EXPECTATIVA DEL AUTOCUIDADO EN EL FUTURO

Sol-gel method is gaining interest over the last decades due to the final properties of its product. The final product is pure, homogeneous and the process temperature is lower when compared to other processes, such as solid state synthesis or co-precipitation [2]. The sol-gel process is a versatile process to produce ceramic and glass materials. In general, this process involves the transition of a system from a liquid “sol” (mostly colloidal) into a “gel” phase. The oxide net can be obtained by inorganic polymerisation reactions, which occur in solution.

An important characteristic of the sol-gel process is the possibility to control all stages, which occur during the passage through molecular precursor to the final product. A better control of the process can produce materials with specific characteristics and proprieties [4, 5]. The aim is to prepare a homogeneous solution containing all the cationic ingredients. In the first stage the heat will transform the homogeneous solution into a viscous sol, which contains particles of colloidal dimensions. Maintaining the solution heated, it will be finally transformed into a transparent, homogeneous, and amorphous solid called gel. This gel has no precipitation of any crystalline phase. After the formation of the gel, it will be fired at higher temperatures (<1200 °C) to remove the volatile compounds that might be trapped in the gel pores or chemically-bound with organic groups and to crystallise the final product [1, 2, 4].

The chemistry of the sol-gel process is based on hydrolysis and condensation of the molecular precursor. The versatile precursors used in this kind of synthesis are alkoxide metals (metal-organic compounds), M(OR)n (metal, ethyl, propyl, isopropyl, butyl, tert-

butyl, etc.), e.g. tetraethylorthosilicate (TEOS), Si(OCH2CH3)4, as source of SiO2. These

precursors usually are covalent liquids that are mixed in appropriated ratios, often with an alcohol to promote miscibility between the alkoxide and water. Water is the key reagent for the hydrolyses of alkoxides, which usually occurs in the presence of acid or base conditions as a catalyst to accelerate the reaction [6, 7].

The high electronegativity of the group alkoxide (OR) means that the metallic atom is attacked by the nucleophiles. Hydrolysis occurs by the reaction of the alkoxide with the water, producing a hydroxide group M-OH and an alcohol. The second stage of the sol- gel process is the condensation polymerization of M-OH species, making the formation of -M-O-M-, and the elimination of water. After several stages of condensation, the final result is (M-O)n. There have been other studies of producing oxides by sol-gel process,

for example, producing SiO2, TiO2, SnO2, V2O5, WO3 [2, 7, 8].

2.1.3 Combustion Method

Combustion synthesis is also known as self propagating high temperature synthesis (SHS) and solid state methathesis (SSM). This method is so fast that the reaction occurs in minutes or even seconds. Once the reaction is initiated, the heat provided is enough to generate high temperature to complete the reaction.

Starting materials for this method are known as “fuel” and “oxidants”; there is a wide range of materials that can be prepared by SHS and SSM methods, such as oxides, nitrides, borides and carbides [2].

2.1.4 Glycine Nitrate Process

The glycine nitrate process is based on the exothermic nature of the redox reaction between the fuel (glycine) and the oxidiser (nitrate).

This process is divided into three stages:

• Dissolution of metal nitrates and glycine in water

• Calcination of the end product, after combustion, to burn out the organic material left after combustion

The final product is a clean homogeneous powder of the required stoichiometry [9].

2.1.5 Sol-Gel and Combustion Method

The sol-gel and combustion method is a combination of the traditional sol-gel method and combustion method. In the first part of this method, the solution prepared, called sol, is heated up to form the gel and more heat is supplied in order to induce the combustion. The fuel used in this process is glycine [2, 10-12].

Figure 2.3 is a schematic of all steps of the process used to produce powders. The powders were prepared by a sol-gel and combustion method. Scandia powders (Stanford Materials Corporation, 99.99 % purity) were dissolved in nitric acid (Fisher Scientific, d=1.42, 70 %) and hydrogen peroxide (Fisher Scientific, >30 % wv). The ratio between nitric acid and hydrogen peroxide amount is 4:1. Separately, cerium (IV) nitrate hexahidrate (Aldrich, 99 % purity), yttrium (III) nitrate hexahidrate (Aldrich, 99.9%) and zirconia acetylacetonate (IV) (Aldrich, 98% purity) were dissolved in distilled water. The first solution, scandia in nitric acid, is added to the second solution; after the two solutions were mixed glycine is introduced. The amount of glycine added was twice of the initial molar fraction of the metals. Ammonia is introduced until the pH is approximately 6. When the solution is ready, the temperature was increased until 150 °C and left until the gel is formed. After the gel is formed the temperatures is increased until 270 °C, and wait until the gel burned off.

Figure 2.3: Process for producing powders by Sol-Gel and Combustion Method [12]

2.2 Calcination

Calcination uses temperature and time to purify and homogenise powders or pellets. Materials are exposed to high temperature to remove water, to give physical and chemical stability and absorbent properties of the materials [13].

Figure 2.4 shows the scheme of the thermal cycle used by my initial method and on Figure 2.5 shows the scheme of a thermal cycle that replaces the thermal cycle on Figure 2.4. To calcine, the powders are introduced in the furnace at room temperature. Then the temperature is increased by 2 °C/min, until reaching 1000 °C, where a dwell of 20 hours

is performed. After cooling, the powders are ground and introduced again in the furnace. Starting at room temperature, the furnace is heated by 5 °C/min until reaching 1000 °C, at this temperature a dwell of 3 hour is performed. After the dwell, the powders are removed from the furnace and left in air to cool down and are grinded again, as shown on Figure 2.4.

Figure 2.4: Thermal cycle for calcination of my initial method

After the observation of powders at several temperatures calcination, it was decided to calcine the powders with the thermal cycle as shown on Figure 2.5. The powders are introduced in the furnace at room temperature, the temperature is increased until 400 °C with a ramp rate of 0.5 °C/ min, and a dwell of 3 hours is performed. Powders are removed from the furnace at 400 °C, left on air to cool down at room temperature and ground.

Figure 2.5: Thermal cycle for new calcination

For any of the procedures described above, an X-ray diffraction is performed to confirm that the process produces the material with the desired phase.

2.3 Powder Characterisation