salt

https://doi.org/10.1016/j.jaap.2019.104640Get rights and content

Highlights

  • Ba-Prop-Ac films decompose at lower temperatures than powders.

  • Decomposition at high temperatures/inert atmosphere involves radicals.

  • Decomposition at low temperatures is based on oxidation reactions.

  • No hydrolysis reaction occurs, even in humid atmosphere.

  • Decomposition is enhanced by O2 diffusion through the melted precursor.

Abstract

Film and powder samples from a BaAc2 solution in propionic acid/MeOH were decomposed in different atmospheres and their thermal decomposition was characterized by means of thermogravimetry coupled with evolved gas analysis techniques (TG-FTIR, EGA-MS) and chemical and structural methods (EA, XRD, FTIR). The thermal behavior of the films was found to be different than the corresponding powder, in terms of volatiles, kinetics, intermediate phases and purity of final product. The mixed Ba-Prop-Ac salt obtained from solution decomposes to BaCO3 before 400 °C through oxidative degradation, and above 400 °C in inert atmosphere through a radical path releasing symmetrical ketones. Its double melting behavior is also highlighted and its decomposition understood by comparison with BaProp2 and BaAc2 precursors, and put into the context of YBa2Cu3O7-∂ (YBCO) film pyrolysis.

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 (TGsingle bondMS) 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.

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