| Zirconium oxide(ZrO2)and titanium oxide(TiO2)are two very important materials,which have been widely used in catalysis,thermal insulation,chemical industry,aerospace,metallurgy and other fields due to their excellent properties.ZrO2 crystal fibers are the most advanced crystalline fibers material for high temperature insulation.Compared with oxide fibers such as alumina,mullite,quartz,aluminum silicate and non-oxide fibers such as carbon,silicon carbide,and boron nitride,ZrO2 fibers have excellent physicochemical properties owing to the theoretically higher melting point,lower thermal conductivity,lower saturated vapor pressure,better resistance to acid and alkali corrosion,and oxidation resistance.Therefore,ZrO2 fibers can be an excellent thermal insulation and cold insulation material.Our research group has been developing the ZrO2 fibers since 1998.After 20 years of hard work,we have broken through a number of preparation techniques and successfully achieved the development and industrialization of ZrO2 fibers.The ZrO2 fibers have been widely used in space heat insulation tiles,high temperature.resistant crystal furnaces and other high-temperature insulation fields.Meanwhile,the fabricated ZrO2 fibers paper can be used as space materials in the field of high vacuum multi-layer insulation for ultra-low temperature liquid storage.The rutile TiO2 is very popular in the field of infrared heat reflective materials and has been widely used in building insulation,car glass and other heat insulation areas due to its high dielectric constant,strong reflection and scattering ability to light.Meanwhile,TiO2 fibers can also be used as heat-insulating fibers in the field of heat insulation,such as in the field of aerogels and storage of cryogenic liquids.However,we have discovered that the thermal insulation performance needs to be improved during the research process.By analyzing the basic physical processes of heat transfer,we know that there are three heat transfer modes:heat conduction,convection,and heat radiation.The main heat transfer mode of ZrO2 and TiO2 fibers is infrared heat radiation under high temperatures and vacuum environment.How to effectively block infrared heat radiation has become a key issue to continue to improve the thermal insulation performance of ZrO2 and TiO2 fibers.Therefore,the purpose of the dissertation is to improve the anti-infrared heat radiation ability of ZrO2 and TiO2 fibers.From the physical mechanism that affects the infrared heat radiation performance,this paper discusses the influence of surface reflection structure and the intrinsic electronic structure on the infrared reflection.The possibility of improving the infrared thermal radiation resistance of ZrO2 fibers is studied.The structural ordered TiO2 fibers and thin films are prepared according to the influence of lattice orientation on infrared reflection.The main results are as follows:1.Applying the high-reflective films on ZrO2 fibersDue to the low density,the high transmission and low reflection to the thermal radiation,the radiative thermal conductivity of the material is greatly improved.By applying a high-reflective film on the fibers to increase the reflectivity,it is found that a high refractive index film can improve the heat radiation resistance of the fibers.(1)The TiO2 thin film was prepared on ZrO2 fibers by sol-gel pulling method,and nano-TiO2 and CeO2 thin films were constructed on ZrO2 fibers via one-step hydrothermal method.The micro-morphology and structure of the films were characterized by XRD,EDS and SEM,which showed that the high-reflective films could be evenly coated on the surface of the fibers,and the film prepared by hydrothermal method had better uniformity.In addition,the thickness of the film prepared via a hydrothermal method could be controlled by adjusting the hydrothermal temperature and time,and the grain sizes of the prepared TiO2 and CeO2 films were about 9 nm and 18 nm,respectively.(2)The mechanical and thermal stability of TiO2 and CeO2 thin films prepared by hydrothermal method were tested.The results showed that the mechanical stability and thermal stability of the films were good.The films were still stable at high temperatures,and there was no cracking phenomenon.(3)The thermal radiation performance of the film prepared by hydrothermal method was measured using the DRS,FTIR methods.It showed that in the visible-near infrared band,the film on the fibers surface could play a role of high-reflective film,and the reflectivity of the fiber could be effectively improved.In the mid-infrared(2.5-10 μm)wavelength range,the infrared reflectance and scattering ability of the fibers could be increased,making the infrared transmittance of the fiber decreased and the radiation heat transfer reduced.By calculating the effective extinction coefficient of the fiber sample,it could be concluded that the TiO2 and CeO2 films could improve the thermal radiation resistance of the fiber at room temperature and high temperatures.At 1000 ℃,the TiO2 film could increase the average effective extinction coefficient of the fiber by 70%;and at 1200 ℃,the CeO2 film could increase the average effective extinction coefficient of the fiber by 77%.The TiO2 thin film prepared by the sol-gel method could effectively reduce the apparent thermal conductivity of fibers by 68%at 800 ℃.2.Preparation and thermal radiation properties of CYSZ(CeO2-Y2O3-ZrO2)fibersThe electronic structure of the material can affect the infrared reflection.The higher conductivity the material has,the higher reflectivity the material has.In order to improve the electrical conductivity of ZrO2 fibers,higher oxygen ion conductivity and lower conductance activation energy can be obtained by doping with variable-valence Ce ions,and the electrical conductivity of the material can be increased,thereby increasing the thermal radiation resistance of the fibers.(1)YSZ fibers with different Ce doping concentrations(0,5,10,20 mol%)were prepared by sol-gel combined with spin-drying method using polyacetylacetonatozirconium(PAZ),Ce(NO3)·6H2O and Y(NO3)3·6H2O as raw materials.Seen from XRD and Raman spectra,one could know that a ZrO2 based solid solution was formed when Ce was added in the system.With the increase of Ce doping concentration,the lattice parameters became larger and distorted,and there was a tendency to form a cubic phase.Through XPS testing,it is found that Ce3+ and Ce4+ coexisted in the system when Ce is added,and more oxygen vacancies were induced as the Ce content increased.(2)The Ce addition could significantly increase the mechanical strength of the fibers.The added Ce could induce more oxygen vacancies,which would hinder the movement of grain boundaries and inhibit grain growth,leading to grain refinement that could increase the mechanical strength of fibers;when the Ce doping amount was 20%,due to the self-spread burning of NO3-ions,a large number of pores appeared in the fibers,resulting in a decrease in the mechanical strength of the fibers.(3)The added Ce could reduce the heat radiation transmission and thermal conductivity.The more oxygen vacancies produced by Ce doping would cause the fiber to have higher oxygen ion conductivity and lower conductance activation energy,which could increase the conductivity of the material,and therefore increasing the infrared reflection ability and reducing the infrared transmittance of the fibers.In addition,the lattice distortion and grain refinement caused by added Ce could cause phonon-boundary scattering and phonon-vacancy scattering effects,which could reduce the phonon heat transfer and improve the heat insulation effect of the material.3.The growth of structural ordered TiO2 fibers and thin film and the mechanism exploration.For materials,when the crystal structure is relatively perfect,the concentration of grain boundaries,impurities and defects will be small,and then the absorption of infrared light is weaker,so the total reflection is enhanced.In order to obtain TiO2 materials with high orientation crystal structure,structural ordered layered TiO2 fibers and films were prepared.In the process of structure transformation,the structure changed from disordered structure to ordered structure,and the transformation process and transformation mechanism were analyzed and explored.(1)Preparation of structural ordered TiO2 fibers and exploration of transformation mechanism for ordered structure.Precursor fibers were prepared from polytitaniumacetate(PET),and the microstructures of fibers heat-treated at 500℃ and 1550℃ is characterized by SEM and TEM.It was found that the structural ordered TiO2 fibers were obtained after heat treatment at 1550 ℃.The fibers composed of randomly oriented nanocrystals at 500 ℃ were transformed into TiO2 fibers with uniform lattice orientation,and the cross-section of the fibers was a layered structure along the fiber axis.(2)Proposal of a stepwise collapse and nanocrystal unit growth mechanism.The structural change of fiber with intermediate temperature was analyzed and the generation of steps were observed,and these steps were not caused by screw dislocations.Therefore,we proposed a stepwise collapse and nanocrystal unit growth mechanism.Fibers consisted of particles,while small particles consisted of nanocrystals with high specific surface energy and high activity.The nanocrystals became soft and caused landslides after heat treatment,forming a series of steps.During the extension of the steps,the nanocrystals reduced the specific surface energy by orderly rearrangement.The combination of surface and surface was the most stable combination,which would cause the appearance of kink and step,and the nanocrystals were preferentially arranged at these positions.The extension of the steps and the rearrangement of the nanocrystals improved the orientation structure,leading to the formation of the fibers with uniform lattice orientation and layered structure.(3)Growth of structural ordered TiO2 thin films and verification of stepwise collapse mechanism.TiO2 thin films tending to single crystals were obtained by spin coating combined with high pressure vapor method.During the transformation of the ordered structure,the formation of steps was also observed,and due to the step extension caused by landslides and the nanocrystals during step extension,a layered structural ordered TiO2 film was finally formed.It showed that the ordered structural transformation of the film could still be explained by stepwise collapse and nanocrystal unit growth mechanism.During the structural transformation of ZrO2 and BaZrO3 films,the steps caused by stepwise collapse were also observed.Meanwhile,the ZrO2 films were transformed into a layered structure with uniform orientation distribution at 1600 ℃,which indicated the proposed stepwise collapse and nanocrystal unit growth mechanism could still be applied to other films. |
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