| Solar thermal power generation is an important approach to solve the current energy crisis,and how to improve the efficiency of thermal power generation has attracted much attention.Heat transmission pipeline?HTP?is the kernel component of the power plant,and serves to transmit the absorbed solar heat,of which the power efficiency is highly dependent on the stability of the pipeline.To meet the needs for the 3rd generation of solar thermal power?the heat-transfer medium is high-temperature air?,cordierite-mullite-corundum composite ceramics used for solar heat transmission pipeline were fabricated via pressureless sintering using?-Al2O3,mullite,Suzhou kaolin,talc and industrial alumina as raw materials,which could replace the metal alloy pipe with poor high-temperature resistance and corrosion resistance for the 1st and 2nd generation of solar thermal power.The relationships between batch formula composition,preparation techniques,microstructure and performances of the composite ceramics were investigated by using modern testing technologies including TG-DSC,XRD,SEM,FE-SEM and TEM,etc.The mechanism and methods to improve the density,high-temperature performances and thermal properties of the ceramics with added Sm2O3 were also discussed.Furthermore,simulation analysis of the effects of the composite ceramic pipelines with different structures on temperature field,pressure field and velocity field were implemented by Ansys Workbench software to determine the best pipeline design.Finally,composite ceramic pipelines used for solar thermal power generation were prepared by extrusion,and bonded with the pipeline adhesives of cordierite glassy ceramic,whilst the bonding mechanism was studied.The main research results are listed as follows:?1?According to the requirements of the microstructure and performances of solarheattransmissionpipeline,in-situsynthesizedcordieritebonding mullite-corundum composite ceramics of series A were prepared.The relationships between formula composition?controling the mass ratio of corundum,mullite and in-situ synthesized cordierite?,microstructure,performances,sintering temperature were studied,and the thermal shock resistance mechanism was investigated.Results show that in the sintering temperature of 1340-1500°C,ceramics had good high-temperature performance,while the samples were hard to approach porcelain only when the sintering temperature was>1460°C.Sample A2?the mass ratios of corundum,mullite and cordierite were designed as mass ratio of 40%,40%and 20%?sintered at 1500°C had the optimal performances as the water absorption,apparent porosity,bulk density,bending strength and thermal expansion coefficient were 0.10%,0.37%,3.01 g·cm-3,114.07 MPa and 7.39×10-6°C-1?RT-800°C?,respectively.After30 thermal shock cycles?1100°C-RT?,and the bending strength of sample A2 was123.23MPa after 30 thermal shock cycles?1100°C-RT?,which was increased by8.03%with better thermal shock resistance.The phase compositions of samples before and after thermal shock were corundum,mullite,spinel and?-quartz.The FE-SEM analysis of the samples also revealed the generation of a small amounts of cordierite?1-5 wt%?.As compared with the formulae with higher kaolin and talc content when?-Al2O3 and mullite content in formulae increased,a greater density could be obtained owing to a smaller grain size and larger specific surface area,which became unfavorable for cordierite generation.A large number of well-developed columnar corundum crystals and bulk mullite crystals interwove together to constitute a stable structure in the sample.The mechanism of thermal shock reisitance revealed that the increasing mullite content resulted in enhanced stress field during several thermal shocks and the crack deflection occurred to consume the thermal shock stress energy,thus improving thermal shock resistance of the sample.?2?To improve the density,reduce the sintering temperature and increase the in-situ synthesized cordierite content of cordierite-mullite-corundum composite ceramics,Y2O3,Sm2O3,MnO2 and V2O5,etc.as sintering aids were added to formula A2,and the comparative study of the structure,performances and technology economy showed that the added Sm2O3 presented the optimal improvement for ceramics.Series AS was designed by adding different content of Sm2O3 into formula A2,and the densification mechanism was also discussed.Result indicated that the added Sm2O3 with a range of 0.5-5wt%could significantly decrease the sintering temperature by 20°C-120°C and improved density and bending strength.Sample AS3?adding 3wt%Sm2O3?sintered at 1420°C had the optimal performances as the water absorption,apparent porosity,bulk density and bending strength were 0.03%,0.10%,3.16 g·cm-3 and 123.48MPa.The phase compositions were corundum,mullite,cordierite,spinel,?-quartz and Sm2Si2O7,and in-situ synthesized cordierite content increased to 14.4%.It was suggested that Sm3+existed in glassy phase and grain boundaries.The introduction of Sm2O3 could promote not only the liquid-phase sintering,but also crystal generation at the grain boundaries which reduced the grain boundary migration rate and suppressed grain growth,leading to a denser structure.?3?The functional mechanism of added Sm2O3 on high-temperature performances and thermal properties of cordierite-mullite-corundum composite ceramics had been studied.Results showed that sample AS3 with 3wt%added Sm2O3hadbetterhigh-temperaturecreepresistance,thermalshockresistance,high-temperature stability and lower thermal conductivity as comparied with sample A2.The high-temperature deformation and bending strength of sample AS3 after serving 100h at 1100°C were 0.2mm and 157.74MPa with an increase of 27.75%.Sample AS3 had also the lowest thermal expansion coefficient as 5.96×10-6°C-1 and the best thermal shock resistance.After 30 thermal shock cycles?1100°C-RT?,the bending strength was 147.81MPa with an increase of 19.70%,and bending strength after 100 thermal cycles?200°C-1100°C?was 147.97MPa,which was also increased by 19.83%.The thermal diffusivity,specific heat capacity and thermal conductivity of sample AS3 respectively were 2.86 mm2·s-1,0.75 J·?g·°C?-1and 6.81 W·?m·K?-1.The mechanism of high-temperature creep resistance could be explained that Sm2O3 was enriched at the grain boundaries to generate a new phase,which shew inertness of mass transfer and reduced the diffusion rate for achieving grain refinement,resulting in the reduce creep rate.Excellent high-temperature properties of the composite ceramics will ensure the working safely and effectively of heat transmission pipeline,which were believe to be that?1?during thermal shock,thermal cycling and high-temperature service,the glassy phase inside sample AS3 was distributed evenly to promote micro-crystalline precipitation with foming a pinning effect;?2?the content of cordierite with low thermal expansion coefficient and mullite with high-temperature stability increased,which was beneficial to enhance the strength and thermal stability.The added Sm2O3 would slow down the phonons propagation speed and exacerbate the lattice scattering,declining the thermal conductivity of the composite ceramics,which could effectively inhibit the radiating of composite ceramic pipeline materials and improve the power efficiency.?4?Cordierite-based glass ceramic pipeline adhesives for bonding the heat transmission pipilines of solar thermal power generation were prepared.The effects of batch formula composition and microstructure on bonding behavior,thermal shock resistance,thermal stability and phase composition of the adhesives were analyzed,and the bonding mechanism was studied.Cordierite glass ceramics of series B were produced by using Shuzhou kaolin,talc and industry alumina as the main raw materials and TiO2 as nucleating agent.It was demonstrated that sample B1?adding1wt%TiO2?through the heat treatment of nucleating at 800°C for 2h and then crystallizing at 950°C for 2h obtained the optimal performances and crystallization state.The water absorption,porosity,bulk density,bending strength and thermal expansion coefficient were 0.78%,1.95%,2.50g·cm-3,66.07MPa and 5.20×10-6°C-1.The crystalline phase was only cordierite,and the cordierite crystals were combined tightly with the average particle size of 0.225?m.The high melting point?>1300°C?of the cordierite glass-ceramics was not suitable for the requirement of pipeline connection,and then fluxes were introduced to lower the melting point and prepare pipeline adhesives of series BR with good physical and compatibility with pipeline.It was indicated that adhesive BR34?cordierite glass-ceramic 60wt%,flux BF24240wt%?had the best bonding property with a shear strength of 10.26MPa,which was far exceeded the industrial standard?JC/T 547-2005?of 1MPa.Phase compositions after heat treatment were cordierite,albite and?-cristobalite.Adhesive BR34 also had good thermal shock resistance and thermal cycle property.The shear strength of samples was 8.51MPa after 30 thermal shock cycles,and increased to 26.93MPa with an increase rate of 162.48%after 100 thermal cycles?200°C-1100°C?.During the thermal shock and thermal cycle processes,albite melted to form liquid to fill up rapidly the pores of the composite ceramic pipeline surface.Cordierite crystals precipitated in pores,which effectively improved thermal shock resistance and thermal cycle property.The bonding mechanism analysis showed that alkaline oxides in adhesive shifted to the ceramic pipeline,i.e.the glass of adhesive filled up the pores in ceramic pipeline surface and penetrated inside.Cordierite crystals precipitated in the permeate parts after cooling to form a"riveting",making them tightly combined.?5?In order to provide the optimal design parameters of heat transmission pipeline,ceramic heat transmission pipelines with different diameters?30mm,40mm,50mm?,different shapes?circular,hexagonal,octagonal?and different volume magnification times?2.5,5 times?were designed.Simulation analysis of the temperature field,pressure field and velocity field inside composite ceramic pipelines.Results indicated that when the pipeline material,heat transfer medium and pipeline volume were determined as cordierite-mullite-corundum composite ceramics?AS3?,high-temperature air and 884224 mm3,respectively,the pipeline possessed the highest heat transfer efficiency as it had an appropriate diameter?40mm?and the suitable shape of circular.Furthemore,the heat transfer efficiency of the composite ceramic pipelines was improved with the increased volume.?6?For large-scale industrial production of solar ceramic heat transmission pipeline material.cordierite-mullite-corundum composite ceramic heat transmission pipeline for solar thermal power was prepared by extrusion and heat treatment.Physical properties,air tightness,corrosion resistance,thermal shock resistance and high-temperature stability of ceramic pipeline materials were characterized.Research results showed that the pipelines sintered at 1420°C possessed good performances.The water absorption,porosity,bulk density,compressive strength and bending strength were 0.07%,0.19%,3.02g·cm-3,32.40 MPa and 18.35MPa,respectively.The ceramic heat transmission pipeline material exhibited excellent air tightness,corrosion resistance and thermal stability.The acid or alkali corrosion mass loss was less than0.5%.After 30 thermal shock cycles,the strength had increased by 6.29%,and increased by 12.70%after 100 thermal cycles.It is believed that the cordierite-mullite-corundum composite ceramiccan meet the requiements of heat transmission pipeline in the solar thermal power generation,and the study provides theoretical basis for the industrialization of the composite ceramic heat transmission pipeline materials. |
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