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Controlled synthesis of BGN is critical to their effective use in biomedical applications since BGN characteristics, such as morphology and composition, determining the properties of BGN, are highly related to the synthesis process. In the last decade, numerous investigations focusing on BGN synthesis have been reported. BGN can mainly be produced through the conventional melt-quench approach or by sol-gel methods. The latter approaches are drawing widespread attention, considering the convenience and versatility they offer to tune the properties of BGN.

According to the types of precursors, the sol-gel process can be divided into two types: organic and inorganic.

The organic route is that the sol-gel is prepared by metal alkoxides which can be considered as a two-step inorganic polymerization. The first step is the hydrolysis of alkoxide ligands to yield hydroxylated metal centers [ 7 ]:. The second step is the condensation of the hydroxylated species to form oxypolymers, involving an oxylation reaction which creates oxygen bridges and expels XOH species as follows [ 7 ]:. The macroscopic properties of the material are determined by its microstructure, and the microstructure is determined by the conditions of the material.

Generally speaking, the sintering properties of the powder vary in different preparation methods. While using large surface area and high surface activity of monodisperse ultrafine ceramic powder, due to short diffusion distance and only need lower sintering temperature and activation energy. Factors that influence the properties of final products, acquired by sol-gel technique, include hydrolysis ratio, acidity of the hydrolyzing agent, gelation condition, drying condition and procedures and types of solvent.

The nature of the precursor can significantly affect the sol-gel reaction kinetics and final products. The hydrolysis ratio, as defined by Eq. The lower amount of water added is easy to form the product of low crosslinking, and the viscosity of the sol is increased; the higher amount of water added is easy to form a highly cross-linked product, and the viscosity is decreased. Therefore, the amount of water has an important effect on the structure of the product and the viscosity and gelation time.

The hydrolysis ratio also has a great influence on the hydrolysis and condensation rate. Both acids and bases can catalyze sol-gel reactions. The hydrolysis and condensation reactions can be explained by SN2 nucleophilic substitution reaction mechanism. Take alcohol as an example, it is not easy to hydrolyze silicon alkoxide, and its hydrolysis and polymerization are usually catalyzed by acid or alkali.

The inorganic acid can make the partially negatively charged alkoxy matrix, so that it is easy to break away from the silicon atom. Alkali provides nucleophilic hydroxyl OH in alkali-catalyzed hydrolysis, and Si—OH loss of protons, thereby accelerating polymerization. In the presence of excess water, because of the acid-catalyzed hydrolysis reaction, Si OH can be formed. The rate of polymerization is faster than that of hydrolysis under the condition of alkali catalysis.

Acid or base catalysis is not the only catalyst.

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Some nucleophiles, such as NaF or 2-methylpyridine, also have the ability to improve the reaction rate. Zinc oxide ZnO nanoparticles were prepared and synthesized via sol-gel method, using citric acid as a precursor. The impact of pH on the particle size was investigated. Three different pH 3. The pH value of the sol-gel solution will change when added with acids and alkalis to catalyze the reactions. Experimental results show that the pH value and hydrolysis ratio have a synergistic effect on the morphologies of the products.

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Generally, increasing temperature results in both hydrolysis and condensation rate increase [ 6 , 7 ]. The sol-gel process provides samples with high purity, homogeneity, and structure of easy control. The most important advantages of the sol-gel process in the preparation of functional materials are as follows:. In all stages, the temperatures required are low and close to room temperature. Then, thermal decomposition of organic material and any entrapped species is minimized leading to high purity and stoichiometry [ 2 ]. As the organometallic precursors for different metals are miscible, the homogeneous sol solutions are easily achieved [ 2 ].

Since the sol-gel is initiated by the reaction of the solution, the materials are very uniform and easy to modify, which is crucial for controlling the physical and chemical properties of the material. The chemical conditions are mild in sol-gel process.

2.3.4 Sol–Gel Process

Hydrolysis and condensation are catalyzed by acid or alkali under mild pH conditions [ 2 , 9 ]. Highly porous and nanocrystalline materials can be synthesized by this method [ 2 ]. Colloid particle size and pore size, porosity, and chemistry of the final product can be optimized by chemical treatment of the precursors, controlled rates of hydrolysis, and by condensation [ 2 ]. Incorporating several components in a single step or in two steps [ 2 ].

Production of samples in different physical forms [ 2 , 10 ]. Starting from the same raw material, changing the process can get different products, such as fiber, powder, or film and composite materials. The negative factors of sol-gel processing include high precursor cost, overall high cost, and environmental problems associated with the disposal of large quantities of organic by-products.

A common theme out of the negative comments was wet chemical processing using alkoxides as precursors [ 11 ]. The sol-gel process is more complex in terms of the difficulty in phase control, which is the different chemical and crystal morphology formation at different temperatures. The morphology is relatively simple, generally, spherical particles.

Gelation, drying, and heat treatment take a lot of time to prepare the sample. In the stage of drying and heat treatment, the sample has a great weight loss and residual stress, film prone to cracking and objectively restricting the thickness of thin film and result in film residual porosity. The acrylamide monomer is added in the ratio of to the cation. The mixture at K temperature is heated to polymerize to form a gel at K. The dry gel is crushed into fine powder with a mortar, placed in a muffle furnace with different calcination temperature for 10 h, with the furnace cooling to room temperature to get fine powders of YFeO 3.

The powder samples and a small amount of ethanol were mixed after grinding and were compressed at a molding pressure of about Mpa in the muffle furnace sintering at K and K for 2 h. Silver paste dried at K was used as the electrode. The sample thickness was about 1. The capacitance and loss were measured by Agilent E LCR analyzer and TH capacitance measuring instrument, and the temperature and magnetic field were controlled by ET low-temperature measuring system.

What is sol-gel processing?

The remanent magnetization Mr and coercive field H are 0. The magnetization of h-YFO is very small and the coercive field is much larger, so it is possible to exclude the influence of impurities on the magnetic properties of the samples, which is characterizing the magnetic properties of h-YFO.

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It can be seen from the figure that the magnetization of h-YFO is gradually reduced from 0. It is also shown that the influence of impurities on the magnetic properties is small. From the extension direction of the loop line, o-YFO has a certain antiferromagnetic property. M-H hysteresis curve of o-YFO powder.

The causes of the abnormal YFeO 3 hysteresis loop are produced by the interaction of the same iron ions, the interaction with the external magnetic field, the DM antisymmetric interaction, and the anisotropy effect of the magnetic crystal. The adjacent two iron ions in YFeO 3 is of a slight distortion, rather than a strict spin antiparallelly which reveals overall magnetic anisotropy.

When the external magnetic field is too small to overcome the anisotropic magnetic field of the magnetic material, the magnetic behavior will be different. Huesing, Synthesis of Inorganic Materials , 2nd edition. Wiley-VCH, Weinheim Alain C. H Schmidt, H. Scholze and A. Kaiser, Journal of Non-Crystalline Solids , 63, pp. Water glass solutions are complex mixtures of monomeric and oligomeric silicates such as sodium metasilicate Na 2 SiO 3 , sodium orthosilicate Na 4 SiO 4 , and sodium pyrosilicate Na 6 Si 2 O 7. The sol-gel process starts with metal alkoxides which undergo a series of hydrolysis and condensation reactions to give sols and gels.


A sol derived from metal alkoxides is a dispersion of colloidal polymeric particles in a liquid. The particles have typically an amorphous and porous structure and are often in the range of 2 nm to 0.

When the sol particles aggregate and condense via residual hydroxyl groups, they form a three-dimensional network. The stability and properties of the wet gel depend on the type of precursor and number of metal oxide bonds M-O-M formed.

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  8. Alkoxides are either polymerized neat or in an organic solvent while alkali metal silicates water glass are always polymerized in water. It is assumed that hydrolysis of siloxanes proceeds by bimolecular nucleophilic substitution reactions involving pentacoordinated intermediates or transition states. Electron withdrawing substituents such as -OH and -OSi increase the hydrolysis rate under alkaline conditions whereas electron withdrawing groups decrease the rate. Therefore, the hydrolysis rate increases with the extent of hydrolysis and condensation.