The Study On Cordierite-Spodumene Composite Ceramics For Solar Heat Transmission Pipeline

Utilization of solar energy is an important approach to solving energy crisis and optimizing energy structure.At present,various countries have increased development of solar thermal power generation technology.As the critical component of solar thermal power generation system,heat transmission pipeline not only connects with solar receiver,thermal storage and power generation but it also transports high temperature heat energy,which directly affect generating efficiency and operation stability of the system.However,heat resistant alloy pipelines used in solar thermal power generation have low serving temperature and easy deformation at high temperature nowadays.The alloy pipelines can’t satisfy the operating requirements of high temperature solar heat transmission pipeline,of which severely limited the development of the solar thermal power technology.This study innovatively prepared cordierite-spodumene composite ceramics for solar heat transmission pipeline with excellent performances via pressureless sintering from industrial Al2O3,Suzhou kaolin,Guiguang talc and spodumene.Relationships of composition,preparation technology,microstructure and performance of the composite ceramic pipeline materials were studied.The causes of andalusite and zirconite increasing thermal shock resistance of the composite ceramics were researched,and the mechanism of Y₂O₃ improving the high temperature stability was discussed.Cordierite-spodumene glass ceramic binders with high bonding strength and good physicochemical matching ability to the composite ceramic matrix were fabricated.The gas tightness,mechanical strength,thermal shock resistance and thermal cycling performances of the composite ceramic pipelines after bonding by glass ceramic binders were studied.Ansys Workbench 15.0 software was utilized to build the mathematics physical models of the composite ceramic pipelines.The effects of pipeline dimensions on heat transfer property of the composite ceramic pipeline were analysized.The main results of the study are listed as follows:（1）Dense cordierite-spodumene composite ceramics with high mechanical strength and excellent thermal shock resistance were prepared by industrial Al₂O₃,Suzhou kaolin,Guiguang talc and spodumene as raw materials.Theα-cordierite was in-situ synthesized by industrial Al2O3,Suzhou kaolin and Guiguang talc.Spodumene could dissolve free silicon in liquid phases at high temperature,which reduced viscosity of the liquid phases to promote the densification,resulting in strength of the composite ceramics increasing.Especially,the composite ceramic with 10 wt%spodumene sintered at 1380oC exhibited the best performances,the water absorption,open porosity,bulk density,bending strength and thermal expansion coefficient（from room temperature to 800oC）were 0.49%,0.71%,2.16 g·cm^-3,102.88 MPa and4.24×10^-6oC^-1,respectively.After 30 thermal shock cycles（wind cooling from 1100oC to room temperature）,the submicron-sizedβ-spodumene grains with a grain diameter of 0.2⁰.8μm generated at the grain boundaries.It could reduce thermal expansion coefficient of the ceramic materials and prevent crack growth of the intergranular fracture,which endowed the ceramic sample with 96.29 MPa residual strength and 6.59%strength loss rate after thermal shock.（2）Proper content of andalusite and zirconite adding enhanced bending strength and thermal shock resistance of the cordierite-spodumene composite ceramics.The rod-like mullite grains generated by mullitization of andalusite could prevent the crack growth of intergranular fracture,which improved mechanical strength and thermal shock resistance of the composite ceramics.However,when the additive content of andalusite was more than 5 wt%,too much cristobaite generating by mullitization increased the viscosity of the liquid phases to inhibit the generation ofα-cordierite grains,resulting in the performances of the ceramic materials decreasing.Compared with andalusite additive,zirconite not only increased strength and thermal shock resistance of the cordierite-spodumene composite ceramics but it also decreased the sintering temperature,which was a perfect additive for the composite ceramics.The optimum content of zirconite was 15 wt%,the ceramic sample sintered at 1360oC exhibited the best performances,the water absorption,open porosity,bulk density and bending strength were 0.03%,0.07%,2.31 g·cm^-3 and 115.25 MPa,respectively.After 30 thermal shock cycles（wind cooling from 1100oC to room temperature）,the strength of the sample was increased by 4.10%.Zircon grains diffusely distributing atα-cordierite grain boundaries could induce the deflection and furcation of the cracks to enhance bending strength of composite ceramics.Meanwhile,a small quantity of Zr⁴⁺ions existing in high temperature liquid phase promoted the amorphousβ-spodumene generating after thermal shock.It increased the combining capacity ofα-cordierite grains to improve thermal shock resistance of the composite ceramics.（3）Doping Y2O3 effectively improved density,thermal shock resistance and high temperature creep resistance of the cordierite-spodumene composite ceramics.Y₂O₃accelerated the reaction of MgO,Al2O3 and SiO2 in the raw materials,which decreased the generation temperature of high temperature liquid phases to promote densification of the ceramics.Furthermore,the generation of yttrium silicate increased the combining capacity ofα-cordierite grains,resulting in the improvement of mechanical strength and high temperature stability of the composite ceramics.The optimum content of Y₂O₃ was 7 wt%,the sample sintered at 1340oC exhibited the best performances,the water absorption,open porosity,bulk density and bending strength were 0.04%,0.09%,2.40 g·cm^-3 and 122.62 MPa,respectively.The residual strength of the sample increased to 132.44 MPa after 30 thermal shock cycles（wind cooling from 1100oC to room temperature）,and the creep index of the ceramic material decreased by 30.77%after serving at 1100oC for 500 h.（4）Using industrial Al₂O₃,Suzhou kaolin,Guiguang talc and spodumene were used as raw materials,cordierite-spodumene glass ceramic binders with high bonding strength,good physicochemical matching ability to composite ceramic matrix and excellent thermal shock resistance were fabricated by adding TiO₂ nucleation agent and frit 1 flux.TiO2 induced heterogeneous nucleation of high temperature liquid phases to promote the crystallization ofβ-spodumene solid solution andα-cordierite,which increased strength of the glass ceramics.High temperature flux could reduce softening temperature of the glass ceramics and improve chemicophysics matching between the binder materials and the ceramic matrix.As 1 wt%TiO₂ and 30 wt%frit1 introducing,the shear strength of the glass ceramic binder was 6.46 MPa,of which was higher than the industry standard of binder material with 1 MPa.Moreover,the shear strength of the binder sample increased by 25.70%after 30 thermal shock cycles（wind cooling from 1100oC to room temperature）.（5）Basic on the optimum formula in（3）,cordierite-spodumene composite ceramics for solar heat transmission pipeline were prepared via extrusion molding by adding appropriate plasticizer CMC,lubricant and others.The composite ceramic pipelines sintered at 1340°C had 0.09%water absorption,0.19%open porosity,2.36g·cm^-3 bulk density,29.76 MPa compressive strength and 19.49 MPa bending strength at bonding location.Strength of the composite ceramic pipeline was higher than the standand of SHA pipelines with 10 MPa.Outside surface of the pipelines hadn’t red-ink overflow after placing for 200 h by fluid leakage method,which showed the composite ceramic pipelines with favourable gas tightness.After 30thermal shock cycles（wind cooling from 1100oC to room temperature）,bending strength of the composite ceramic pipelines was increased by 9.78%.Furthermore,the strength of the bonding location enhanced to 21.62 MPa after 500 h thermal cycles from 200°C to 1100°C.It was indicated that the cordierite-spodumene composite ceramic pipelines had outstanding thermal shock resistance and thermal cycling performance.（6）Ansys Workbench 15.0 software was used to build mathematics physical models of cordierite-spodumene composite ceramics for solar heat transmission pipeline,the effect of pipeline dimensions on heat transfer performances of the composite ceramic pipelines was studied.When the pipeline material and heat transfer medium were invariable,wall thickness reducing,diameter enlargement and length shortening improved the heat transfer performances of the composite ceramic pipeline.At the same time,isovolumetric magnifying was benefited to decrease the heat loss and pressure loss,which enhancing heat transfer efficiency and high temperature stability of the pipeline.It provides the theory for the practical application of cordierite-spodumene composite ceramic for solar heat transmission pipeline.