Solution combustion synthesis of nanomaterials

doi:10.1016/j.proci.2006.07.052

Proceedings of the Combustion Institute

Volume 31, Issue 2, January 2007, Pages 1789-1795

https://doi.org/10.1016/j.proci.2006.07.052 Get rights and content

Abstract

Solution combustion (SC) is an effective method for synthesis of nano-size materials and it has been used for the production of a variety (currently more than 1000) of fine complex oxide powders for different advanced applications, including catalysts, fuel cells, and biotechnology. However, it is surprising that while essentially all of the studies on SC emphasize the characterization of the synthesized materials, little information is available on controlling combustion parameters and the reaction mechanisms. This paper is devoted to the analysis of the combustion parameters for different SC reaction modes. First, the conventional volume combustion synthesis mode, which involves uniform reaction solution preheating prior to self-ignition, is briefly discussed. Second, for the first time, results of detailed experimental studies on steady-state self-propagating mode of SC synthesis of nano-powders are presented. Finally, the so-called solution + impregnation combustion mode is considered. The relationship between combustion parameters and product microstructures are emphasized. These results are crucial not only from the application stand-point, but more importantly lead to methodological benefits, allowing application of the developed approaches to investigate steady state heterogeneous combustion waves in new classes of reaction systems.

Introduction

It is well recognized that combustion-based techniques, known in the literature as combustion synthesis (CS) or self-propagating high-temperature synthesis (SHS) are an effective energy saving method for the synthesis of a variety of advanced materials [1], [2], [3], [4], [5], [6], [7]. SHS mode is characterized by the fact that once the initial exothermic reaction mixture is ignited by means of an external thermal source, a rapid (typically from 0.1 to 10 cm/s) high-temperature (1000–3000 °C) reaction wave propagates through the heterogeneous mixture in a self-sustained manner leading to the formation of the solid material without involving any additional energy. In the case of another CS mode, the so-called volume combustion synthesis (VCS), the entire sample is heated uniformly until the reaction self-initiates throughout the volume. This synthesis mode is less controllable and is used for weakly exothermic reactions that require preheating prior to ignition. However, in the conventional CS scheme, the initial reaction medium is a powder mixture with characteristic scale of heterogeneity in the range 1–100 μm. This feature, coupled with high reaction temperatures (>2000 K), makes it difficult to synthesize nano-size materials with high surface area, although it does provide a rapid way to form product with a well-crystalline structure.

A combination of CS and reactive solution approaches leads to solution combustion (SC) synthesis methods (cf. [8], [9], [10], [11], [12], [13], [14]). Typically, SC involves a self-sustained reaction in solutions of metal nitrites and different fuels, which can be classified based on their chemical structure, i.e. the type of reactive groups (e.g. amino, hydroxyl, and carboxyl) bonded to the hydrocarbon chain [13]. The reaction between fuel and oxygen containing species, formed during the decomposition of the nitrite species, provides high-temperature rapid interaction. In a typical scheme, an initial liquid solution of desired reagents, after preheating to a moderate temperature (∼150–200 °C), self-ignites along the whole volume (VCS mode) leading to the formation of fine solid products with tailored composition.

The following features of SC contribute to the unique properties of the synthesized products. First, the initial reaction media being in the liquid state (e.g. aqueous solution) allows mixing the reactants on the molecular level, thus permitting precise and uniform formulation of the desired composition on the nano scale. Second, the high reaction temperature (T_c) ensures high product purity and crystallinity. This feature allows one to skip an additional step, high-temperature product calcinations which typically follow the conventional sol-gel approach, to achieve the desired phase composition. Third, short process duration and the formation of various gases during SC inhibit particle size growth and favor synthesis of nano-size powders with high specific surface area (cf. [8], [9], [10], [11], [12], [13], [14]).

Thus it can be concluded that SC is an effective method for synthesis of nano scale materials and it has been used for the production of a variety (currently more than 1000) of fine complex oxide powders for different advanced applications including catalysts, fuel cells, and biotechnology (cf. [[10], [12]]).

However, it is surprising that while essentially all of the studies, including cited above, emphasize the characterization of the synthesized materials, little information is available on the controlling combustion parameters and the reaction mechanisms. It is important to note that this class of combustion systems involves a variety of unique features that cannot be described based on existing combustion models. Indeed, intense heat transfer in the initial liquid reaction media and products gasification coupled with the rapid chemical reaction and formation of solid nano particles make unique conditions for the SC wave propagation.

Analysis of the relevant literature also gives the answer to the question: why are so few results found regarding the SC mechanism? Since in all works only the VCS mode was used, which produces a spontaneous and extremely rapid reaction self-initiation, it was difficult to monitor the reaction media evolution taking place in the time range of approximately several seconds at a temperature of approaching 1000 °C. In addition, the experimental conditions of vigorous gas phase evolution and solid product eruption, hindered observation of the reaction mechanism.

Our preliminary thermodynamic calculations (see examples below) revealed that most of the solution combustion mixtures (e.g. metal nitrites and such fuels as glycine, urea etc.) are not weakly exothermic and, thus, the self-propagating (SHS) mode is not prohibited for these systems. In this work for the first time, results of detailed experimental studies on the steady-state self-propagating mode of solution combustion synthesis of nano-powders are presented. These results are crucial not only from the application stand-point, but more importantly lead to methodological benefits, allowing application of the developed approaches to investigate steady-state heterogeneous combustion waves in new classes of reaction systems.

Section snippets

Experimental

Combustion in a ferric nitrite–glycine system to synthesize iron oxide is used as an example in this paper. However, the trends presented work for a variety of other systems, including the synthesis of complex oxides such as perovskites and cuprates.

Ferric nitrite, Fe(NO₃)₃·9H₂O (98%, Alfa Aesar; b.p. 125 °C) was used as an oxidizer and glycine, CH₂NH₂CO₂H, (98%, Alfa Aesar; m.p. 262 °C) as a fuel. Under equilibrium conditions, the reactions in these systems can be represented as follows: $Fe ({NO}_{3})_{3} +$

Results and discussion

First, the thermodynamics of the reaction system are analyzed. Second, the conventional volume combustion synthesis mode, which involves uniform reaction solution preheating prior to self-ignition, is briefly discussed. Third, some characteristics of the self-propagating mode, when a locally initiated reaction propagates in the form of a combustion wave along the homogeneous solution media, are presented. Finally, the so-called solution + impregnation [15] combustion mode is considered. The

Concluding remarks

Three different modes for solution combustion synthesis—VCS, SHS, and Impregnation CS—are discussed focusing on the combustion characteristics of the processes. It was shown that controlling the combustion parameters allows for the influence of the microstructural characteristics of the product. The nano-powders with high specific surface area were synthesized under optimum conditions. The main goal of the paper is to attract attention to the study of the fundamental mechanisms of the solution

Acknowledgments

This work was supported by the U.S. Army CECOM RDEC through Agreement AAB07-03-3-K414. Such support does not constitute endorsement by the U.S. Army of the views expressed in this publication.

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Solution combustion synthesis of nanomaterials

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Concluding remarks

Acknowledgments

Mater. Sci. Rep.

Prog. Mater. Sci.

Adv. Chem. Eng.

Curr. Opin. Solid State Mater. Sci.

Sep. Purif. Tech.

Combust. Flame

J. Catal.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

Dokl. Chem.