CN1915839A - Method of burning gel of stearic acid for preparing Nano LaCo03 in type of perovskite - Google Patents

Abstract

A method for preparing perovskite nanometer LaCoO ₃ . The perovskite nanometer LaCoO ₃ is prepared by a stearic acid gel combustion method. The specific scheme is: respectively weigh lanthanum nitrate, cobalt nitrate and stearic acid according to the molar ratio of 1:1:9.5～1:1:12.0. Under the condition of heating in an oil bath, the stearic acid is first dissolved, and under constant temperature magnetic stirring, the solid mixture of lanthanum nitrate and cobalt nitrate is dissolved in the molten stearic acid, and the temperature is controlled at 110-120°C to form a gel. The gel was placed in a muffle furnace and burned at 450°C. After burning, the product is taken out and ground, and then calcined in a muffle furnace at 850°C for 3 hours to obtain perovskite LaCoO ₃ with a thickness of 10nm-35nm. The method is fast, simple, convenient, and easy to realize industrialization.

Description

Preparation of Perovskite Nano LaCoO3 by Stearic Acid Gel Combustion

technical field

The invention belongs to the technical field of preparation of perovskite-type nano LaCoO ₃ , in particular to preparing perovskite-type nano-LaCoO ₃ by a stearic acid gel combustion method.

Background technique

In recent years, nanoscience and nanotechnology are booming. The miniaturization of the particle size of nanomaterials produces small size effects, surface effects, interface effects, quantum size effects, and macroscopic quantum tunneling effects that bulk materials do not have, making them useful in magnetism, light, electricity, and biomedicine. It presents a series of properties that conventional materials do not have. Nanomaterials have broad application prospects in the fields of magnetism, electronics, optics, high-density sintering, photocatalysis, sensing, especially biology and medicine.

LaCoO ₃ is a perovskite-type rare earth composite oxide, which can be used as a catalyst for environmental pollution treatment, oxygen sensor, solid fuel cell, etc. Most of its properties depend on the production method of the powder. So a good synthesis method often greatly changes its physical and chemical properties. The nanonization of LaCoO ₃ can make its physical and chemical properties better.

The method of preparing LaCoO ₃ usually adopts high-temperature solid-phase method, chemical co-precipitation method, sol-gel method, hydrothermal synthesis method and low-temperature combustion synthesis method. The samples prepared by the high-temperature solid-phase method have excellent properties, high mechanical strength, good activity and anti-poisoning ability, but the particle size and uniformity of the products prepared by this method are poor, and impurities are easily introduced. Too high will easily cause sintering and agglomeration of the product. The perovskite powder prepared by the chemical co-precipitation method has a high specific surface area and reactivity, but the uniform dispersion of the reactants at the molecular level cannot be achieved. The preparation of perovskite-type rare earth composite oxides by sol-gel method has the following advantages: (1) the reaction temperature is low, and the reaction process is easy to control; (2) the uniformity and purity of the product are high (the uniformity can reach the molecular or atomic level); ( 3) The stoichiometry is accurate, easy to modify, and the range of doping is wide (including the amount and type of doping). However, the time required for preparation is long, and it is not easy to realize industrialization.

Combustion synthesis is proposed relative to self-propagating high-temperature synthesis. In 1988, Patil and others used saturated aqueous solution of metal salt (oxidant) and organic fuel (reductant) as raw materials for the first time, dissolved each raw material in water, and then placed the Pyrex hard glass container containing the solution on a hot plate or muffle In the furnace, heated to 573 ~ 773K, the solution boils, concentrates, smokes, and then catches fire and burns rapidly to obtain foamy loose oxide superfine powder. The basic principle of this process is: the mixture of oxidant and fuel used has exothermic characteristics, and can spontaneously undergo oxidation-reduction reaction under a certain temperature induction, and finally obtain the desired product. The particle size and distribution of the obtained product are closely related to the type of fuel used and synthesis conditions.

The gel-combustion synthesis method is a low-temperature combustion chemical method that organically combines the sol-gel wet chemical synthesis method and the self-propagating combustion synthesis method. It is fast, simple, convenient, and easy to realize industrialization. At present, the organic fuels used in the preparation of LaCoO ₃ and its dopants by low-temperature combustion method are aminoacetic acid and glycine. The resulting product has a larger particle size.

The stearic acid gel combustion method we adopt is based on the low melting point of stearic acid (68-70 ° C), and itself can also be used as a solvent for metal salts, and stearic acid has both a compounding agent and a surfactant. Double function, no or only a small amount of water participates in the synthesis process, thus preventing the hydrolysis and precipitation of metal ions and reducing the agglomeration of products. In addition, the solid particles formed by the impact of a large amount of gas generated during the combustion process can not only make the particle size of the powder smaller, but also prevent the agglomeration of the particles. Therefore, under suitable conditions, nano-powder with uniform particle size distribution and small particle size can be obtained. It is expected to greatly improve various physical and chemical properties of the product.

Contents of the invention

The invention overcomes the shortcomings of large particle size and uneven distribution of _LaCoO3 synthesized by the current gel combustion synthesis method, adopts a new fuel stearic acid, and prepares perovskite-type nano LaCoO by stearic acid gel combustion method ₃ . The optimal conditions for the preparation of perovskite-type nano-LaCoO ₃ from stearic acid are provided.

The present invention adopts following scheme to realize, specifically comprises the following steps: (1) under constant temperature oil bath heating condition, stearic acid is dissolved (2) lanthanum nitrate, the solid mixture of cobalt nitrate is dissolved in molten stearic acid , under constant temperature magnetic stirring, control a certain temperature, and react for a certain period of time to make the reaction system into a gel. (3) After the gel continues to dry (dehydration), the gel is burned at a certain temperature. After burning, the product is taken out and ground, put into a muffle furnace, and calcined at a certain temperature for a certain period of time to obtain a particle size distribution. Uniform LaCoO ₃ powder.

The suitable conditions for preparing perovskite-type nano-LaCoO ₃ by stearic acid gel combustion method are: weighing lanthanum nitrate, cobalt nitrate and stearic acid in a molar ratio of 1:1:9.5 to 1:1:12. Under the condition of heating in an oil bath, the stearic acid is first dissolved, and under constant temperature magnetic stirring, the solid mixture of lanthanum nitrate and cobalt nitrate is dissolved in the molten stearic acid, and the temperature is controlled at 110-120°C to form a gel. The gel was placed in a muffle furnace and burned at 450°C. After combustion, the product is taken out and ground, and then calcined in a muffle furnace at 850°C for 3 hours to obtain a LaCoO ₃ powder of 10nm-35nm.

The molar ratio of lanthanum nitrate, cobalt nitrate and stearic acid affects the particle size, purity and morphology of the powder. When the molar ratios of lanthanum nitrate, cobalt nitrate, and stearic acid are 1:1:9.5 to 1:1:12, LaCoO ₃ powders with a thickness of 10nm-35nm can be prepared under suitable conditions. The experimental results are shown in Table 1. This is because the amount of stearic acid is too low due to the lack of enough complexing agent molecules, the nitrates cannot be fully complexed, the prepared powder particle size distribution is uneven, and the powder is agglomerated. The greater the amount of stearic acid, the smaller the particle size of the resulting product. The reason for the decrease can be considered to be that more gas is generated with the increase in the amount of stearic acid added, which is conducive to finer powder particle size. However, if the amount of stearic acid is too high, there will be more organic residues in the final product. In order to remove the organic matter, it needs to be calcined for a longer time. On the other hand, it will cause waste. Therefore, the molar ratio of lanthanum nitrate, cobalt nitrate and stearic acid should be 1:1:9.5～1:1:12.

Table 1 The relationship between the molar ratio of lanthanum nitrate, cobalt nitrate and stearic acid and the particle size of LaCoO ₃ powder The molar ratio of lanthanum nitrate, cobalt nitrate and stearic acid 6 9 9.5 12 13 Particle Size Distribution and Morphology Irregular in appearance, with reunion 10nm-100nm, spherical, good dispersion 10nm-35nm, spherical, good dispersion 10nm-20nm, spherical, good dispersion 2nm-10nm, spherical, good dispersion

The temperature control in the oil bath is 110～120 ℃, because the reaction in the constant temperature water bath not only takes a long time (generally more than 12 hours) but also the reaction is incomplete. Using an oil bath as the reaction condition, after repeated control experiments, it was found that the constant temperature oil bath (edible soybean salad oil) between 110 and 120 ° C can complete the reaction at a faster speed, and the gelling effect is good.

The combustion temperature is controlled at 450 °C because the gel sample is put into the muffle furnace and the temperature starts to rise. When the temperature reaches 400°C, it ignites and burns rapidly, concentrates, and burns for 4 to 6 minutes. In order to ensure complete combustion, the combustion temperature is controlled at 450°C.

Experiments have found that when the calcination temperature is lower than 850°C, the perovskite-type LaCoO ₃ powder cannot be completely transformed into a perovskite-type LaCoO 3 powder. For example, if it is calcined at 700°C for 3 hours, the obtained powder contains impurity phases La ₂ O ₃ , Co ₂ O ₃ , such as attached figure 1. When the calcination temperature is 850°C, after 3 hours of calcination, the perovskite LaCoO ₃ powder can be obtained. As attached picture 2.

The present invention has following advantage and effect:

The raw materials used in the present invention are easy to obtain and non-toxic. The adopted technology has the characteristics of quickness, simplicity, convenience, and easy realization of industrialization. Due to the use of stearic acid which can generate a larger amount of gas, the particle size distribution of the obtained product is uniform and the particle size distribution is small.

Description of drawings

Figure 1 is when the molar ratio of lanthanum nitrate, cobalt nitrate and stearic acid (n _La(NO3)3 :n _Co(NO3)2 :n _{CH3-(CH2)16-COOH} ) is 1:1:9.8, oil bath Under the temperature control of 115°C, the resulting gel was burned and ground at 450°C and then calcined at 700°C for 3 hours. The X-ray diffraction pattern of the obtained powder LaCoO ₃ shows that there are La ₂ O ₃ , Co ₂ O ₃ Miscellaneous. The marked Δ represents LaCoO ₃ , the marked × represents La ₂ O ₃ , and the marked O represents Co ₂ O ₃ .

Fig. 2 is when the molar ratio of lanthanum nitrate, cobalt nitrate, stearic acid (n _{La (NO3) 3} : n _{Co (NO3) 2} : n _{CH3-(CH2) 16-COOH} ) is 1: 1: 9.8, oil bath Under the temperature control of 115°C, the resulting gel was burned at 450°C, and after grinding, it was calcined at 850°C for 3 hours. The X-ray diffraction pattern of the obtained powder was completely consistent with the standard pattern PDF 40-1279. It is known that the obtained product is calcium Titanite type LaCoO ₃ .

Figure 3 shows that the molar ratio of lanthanum nitrate, cobalt nitrate, and stearic acid is 1:1:10, under temperature control of 115°C, the resulting gel is burned at 450°C, and the burned product is calcined at 850°C for 3 hours Transmission electron microscope image of the obtained powder

Fig. 4 is that the mol ratio of lanthanum nitrate, cobalt nitrate, stearic acid is 1: 1: 11.2. Under the temperature control of 113°C, the generated gel was burned at 450°C, and the burned product was calcined at 850°C for 3 hours to obtain the transmission electron microscope image of the powder

Detailed ways

Embodiment 1

Weigh a certain amount of lanthanum nitrate, cobalt nitrate, and stearic acid respectively so that the molar ratio is 1:1:10. First, under the condition of constant temperature oil bath heating, stearic acid is dissolved, under constant temperature magnetic stirring, the solid mixture of lanthanum nitrate and cobalt nitrate is dissolved in molten stearic acid, the temperature is controlled at 115 ° C, and the reaction time is 3 hours. into a gel. The gel was burned at 450°C. After burning, the product was taken out and ground, put into a muffle furnace, and calcined at 850°C for 3 hours to obtain nano-powder LaCoO ₃ as shown in FIG. 3 . The shape is spherical, the particle size distribution is uniform, and the size is about 10nm-30nm.

Embodiment 2

Weigh a certain amount of lanthanum nitrate, cobalt nitrate, and stearic acid respectively so that the molar ratio is 1:1:11.2. First, under the condition of constant temperature oil bath heating, stearic acid is dissolved, and under constant temperature magnetic stirring, the solid mixture of lanthanum nitrate and cobalt nitrate is dissolved in molten stearic acid, the temperature is controlled at 113 ° C, and the reaction time is 3 hours. turns into a gel. The gel was burned at 450°C. After burning, the product was taken out and ground, put into a muffle furnace, and calcined at 850°C for 3 hours to obtain nano-powder LaCoO ₃ as shown in Figure 4 . The shape is spherical, the particle size distribution is uniform, and the size is about 10nm-20nm.

Claims (2)

1. A method for preparing perovskite-type nano-LaCoO ₃ , which uses a stearic acid gel combustion method to prepare perovskite-type nano-LaCoO ₃ , specifically comprising the following steps: (1) under a constant temperature oil bath heating condition, first Dissolve the stearic acid. (2) Dissolve the solid mixture of lanthanum nitrate and cobalt nitrate in molten stearic acid, under constant temperature magnetic stirring, control the temperature to a certain temperature, and make the reaction system become gel after reacting for a certain period of time. (3) The gel is burned at a certain temperature. After burning, the product is taken out and ground, put into a muffle furnace, and calcined at a certain temperature for a certain period of time to obtain a LaCoO ₃ powder with uniform particle size distribution. It is characterized in that ① the fuel used is stearic acid. ②The suitable conditions for preparation were screened. 2. according to the described stearic acid gel combustion method of claim 1, prepare the method for perovskite type nano-LaCoO ₃ , its suitable preparation condition is: by molar ratio 1: 1: 9.5～1: 1: 12, weigh respectively The amount of lanthanum nitrate, cobalt nitrate, stearic acid. Under the condition of heating in an oil bath, the stearic acid is first dissolved, and under constant temperature magnetic stirring, the solid mixture of lanthanum nitrate and cobalt nitrate is dissolved in the molten stearic acid, and the temperature is controlled at 110-120°C to form a gel. The gel was placed in a muffle furnace and burned at 450°C. After burning, the product is taken out and ground, and then calcined in a muffle furnace at 850°C for 3 hours to obtain spherical perovskite LaCoO ₃ with a thickness of 10nm-35nm.

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