Reductive/expansion synthesis of zero valent submicron and nanometal particles

Abstract

Abstract

Evaluation of the modified reduction/expansion synthesis in the production of nanoparticulate alumina

“…Transferring the mixed paste of aluminum nitrate and urea from room temperature to a preheated oven involves an extremely fats heating of the mixture, leading to a thermal decomposition of the mixture in a temperature range between 160 to 250 o C, depending of the paste composition [7,12,13,15,16]. Aluminum nitrate decomposition has been reported to start at temperatures as low as 150 o C [15], meanwhile urea decomposes in a first stage between 180 -250 o C to form ammonium and isocianic acid followed by a further decomposition of the isocianic acid to a gaseous mixture of ammonium and carbon dioxide at temperatures above 350 o C [10,12]. In the case of the aluminum nitrate, the initial stage of decomposition below 170 o C, comprises a series of simultaneous reactions such as dehydration, hydrolysis, and destruction of nitrate groups sharing comparable reaction mechanism to processes such as dehydroxylation of hydroxy salts, that also could be formed during the thermal decomposition of hydrolytic processes that take place at temperatures above 170 o C. Therefore, stoichiometric and thermodynamics conditions of the mix at the initial moment of decomposition may lead to the formation of an initial hydroxyl compound of the type Alx(OH)y(NO3), the nitrate molecules decomposes rapidly due to the thermodynamic favorable reaction to the formation of ammonium in the presence of urea.…”

“…The study of simple methods for the production of particulate oxides with high thermal resistance like alumina and alumina based ceramics oxides has aroused great interest recently; in particular, aluminum oxides are of great interest given their important physical characteristics such as high hardness, high stability, high insulation, and transparency. An interesting alternative for the production of metallic particles was proposed by Zea et al [10], involving economic reagents and using simple equipment; this versatile reductive/expansion synthesis (RES) approach not only allows the generation of metallic material but also by tuning the reaction atmosphere allows the formation of oxides. Under this approach (M-RES, modified reductive/expansion synthesis), an initial reductive condition coupled with an oxidant atmosphere and thermal treatment generates aluminum oxides with different crystallographic phases.…”

Modificación del proceso de reducción expansiva para la síntesis de nanopartículas de hierro

“…Estudios anteriores demostraron que la técnica denominada síntesis de reducción expansiva (RES por su sigla en inglés Reduction Expansive Synthesis) genera aglomerados micrométricos fácilmente disgregables de nanopartículas metálicas (Zea et al 2011. Estos resultados mostraron que partículas cero valentes se pueden producir a partir de sales nitrogenadas del metal de interés, por medio de la descomposición súbita de urea.…”

“…Inicialmente el nitrato se descompone en óxido (en ausencia de urea) como es el caso de la síntesis por combustión; cuando las mezclas urea/ nitrato se someten a un calentamiento rápido hay una "convolución" de los procesos de descomposición. De esta forma, la urea se descompone generando gases reductores que permiten la formación del metal más que del óxido (Zea et al 2011). Adicionalmente, la creación de partículas metálicas micro y submicrométricas a partir de precursores de óxido e hidróxido, indican que el uso de precursores óxidos para la completa reducción a micro-partículas, requiere mayor contenido de urea y mayores temperaturas; mientras que con el uso de hidróxidos se logra la reducción a temperaturas más bajas (Luhrs et al 2013).…”

Determination of thermal properties of the ionic system xAgI-(1-x)NH4I by differential scanning calorimetry

“…With these data, temperatures and transition or reaction enthalpies can be obtained. Some recommendations for the operation of the DSC are the following [5]. In this work we determined the thermal properties of the ionic system xAgI-(1-x)NH4I by Differential Scanning Calorimetry.…”