先驱体转化法制备氧化锆毡ZrO2多孔复合材料的力学与热导率性能

1、The thermal conductivity and mechanical properties of highly porous zirconia felt/ZrO2 composites by zirconium oxide precursor impregnationJianping Ai a,b, Guohong Zhou a,*, Zhengjuan Wang a,b, Hailong Zhang a, Xianpeng Qin a,b, Peng Liu a,Shiwei Wang a,*a The State Key Laboratory of High Performanc

2、e Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai , PR Chinab University of Chinese Academy of Sciences, Beijing , PR ChinaAbstract The vacuum pressure impregnation method was adopted to make both low thermal conductivit

3、y and high-temperature resistance porous zirconia felt/ZrO2 composites. Incorporating zirconia felt into porous zirconia matrix to improve the mechanical strength. Meanwhile, lanthanum nitrate as an additive to yttrium-stabilized zirconia to reduce the thermal conductivity of the matrix. The effect

4、of sintering temperature on compressive strength and thermal conductivity were investigated on the basis of porosity, microstructure and composition. The tetragonal phase and the cubic phase co-existed at 1450, while the cubic phase predominated upon 1450 as zirconia ions were replaced by lanthanum

5、and yttrium ions. And the open porosity and size of small pores decreased as the sintering temperature increased from 1450 to 1600 due to the zirconia grains grew up , a decrease of open porosity from 48.1% to 42.5%. Compared with other temperature sintered samples, the 1600-sintered samples always

6、owned higher thermal conductivity value of 0.700.78 W/(m K) at lower temperature, inversely, possessed lower thermal conductivity after 800.1. IntroductionZirconia-based ceramics have high melting point, excellent mechanical strength, low thermal conductivity compared with other ceramics 1-3. It is

7、therefore of interest to consider them as thermal protections. Zirconia-based ceramics show promise as thermal barrier coatings (TBCs) 4-6. TBCs offer advantage in thermal insulating, however, they cannot meet the strength requirement of thermal protections which require not only low thermal conduct

8、ivity but also high strength. Therefore, it is significant to research zirconia-based ceramics as bulk materials which meet both thermal and mechanical requirements.It has been confirmed that thermal conductivity of zirconia-based ceramics is lowered by adding lanthanum oxide to YSZ and introducing

9、pores to form porous ceramics 7-9. Meanwhile, lanthanum oxide added zirconia ceramics show smaller mechanical strength compared with the other oxide added ones 10, and the strength of porous ceramics decreases significantly with increasing porosity 11. There is currently widespread interest in fabri

10、cating porous zirconia ceramics with low thermal conductivity for their applications in bulk thermal protections. The fabrication methods include template methods 12-14, employing pore forming agents 15, 16, impregnation of open-cell polyurethane foams 17, partial sintering 18. However, fabrication

11、of porous ceramics with both high porosity and high strength were rarely reported.In the present study, we employed lanthanum nitrate as an additive to make porous YSZ-based ceramics suitable as thermal insulators. At this time, our innovated zirconia precursor conversion method was adopted to impro

12、ve the mechanical strength by reinforcing porous zirconia matrix with zirconia felt. Incorporating zirconia felt into other porous materials, however, was rarely reported. The added lanthanum ions substituted for zirconium ones and transformed the YSZ ceramic from a tetragonal to a cubic structure h

13、aving lower thermal conductivity. The effect of sintering temperature on compressive strength and thermal conductivity were investigated on the basis of porosity, microstructure and composition.2 Experimental procedure 2.1. Sample preparationCommercially available zirconia felt (Nanjing Yulong new m

14、aterial corporation, Nanjing, China ) was used as starting material. The total porosity and density of this zirconia felt are around 80% and 0.96 g/cm3. XRD pattern was shown in Fig. 1 for zirconia felt, a small amount of monoclinic phase remained in zirconia felt, while the cubic phase predominated

15、. The average diameter of chopped zirconia fiber is around 20 m, as shown in Fig. 2.Fig.1. XRD patterns of raw material about zirconia felt.Fig. 2. SEM micrograph of zirconia felt.The procedure for the preparation of ZrO2 precursor emulsion was as follows: first, inorganic salt ZrOCl2 8H2O (analytic

16、al pure), yttrium nitrate hexahydrate (analytical pure), lanthanum nitrate hexahydrate (analytical pure) were fully dissolved in deionized water by magnetic stirrer, and the mole ratio of atoms La, Y and Zr is 3:5:92. Subsequently, polyethylene glycol (chemical pure) as a dispersant was added to thi

17、s solution. NH4OH (concentration is 2mol/L) as a catalyst was added into the solution while vigorously stirring. Finally, the solution was evaporated a certain content of water while stirring, then was cooled to room temperature. So the preparation of 3La5YSZ precursor emulsion was complete.Fig. 3.

18、A schematic diagram of preparation of porous zirconia felt/ZrO2 composites.The preparation of porous zirconia felt/ZrO2 composites included three stages, as shown in Fig. 3. In the stage 1, 3La5YSZ precursor emulsion mixed with ethanol and polyvinyl alcohol solution to prepare suitable viscosity ZrO

19、2 precursor mixture, and then preforms were infiltrated with ZrO2 precursor mixture in vacuum. After half an hour, impregnation tank was filled N2 to 2 MPa and maintained 2 MPa pressure for 9 h. In the stage 2, the preforms filled with precursor were dried at 6090 for 12 h. In the stage 3, the dried

20、 preforms were pyrolyzed in air at 970 for 2 h. In order to densify the composites, the other eleven impregnation-dry-pyrolysis cycles were repeated. Finally, the preforms of zirconia felt/ZrO2 sintered under the different sintering temperature from 1450 to 1600 for 2 h. The content of zirconia felt

21、 was about 30wt% in the composites.2.2 Characterization The bulk densities and open porosity of the samples were measured according to Archimedes principle with deionized water as immersion medium. The specimens were machined into cube of 10mm10mm10mm to measure the compressive strength. The speed o

22、f the crosshead displacement was 0.2mm/min at room temperature in air. Five specimens were used to determine the average compressive strength.The phase compositions of samples were analyzed by X-ray diffraction (XRD) using a Germany Bruker D8 Focus diffractometer with Cu K radiation (= 0.15418 nm) i

23、n the range of 2=1085. The fracture surfaces of specimens were observed by a scanning electronic microscope (SEM, Mode JSM-6390, Jeol Co., Tokyo, Japan). The thermal diffusivity of porous samples was measured in the range of 100 to 1200 by the laser flash method (LFA427, Netzsch Co., Germany). Typic

24、al dimensions of the disk samples were coated with a thin graphite layer in order to avoid the propagation of the laser radiation through the thickness of the material. The thermal conductivity of porous samples was determined by =cp (1)where is the thermal diffusivity, is the density and cp is the

25、heat capacity. The density was determined by the Archimedes method. The heat capacity of porous samples with a diameter of 5.86 mm and a height of 19.5 mm was measured by high temperature calorimeter instrument (MHTC 96 line, Setaram Co., France) in the range of 100 to 1200，in Ar atmosphere. 3 Resul

26、ts and discussion(a) (b) 3.1 Microstructure and composition(c) Large poreSmall pore(d) Large poreSmall poreFig.4. SEM micrographs of porous zirconia felt/ZrO2 composites sintered at : (a )1450, (b) 1600, (c) and (d) porous zirconia matrix corresponding to (a) and (b).Fig. 4 shows the SEM micrographs

27、 of the porous zirconia felt/ZrO2 specimens sintered at 1450 and 1600. As shown in Fig. 4a and b, the iasolated pores with a size of about 30m are distributed in the zirconia continuous matrix. These large pores, denoted as arrows in Fig. 4a and b, generated by stacking between zirconia particles an

28、d zirconia felt. The size and distribution of large pores relate directly to effectiveness and cycle of impregnation, while little relationship with sintering temperature. The other type pores are small pores relate to sintering temperature with an size range from 1 to 3m formed in the sintered zirc

29、onia matrix. It could be observed from Fig. 4c and d that the well-developed necks between the zirconia particles. And the porosity and average size of small pores decreased as the sintering temperature increased from 1450to 1600 due to the zirconia grains grew up. Remarkablely, a few pull-out and d

30、ebonding zirconia felt are observed on the fracture surface of samples (Fig. 5a and b), which is similar to pullout of continuous carbon fibers may benefit to mechanical properties 19.(a) (b) Pull-out DebondingFig.5. Zirconia felt in the specimen sintered at 1600: (a) pull-out and (b) debondingXRD p

31、atterns are shown in Fig. 6 for porous zirconia felt/ZrO2 composites sintered at various temperatures. The original m-ZrO2 phase existed at all the four sintering temperatures, weak La2Zr2O7 peaks appeared at 1450. The tetragonal phase and the cubic phase co-existed at 1450, while the cubic phase pr

32、edominated upon 1450 as zirconium ions were replaced by lanthanum and yttrium ions.Fig.6. XRD profiles of porous zirconia felt/ZrO2 composites sintered at various temperatures.3.2 Porosity and mechanical strengthFig.7. Effects of sintering temperature on density, open porosity and compressive streng

33、th of porous zirconia felt/ZrO2 composites.The effect of the sintering temperature on density, open porosity and compressive strength of porous zirconia felt/ZrO2 composites is illustrated in Fig. 7. As the sintering temperature increased from 1450 to 1600, the open porosity decreased from 48.1 to 4

34、2.5% due to the process of sintering densification. However, the compressive strength increased from 16.9 to 40 MPa with the increased sintering temperature. The mechanical strength relates directly to the microstructure of zirconia felt/ZrO2 composites. As shown in Fig. 4c and d, with the increase

35、of sintering temperature, adjacent zirconia grains interconnected to form a strong skeleton 3, which led to higher strength of the final products. The compressive strength of porous ceramics also depends on the porosity. A higher sintering temperature led to far less porosity and more densification.

36、 As the porosity increased, the compressive strength decreased remarkably.3.3 Thermal conductivityFig.8. Thermal conductivity (range 1001200) of porous zirconia felt/ZrO2 composites sintered at various temperatures.For porous zirconia ceramics applications in bulk thermal protections, there is neces

37、sary to understand their thermal conductivity. Fig. 8 shows thermal conductivity (range 1001200) of porous zirconia felt/ZrO2 composites sintered at various temperatures. It can be seen that the thermal conductivity of all sintered porous zirconia felt/ZrO2 composites firstly decreased and then incr

38、eased in the range of 100 to 1200. The behavior of thermal conductivity with temperature can divide into two stages. When temperature is not too high, the thermal conductivity of sintered samples decreased with temperature which is characteristic of phonon scattering by defects, impurity and crystal

39、 grain interface 20, 21. When phonon mean wavelength down to size of point defects with temperature increased, phonon scattering effect achieved peak, and then had nothing to do with temperature. However, the gas in porous zirconia felt/ZrO2 composites made an increased contribution to thermal condu

40、ctivity at higher temperature phase due to convection heat transfer among pores. Compared with other temperature sintered samples, the value for the thermal conductivity of 1600-sintered samples always was highest from 100 to 600, but continuous lowest after 800. The thermal conductivity of zirconia

41、-based materials also strongly depends on their microstructure and composition 22. Gas thermal conductivity is lower than solid, so 1600-sintered samples always owned high value for thermal conductivity at lower temperature attribute to highest density. As shown in Fig. 6, the cubic phase predominat

42、ed in 1600-sintered samples, while the tetragonal phase and cubic phase co-existed in 1450-sintered samples. The La-,Y-co-substituted zirconia ceramic from a tetragonal to cubic structure having lower thermal conductivity 7. On the other hand, the porosity and size of small pores decreased as the si

43、ntering temperature increased from 1450to 1600(Fig.4c and d), result in reduction of convection heat transfer among pores. Thus the 1600-sintered samples possessed lowest value for thermal conductivity at higher temperature compared with other temperature sintered samples.4 Conclusions1. Porous zirc

44、onia felt/ZrO2 composites have been prepared by the vacuum pressure impregnation method. As the sintering temperature increased from 1450 to 1600, the open porosity decreased from 48.1 to 42.5% , however, the compressive strength increased from 16.9 to 40.0 MPa . The tetragonal phase and the cubic p

45、hase co-existed at 1450, while the cubic phase predominated upon 1450 as zirconium ions were replaced by lanthanum and yttrium ions.2. The thermal conductivity of sintered samples decreased with temperature when temperature is not too high, however, the gas in porous zirconia felt/ZrO2 composites ma

46、de an increased contribution to thermal conductivity at higher temperature phase due to convection heat transfer among pores. Compared with other temperature sintered samples, the value for the thermal conductivity of 1600-sintered samples always was highest from 100 to 600, but continuous lowest af

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