Arvind Varma

Combustion synthesis of advanced materials

Abstract A variety of advanced materials can be synthesized using solid-state combustion reactions, following two different modes of synthesis, the self-propagating mode and the thermal explosion mode. Combustion synthesis offers many potential advantages over conventional techniques of synthesis, including relatively simple equipment, shorter processing times, lower energy requirements, higher product purities, and the possibility to synthesize metastable phases. Various aspects of combustion synthesis are discussed in this review. They include thermodynamic considerations, theory and mechanisms, and structure-forming processes. The influence of processing variables on reaction progress and structure formation is discussed in detail. Some novel technologies and materials based on combustion synthesis are also described. The principles of chemical reaction engineering, combined with concepts and techniques of materials science, provide a suitable basis for studies of combustion synthesis.

Combustion Synthesis of Advanced Materials: Principles and Applications

Combustion synthesis is an attractive technique to synthesize a wide variety of advanced materials including powders and near-net shape products of ceramics, intermetallics, composites, and functionally graded materials. This method was discovered in the former Soviet Union by Merzhanov et al. (1971). The development of this technique by Merzhanov and coworkers led to the appearance of a new scientijc direction that incorporates both aspects of combustion and materials science. At about the same time, some work concerning the combustion aspects of this method was also done in the United States (Booth, 1953; Walton and Poulos, 1959; Hardt and Phung, 1973). However, the full potential of combustion synthesis in the production of advanced materials was not utilized. The scientijc and technological activity in thejeld picked up in the United States during the 1980s. The signijcant results of combustion synthesis have been described in a number of review articles (e.g., Munir and Anselmi-Tamburini, 1989; Merzhanov, I990a; Holt and Dunmead, 1991; Rice, 1991; Varma and Lebrat, 1992; Merzhanov, 1993b; Moore and Feng, 1995). At the present time, scientists and engineers in many other countries are also involved in research and further development of combustion synthesis, and interesting theoretical, experimental, and technological results have been reported from various parts of the world (see SHS Bibliography, 1996). This review article summarizes the state of the art in combustion synthesis, from both the scientijc and technological points of view. In this context, we discuss wide-ranging topics including theory, phenomenology, and mechanisms of product structure formation, as well as types and properties of product synthesized, and methods for large-scale materials production by combustion synthesis technique.

An in situ diffuse reflectance FTIR investigation of photocatalytic degradation of 4-chlorophenol on a TiO2 powder surface

An in situ FTIR technique has been employed to probe the adsorption and photocatalytic degradation of 4-chlorophenol (4-CP) on TiO2 particles in a gas/solid system. The chemisorbed 4-CP adsorbed on the TiO2 surface corresponds to a monolayer. The reaction has been followed under a number of conditions in order to investigate the roles of oxygen and water in the mechanism of 4-CP degradation. The rate of 4-CP disappearance was greatest under an oxygen atmosphere saturated with water and with UV irradiation. Under these conditions hydroquinone (HQ) was observed as a primary reaction intermediate and carbonates as the final product. Transformation of 4-CP to HQ was seen to occur under a variety of reaction conditions (i.e. O2/UV, N2/H2O/UV), but HQ degradation took place only in the presence of oxygen and UV irradiation. These results illustrate that the interactions of 4-CP at the semiconductor surface are complex and suggest that oxygen is intrinsically involved in the degradation reactions.

Perovskite membranes by aqueous combustion synthesis: synthesis and properties

The objective of this work is to identify optimum synthesis, compacting and sintering conditions in order to achieve a pure phase fully densified La0.8Sr0.2CrO3 (LSC) perovskite membrane. The aqueous combustion synthesis of LSC powders was investigated over a wide range of synthesis conditions by using the metal nitrates (oxidizer)‐glycine (fuel) system. The powders were pressed and sintered to create dense materials, which were characterized. It was shown that depending on fuel/oxidizer ratio, , the reaction can proceed in three different modes: Smoldering Combustion Synthesis (SCS), 0.7, with maximum temperature, Tm 600°C; Volume Combustion Synthesis (VCS), 0.71.2, 1150°CTm1350°C; Self-propagating High-temperature Synthesis (SHS), 1.2 1.6, 800°CTm1100°C. In turn, the characteristics of synthesized powders depend on the combustion mode. The crystalline structure of as-synthesized powders becomes more defined as increases (amorphous for SCS; crystalline for VCS and SHS). The specific surface area decreases slightly when mode changes from SCS ( 25 m 2 g − 1 )t o VCS (20 m 2 g −1 ), however, it increases substantially under SHS conditions (up to 45 m 2 g − 1 ). It was also shown that calcination is beneficial only for SCS powders, while VCS and SHS powders may be sintered directly as synthesized, thus bypassing the time and energy consuming calcination step. The measured oxygen permeation values for the membranes are comparable with the best candidate materials reported in the literature.

Palladium composite membranes by electroless plating technique: Relationships between plating kinetics, film microstructure and membrane performance

Thin supported palladium membranes were prepared using electroless plating technique. The intrinsic plating kinetics were determined for hydrazine-based palladium plating baths. Mathematical models were developed to predict film thickness and plating rate as a function of plating parameters (i.e., reactant concentrations, temperature and time). Analyses of film surface microstructure indicate that grain structure is dependent on both plating chemistry and plating rate. The evolution of film morphology during electroless plating was also investigated. Hydrogen permeation study demonstrated that film structure has a significant influence on the permeation performance of the palladium membrane.

A generalized criterion for parametric sensitivity: application to thermal explosion theory

Abstract A new criterion for parametric sensitivity or thermal runaway is proposed in the context of thermal explosions, which can also be readily utilized for chemical reactors. Criticality is defined as the situation where the normalized objective sensitivity, of the temperature maximum to any of the physicochemical parameters of the model, is a maximum. In fact it is found that, in all cases of practical interest, the region of sensitivity obtained with respect to any of the independent model input parameters is the same. Thus, this criterion predicts a parametrically sensitive or runaway region, which may be called “generalized” since the maximum temperature becomes simultaneously sensitive to small changes of any of the model inputs. The proposed criterion exhibits a highly intrinsic nature and it is firmly based on the rigorous concept of normalized sensitivity. This, together with its flexibility, allows direct extension of the criterion to more general problems, both in thermal explosion and in chemical reaction engineering fields. This is especially so for cases where it may be convenient or desirable to consider the sensitivity of outputs of the model other than the maximum temperature, and/or cases where a temperature profile does not even exist, e.g. in a CSTR. None of these situations can be handled with sensitivity criteria previously available in the literature, since they are all based on the topology of some type of a temperature profile.

Thermal explosion in Ni-Al system: influence of reaction medium microstructure

The thermal explosion (TE) phenomenon was investigated in the Al-Ni system (Al-rich compositions) with various reactant medium microstructures. The parameters of TE (e.g., ignition temperature, reaction times, etc.) obtained for Al particles clad by Ni were compared with those for Al+Ni powder mixtures having different particle sizes. The heating rate was varied in the range 100 to 900 K/min, and its influence on TE characteristics was also studied. The results show that ignition process is related to either phase (melting of reactant or first product formed) or microstructural (breaking of the Ni shell in case of clad particles) transformations and depends on the reactant medium microstructure. The latter influences the process of initial product formation, which in turn affects the interaction rate during the post-ignition stages. The obtained results were discussed and compared with those reported in the literature. In general, it is suggested that TE in heterogeneous gasless systems differs from explosions in conventional homogeneous media. In the latter, the operating conditions define ignition temperature, while in the former, it is related to the system phase diagram and thus is a characteristic property of the system.

Optimal Distribution of Catalyst in Pellets

Abstract A large fraction of the chemical and refinery processes are catalytic in nature. While the worldwide sales of catalysts are only about

Metal composite membranes: Synthesis, characterization and reaction studies

4 billion annually, the economic impact of catalysis comes from the fact that approximately

Some factors influencing the formation of reaction-bonded silicon nitride

200 worth of products are manufactured for every