Junguo Li

Mechanism of Sintering YAG/ZrB2 Multiphase Ceramics with Spark Plasma Sintering

Although ZrB2 has some excellent performances, it is difficult to prepare the high relative-density ZrB2 ceramics, which limits the application of ZrB2 materials. To obtain higher relative-density ZrB2 ceramics for increasing the good oxidation resistance, YAG/ZrB2 multiphase ceramics is prepared with the spark plasma sintering, the sintering mechanism of preparing YAG/ZrB2 multiphase ceramic materials is investigated. Al2O3 reacts with Y2O3 to form YAG above 700°C during the sintering process. YAG is filled and melted the space of ZrB2 particles to bind the ZrB2 particles during reaction and sintering process, which increases the density of YAG/ZrB2 multiphase ceramic materials. The YAG/ZrB2 multiphase ceramic materials was successfully prepared under sintering temperature of 1700°C, holding temperature time for 4 min, and sintering pressure of 20 MPa; the density is 5.63 g/cm3, the relative density is 99.78%.

Synthesis and characterization of Al(OH)3-Y(OH)3-ZrB2 composite particles

In order to decrease the oxidization and improve the strength of ZrB2 at high temperature, the surface of ZrB2 was coated with Al(OH)3 and Y(OH)3 to synthesize Al(OH)3-Y(OH)3-ZrB2 composite particles. Through TEM, EPMA and SEM analysis, when pH was about 9, Al(OH)3-Y(OH)3-ZrB2 composite particles were successfully synthesized by co-precipitation methods. The coating layer was about 25 nm, which has compaction and conformability. Dispersibility of coated ZrB2 with Al(OH)3 and Y(OH)3 particles were very good. Synthesis of Al(OH)3-Y(OH)3-ZrB2 composite powder will lay the foundation for preparing high-performance YAG/ZrB2 and Al2O3-YAG/ZrB2 multiphase ceramic materials.

Influence of Ultrasonic on the Dispersibility of ZrB2 Particles

To obtain the better ZrB2/Al(OH)3–Y(OH)3 core-shell composite particles, ZrB2 particles must be adequately dispersed during the coating process. In this article, the dispersibility of ZrB2 particles in the ZrB2 suspension is investigated via the sedimentation method. Through test and analysis, the dispersibility is rapidly increased with prolonging the ultrasonic dispersion time before 10 minutes. After 30 minutes, the sedimentation grads using ultrasonic dispersion basically reach the sedimentation balance, but the grads using mechanical agitating dispersion does not basically change after 5 minutes. The ultrasonic dispersion is one of more effective ways between ultrasonic and mechanical agitating dispersion, the dispersion time for 10 minutes is chosen in the dispersibility of ZrB2 particles via ultrasonic dispersion.

Influence of Sintering Conditions on Densification of YAG—ZrB2 Multi-phase Ceramics with Spark Plasma Sintering

Zirconium diboride (ZrB2) is difficult to prepare as high-density ceramics, which limits its application. To obtain high-density ZrB2 ceramics for improving its high-temperature properties, ZrB 2 particles were coated with Al2O3—Y 2O3 particles to prepare 20%YAG—ZrB2 ceramics with SPS. The optimum sintering condition is a temperature of 1700°C, a pressure of 20 MPa, and a holding time of 4 min. The relative density of prepared 20%YAG—ZrB2 multi-phase ceramics is 97.2% under optimization sintering technology, which indicate that high density YAG—ZrB 2 multi-phase ceramics are successfully prepared.

THERMODYNAMIC AND KINETIC ANALYSES OF SYNTHESIZING ZrB2@Al(OH)3–Y(OH)3 CORE–SHELL COMPOSITE PARTICLES

ZrB2 has some excellent performances, but it is easily oxidized at high temperatures to impact the high-temperature strength, which restricts its applied range. To protect from the oxidization and improve the strength of ZrB2 at high temperature, the surface of ZrB2 particles is coated with the Al(OH)3–Y(OH)3 shell to synthesize ZrB2@Al(OH)3–Y(OH)3 core–shell composite particles. Through the thermodynamic and kinetic analyses of the heterogeneous nucleation and homogeneous nucleation, the concentration product of precursor ion (Y3+ or Al3+) and OH- (Qi) must be greater than the solubility product (Ksp), respectively; the conditions of Y3+ and Al3+ are reached to produce Al(OH)3–Y(OH)3 shell on the ZrB2 surface between the Y3+ line and the AlO2- line. Through TEM and XRD analyses, ZrB2@Al(OH)3–Y(OH)3 core–shell composite particles are successfully synthesized by the co-precipitation method, the shell layer quality is better at pH = 9, which established the foundation for preparing high-performance YAG/ZrB2 and Al2O3–YAG/ZrB2 multiphase ceramic materials.

INTRODUCTION OF COATING TECHNOLOGY OF SUPERFINE PARTICLE SURFACE

With the fast development of new materials investigation, attention is paid to. The performance of superfine powders, which must be modified on the surface to acquire some points. Coating technology of particles is one especial method of surface modification. In this paper, coating methods of particles are classified into solid state, liquid state, and gaseous state, main methods and mechanisms during current time are reviewed, respectively, and some research examples are listed. The choice of diversified coating technologies is decided synthetically based on powder materials, performance of the modified substance, and application of coated powders. In the future, the researches of the core-shell modification mechanism, coated particles with an ordered arrangement coating layer, a new surface active agent, the facilities of suiting surface modification, and the evaluation methodology of the surface coating effect are very exigent and necessary for the preparation and application of superfine powders.

Effect of YAG Content on the Properties of YAG–ZrB2 Ceramics

ZrB2 belongs to a class of ceramics defined ultra-high-temperature ceramics (UHTCs) with extremely high melting temperatures, but ZrB2 ceramics are difficult for densification. To make high relative density and good high-temperature property ZrB2 ceramics, in this article, the effect of YAG content on the property of YAG–ZrB2 multiphase ceramics is investigated. The sintering shrinkage is increasing with the increasing of the YAG content from 10% to 40%. The relative density increases with the increase of the YAG content from 10% to 40%, but the rate of increasing relative density from 10% to 30% is higher than that of 40%. When the content of YAG is higher than 30%, fracture toughness increases a little. The YAG content is more, and the weight gain is less, which shows that the oxidation layer thickness is thinner.

INFLUENCE OF SYNTHESIS CONDITIONS ON Al(OH)3–Y(OH)3/ZrB2 COMPOSITE PARTICLES

Although Zirconium diboride (ZrB2) is a desirable combination with some good properties, it is easily oxidized in the high-temperature air to impact high-temperature properties, which dwindles the applied range. In order to decrease oxidization and improve the high-temperature properties of ZrB2, the surface of ZrB2 is coated with Al(OH)3–Y(OH)3 to synthesize Al(OH)3–Y(OH)3/ZrB2 composite particles. In this paper, the conditions of synthesizing Al(OH)3–Y(OH)3/ZrB2 composite particles by the co-precipitation method are investigated. Al(OH)3–Y(OH)3/ZrB2 composite particles are synthesized under different conditions, but the conditions of synthesizing Al(OH)3–Y(OH)3/ZrB2 composite particles with the better coating quality require pH = 9, the appropriate concentration (Al3+ = 0.017 mol/L, Y3+ = 0.01 mol/L) of composite solution, reaction time of 60 min, titration speed of 0.05 ml/s, using the dispersant in the ZrB2 suspension and the ultrasonic dispersion, respectively.

Effect of Conditions on the Dispersibility of ZrB2 Particles

ZrB2@A1(OH)3‐Y(OH)3 core‐shell composite particles are synthesized by co‐precipitation method for strengthening the antioxidation of ZrB2 at high temperature. To reach better A1(OH)3‐Y(OH)3 composite shell and higher coating ratio on the ZrB2 particles surfaces, ZrB2 particles must be adequately dispersed in the ZrB2 suspension during the coating process. The dispersibility of ZrB2 particles under different conditions is investigated by the sedimentation method. Through test and analysis, the optimum conditions of the dispersibility of ZrB2 particles in the ZrB2 suspension are sedimentating for 15 minutes, ultrasonic dispersion for 10 minutes, the lower ZrB2 concentration, pH=9, the dispersant content for the 2% volume of ZrB2 suspension, and using the polyelectrolyte dispersant, respectively.

Mechanism of Synthesizing Al(OH)3‐Y(OH)3‐ZrB2 Composite Particles

To decrease the oxidation and improve the strength of ZrB2 at high temperatures, the surface of ZrB2 is coated with Al(OH)3 and Y(OH)3, which results in the production of Al(OH)3‐Y(OH)3‐ZrB2 composite particles. The pH value through analysis of theory and calculation must be greater than 8.3 provided that Al(NO3)3 and Y(NO3)3 both reached the saturated concentration of heterogeneous nucleation or homogeneous nucleation to co‐precipitation of Y3+ and Al3+ precursors on the surface of ZrB2 particles. When pH is 9, Al(OH)3‐Y(OH)3‐ZrB2 composite particles are synthesized successfully and the best coating quality with Al(OH)3 and Y(OH)3 by co‐precipitation method, which established the foundation for preparing high‐performance YAG/ZrB2 and Al2O3‐YAG/ZrB2 multiphase ceramic materials.