Citric acid-ethylene glycol precursor

YBa2Cu20 powder obtained from the citric acid/ethylene glycol precursor. After annealing in oxygen at 950 °C for 5 h the sample... 

Another potential source of processlble precursors Is the citric acid/ethylene glycol system which has been esiployed previously in the preparation of highly dispersed perovskite, spinel and related complex oxides. This method provides soluble, metal-organic, polymer precursors which have been used for the fabrication of oxide thin films as well as for the production of oxide powders with excellent homogeneity, good stoichiometry control and uniform sizes at relatively low temperatures(13,14). 

An idea to use polybasic hydroxy carboxylic acids in syntheses of oxides goes back to Pechini [3], Evaporating solutions of metal salts in citric acid at presence of ethylene glycol he obtained a polymeric resin as a precursor of target oxides. Then this process was extensively used to manufacture various ceramic oxide powders in several publications [4-8],... 

A variation of the precursor method is that the mixed precursors are polymer complexes. A powder of composition Ceo,i2Zro,8802 has been synthesized at mild temperatures (873 - 1073 K) by a polymer complex solution method using polyvinylalcohol (PVA) and by a polymerizable complex method using ethylene glycol. In these methods, a solution of polyvinylalcohol (PVA) (or ethylene glycol), citric acid, and metal ions are polymerized to form a gel precursor with randomly distributed cations. Heating of these precursors at a mild temperature produces mixed oxides that are compositionally homogenous at an atomic level. 

The precursors are syntliesized by a sol-gel process. This metliod that was developed and accorded to Baythoun et al. [16], consists in dissolving, gelating, drying and activation steps. The salts are dissolved, in either water (hereafter first method), or ethylene glycol as solvent (second metliod) with citric acid. 

To further understand the synthesis mechanism used in this work, which uses gelatin instead of ethylene glycol, it is necessary to review the traditional polymeric precursor method proposed by Pechini (Pechini, 1967). The Pechini method involves the formation of stable metal-chelate complexes with certain alpha-hydroxycarboxyl adds, such as dtric acid, and polyesterification in the presence of a polyhydroxy alcohol, such as ethylene glycol, to form a polymeric resin. The metal cations are homogeneously distributed in the polymeric resin, which is then calcined to yield the desired oxides. The most common materials used as source of cations are nitrate salts since they can be fully removed at low temperatures (400 - 500 °C). The synthesis mechanism of the modified Pechini method used in this work can be explained in three basic steps, as shown in Fig. 2. It stands out by its simplicity and low cost, using only citric acid, gelatin and metal nitrates as reagents. 

Ti(OBut)4, ZrCX i2.8H20 (Polymeric precursor) Mixing with ethylene glycol, Ti(OBut)4, ZrOCl2.8H20 and citric acid, at 110-120 °C Bianco etal, 1998... 

Polymerizable compound route (PCR) based on the use of metal salts such as precursors, citric acid, and ethylene glycol as a chelating agent such as polyesterificatiOTi... 

Usually an a - hydroxy-carboxylic acid, e.g., citric acid, is used to form chelates with the metal cations from the salt precursor. In the presence of a polyhydroxy alcohol, e.g., ethylene glycol, these chelates react with the alcohol to form ester and water as products. When the mixture is heated a polyesterification occurs in the liquid solution creating a homogeneous colloidal solution (sol), in which irais are uniformly distributed in the organic polymer matrix. Upon removal of the solvent, an intermediate resin is formed. Finally, the oxide structure is obtained by removal of organic substances by means of calcination [157]. 

In the present study, synthesis of doped ATLS with general formula Laio-x-zAxSi6-yBy027-x/2-y/2-3z/2 (A = Sr B = Al, Fe x = 0- 3 y = 0- 1.5 z = 0 0.67) has been conducted via mechanical activation (MA) of solid precursors and using polymerized ethylene glycol-citric acid polyester precursors (Pechini (Pe)) route. To find optimal parameters of the synthesis, the effect of the dopant nature and its parent compoimds, milling duration, temperature of MA products annealing on the doped ATLS formation has been studied. Phase, structural and microstructural studies have been carried out to clarify the mechanism of Al and Fe-doped lanthanum silicates formation in the mixtures with different parent compounds. 

Pechini route. The solution of corresponding nitrate salts and tetraethoxysilane in ethanol was mixed with complexation/polymerization agents citric acid (CA), ethylene glycol (EG) and ethylenediamine (ED). The molar ratios of CA EG metal ions (Me) in the solution were 3 15.2 1 when only EG was used as polymerization agents [52]. When both EG and ED were used, the molar ratios of CA EG ED Me were 3.75 11.25 3.75 1. The mixed viscous solution was heated until the formation of a transparent polymeric gel which was then calcined at 50-1200°C. In some cases, the precursors calcined at 600°C were subjected to the mechanical activation for 5 min before further calcination. The composition and synthesis conditions for some samples are presented in Table 2. 

Perovskites were synthesized via Pechini (Pe) method. In this procedure, the aqueous solution of corresponding nitrate salts was mixed with solution of citric acid (CA) in ethylene glycol (EG). Then ethylenediamine (ED) was added dropwise imder continuously stirring at room temperature followed by further stirring for 1 h. Molar ratios of CA EG ED metal ions (Me) were 3.75 11.25 3.75 1. The mixed viseous solution was heated at 80°C for 24 h. The transparent polymeric gel formed was calcined up to 600°C for decomposition of the polymeric precursors. After that the sample was ground and finally calcined at 700 °C for 1 h as well as at 900 °C for 5 h and 1100°C for 3 h. Table 1 presents some characteristics of perovskite materials after calcination at 700 °C. 

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