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Gadolinium zirconeate - For thermal barrier coating

Views: 0     Author: Site Editor     Publish Time: 2022-11-18      Origin: Site

Basic information of gadolinium zirconate

CAS No: 11073-79-3

Molecular formula :Gd2Zr207

Molecular weight :608.98

Properties: White powder, does not dissolve dry water.

Density: 632a/cm3,

Melting point: 2570℃

Insoluble in acid, airtight store.

Purity :99.9% 99.99% 99.999%

Introduction of gadolinium zirconate material

Gadolinium zirconate (Gd2Zr2O7), a rare earth zirconate, is used in thermal barrier coatings, nuclear waste cured substrates, solid oxide battery electrolytes and other fields. In 2004, Vassen et al. first reported the application of rare-earth zirconate in thermal barrier coatings. After this, it was found that rare-earth zirconate has the lowest thermal conductivity of many ceramic materials with lower thermal conductivity than YSZ, while gadolinium zirconate has the lowest thermal conductivity and the highest thermal expansion coefficient of A2B2O7 rare-earth zirconate. Because of its excellent heat insulation performance and high temperature stability. In recent years, there have been many reports about gadolinium zirconate in the field of thermal barrier coating. With the strong demand for new thermal barrier coating, the research of gadolinium zirconate material is in the ascendant.

Structure of gadolinium zirconate

Gadolinium zirconate has the same crystal structure as pyrochlorite, so it is also called pyrochlorite type compound. It can undergo a phase transition at high temperatures, transforming into defective fluorite structure. The temperature of the phase transition of gadolinium zirconate is 1530℃, much higher than that of YSZ (about 1200℃). The crystallographic characteristics of pyroxite and fluorite structures are described in detail in the literature. Both of them are facial-centered cubic structures, in which pyroxite structures belong to Fd3m(227) space group, and defective zonite structures belong to Fm3m(225) space group. FIG. 1 shows the arrangement of anions in the two structures. It can be seen that in the pyrochlorite structure, cations usually occupy the 16d position and can form a cube with 8 O2- coordination. Zr4+ is located at 16c and can form octahedron with 6 O2- coordination. However, in the fluorite structure, O2- has only one crystallographic position and is located at the center of the surrounding cations.

Ordered and disordered transformation of gadolinium zirconate materials

Gadolinium zirconate at low temperature can be regarded as an ordered defective fluorite structure. With the increase of temperature, the disorder of the pyroxite structure increases. After reaching a certain transition temperature, the crystal structure begins to change from order to disorder, and finally forms a disordered defective fluorite structure.

best Gadolinium metal - FUNCMATER

The transition temperature of A2B2O7 type rare-earth zirconate is related to the ionic radius ratio between rare-earth cation and zirconium ion. With the increasing of rare-earth cation radius, the transition temperature increases gradually. According to the literature reports, under the standard atmospheric pressure, the condition of rare earth zirconate forming stable pyroxite structure is as follows :1.46<r(A3+)/r(Zr4+)<1.78; When r(A3+)/r(Zr4+)<1.46, the defective fluorite structure is formed. Through computer simulation, Rushton et al. predicted the order and disorder transition temperatures of various rare earth zirconate materials from the perspective of cluster formation energy. The results are shown in Figure 2. With the increase of the cationic radius of rare earth, the disorder energy and temperature required for the order and disorder transition gradually increased, which was consistent with the aforementioned change law.

Gadolinium zirconate is not only affected by temperature and structure, but also by pressure. Zhang et al. used X-ray diffraction combined with Raman spectrum. The effect of high pressure on the transformation behavior of gadolinium zirconate at room temperature was studied. The results show that when the pressure reaches 15.3GPa, the ordered pyroxite structure begins to transform to the disordered defective fluorite structure, which indicates that a certain pressure will increase the structural disorder and lead to the transformation of the material structure.

Thermal properties of acid materials

Gadolinium zirconate exhibits excellent thermal properties at high temperatures, and the results are shown in Table 1 when compared with common thermal barrier coatings. In fact, there are some differences in the thermal conductivity and thermal expansion coefficient of gadolinium zirconate in different reports, mainly due to the preparation technology and testing conditions. It is caused by different material density, but no matter which experimental scheme is chosen, the thermal conductivity of zirconic acid coating is lower than YSZ, and the phase stability is good at high temperature. This excellent thermal insulation performance is due to the crystal structure of gadolinium zirconate. There is an oxygen vacancy in each molecular unit. The high concentration of oxygen vacancy enhances phonon scattering and reduces the mean free energy of phonons, thus reducing the thermal conductivity of gadolinium zirconate.

Gadolinium zirconate powder preparation

Powder raw material is the basis of thermal spraying to prepare thermal barrier coating. Particle size, composition, morphology, fluidity and agglomeration bonding strength of powder directly determine the performance of thermal barrier coating. Therefore, in order to obtain high performance thermal barrier coating, it is necessary to select the preparation process of powder, and explore the best process parameters in the preparation process, so as to achieve the purpose of controlling the performance of powder.

There are two common methods of powder preparation: solid phase method and liquid phase method. The solid-phase method is the direct reaction of raw materials in the solid-phase state to obtain the required solid compounds. According to its processing technology characteristics can be divided into mechanical crushing method and high temperature solid phase method two categories. Mechanical pulverization method is the raw materials directly pulverized and ground into ultrafine powder with the crusher, and the high temperature solid phase law is the metal salt or metal oxide according to the formula proportion of fully mixed, after grinding and then calcination solid reaction, directly or then grinding to get ultrafine powder. The advantages of this method are low cost, simple process, no agglomeration of the powder, good filling, the disadvantages are slow reaction rate, large particle size of the powder, easy to mix impurities, serious segregation of components, etc.

Liquid phase method refers to the raw material dissolved in solution, through various reactions to obtain the target product method, mainly used for the preparation of oxide or composite oxide ultrafine powder. This method usually requires the selection of one or more suitable soluble metal salts, calculation and preparation of the solution according to the composition of the target material, so that each element in the solution is in ionic or molecular state. At this time, appropriate precipitator is added to the solution or evaporation, sublimation, hydrolysis and other operations are adopted to make the metal ions uniformly precipitate or crystallize out. Finally, the precipitated or crystallized products are decomposed by dehydration or heating to obtain the required powder raw materials. According to the technological differences in the preparation process, the liquid phase method can be divided into the following three kinds.

Precipitation method: refers to the solution contains two or more components (cations), after adding the precipitator, the precipitation reaction to obtain the composition of homogeneous precipitation, calcination to obtain the target powder method. The reaction temperature of precipitation method is lower, the particle size of powder obtained is smaller, the composition, the performance is uniform, suitable for mass production.

Hydrothermal method: in a closed pressure vessel, the original powder dissolved and recrystallized in the environment of high temperature and high pressure, can be prepared with controllable morphology, particle size, grain integrity, compact light powder. In addition, the powder prepared by hydrothermal method does not need to be calcined at high temperature, which avoids the problems of grain growth, impurity introduction and defect formation in the process of calcination, so the prepared powder has higher sintering activity. However, the technology of hydrothermal method is more difficult, which is only suitable for preparing a small amount of powder and difficult to be used for large-scale production.

Sol-gel method: using compounds containing high chemical active components as precursors, they are mixed evenly in the liquid phase state, and hydrolysis, condensation reaction, to get a short time stable sol system. Because the colloidal particles have the tendency to automatically bond larger, after a period of time, the colloidal particles are polymerized, solidified to form gel, and then the obtained gel can be dried and sintered to obtain the target powder. The powder prepared by sol-gel method has small particle size and controllable composition, but the preparation process is long, the raw material cost is high, and it is also difficult to be used in mass production.

The powder produced by solid or liquid phase method has poor fluidity, small particle size, and is difficult to be sent into the central high temperature area of the flame flow, and it is easy to be blown and ablated in the spraying process, so it can not be directly used with thermal spraying. Therefore, the powder needs to be further granulated to meet the size requirements of thermal spraying micron particles through the process of ball milling mixing, spray drying agglomeration and high temperature sintering.