Increasing the phonon scattering center by adding nanoparticles to thermoelectric materials is an effective method of regulating the thermal conductivity.In this study,a series of Ca3 Co_(4)O_(9)/x wt.%CNTs(x=0,3,5,7,...Increasing the phonon scattering center by adding nanoparticles to thermoelectric materials is an effective method of regulating the thermal conductivity.In this study,a series of Ca3 Co_(4)O_(9)/x wt.%CNTs(x=0,3,5,7,10)polycrystalline ceramic thermoelectric materials by adding carbon nanotubes(CNTs)were prepared with sol-gel method and cold-pressing sintering technology.The results of x-ray diffraction and field emission scanning electron microscopy show that the materials have a single-phase structure with high orientation and sheet like microstructure.The effect of adding carbon nanotubes to the thermoelectric properties of Ca_(3)Co_(4)O_(9)was systematically measured.The test results of thermoelectric properties show that the addition of carbon nanotubes reduces the electrical conductivity and Seebeck coefficient of the material.Nevertheless,the thermal conductivity of the samples with carbon nanotubes addition is lower than that of the samples without carbon nanotubes.At 625 K,the thermal conductivity of Ca3 Co_(4)O_(9)/10 wt.%CNTs sample is reduced to0.408 W·m^(-1)·K^(-1),which is about 73%lower than that of the original sample.When the three parameters are coupled,the figure of merit of Ca_(3)Co_(4)O_(9)/3 wt.%CNTs sample reaches 0.052,which is 29%higher than that of the original sample.This shows that an appropriate amount of carbon nanotubes addition can reduce the thermal conductivity of Ca_(3)Co_(4)O_(9)ceramic samples and improve their thermoelectric properties.展开更多
The tuning of electron and phonon by ion doping is an effective method of improving the performances of thermoelectric materials.A series of lower concentration K-doped Ca_(3-x)K_(x)Co_(4)O_(9)(x=0,0.05,0.10,0.15)poly...The tuning of electron and phonon by ion doping is an effective method of improving the performances of thermoelectric materials.A series of lower concentration K-doped Ca_(3-x)K_(x)Co_(4)O_(9)(x=0,0.05,0.10,0.15)polycrystalline ceramic samples are prepared by combining citrate acid sol-gel method with cold-pressing sintering method.The single-phase compositions and plate-like grain morphologies of all samples are confirmed by x-ray diffraction and field emission scanning electron microscope.The effects of lower concentration K doping on the thermoelectric properties of the material are evaluated systematically at high temperatures(300–1026 K).Low concentration K doping causes electrical conductivity to increase up to 23%with little effect on the Seebeck coefficient.Simultaneously,the thermal conductivity of K-doped sample is lower than that of the undoped sample,and the total thermal conductivity reaches a minimum value of approximately1.30 W·m^(-1)·K^(-1),which may be suppressed mainly by the phonon thermal conduction confinement.The dimensionless figure-of-merit ZT of Ca_(2.95)K_(0.05)Co_(4)O_(9)is close to 0.22 at 1026 K,representing an improvement of about 36%compared with that of Ca_(3)Co_(4)O_(9),suggesting that lower concentration K-doped Ca_(3)Co_(4)O_(9)series materials are promising thermoelectric oxides for high-temperature applications.展开更多
Thermoelectric(TE)performance of Ca_(3)Co_(4)O_(9)(CCO)has been investigated extensively via a doping strategy in the past decades.However,the doping sites of different sublayers in CCO and their contributions to the ...Thermoelectric(TE)performance of Ca_(3)Co_(4)O_(9)(CCO)has been investigated extensively via a doping strategy in the past decades.However,the doping sites of different sublayers in CCO and their contributions to the TE performance remain unrevealed because of its strong correlated electronic system.In this work,Sr and Ti are chosen to realize doping at the[Ca_(2)CoO_(3)]and[CoO_(2)]sublayers in CCO.It was found that figure of merit(ZT)at 957 K of Ti-doped CCO was improved 30% than that of undoped CCO whereas 1 at% Sr doping brought about a 150% increase in ZT as compared to undoped CCO.The significant increase in electronic conductivity and the Seebeck coefficient are attributed to the enhanced carrier concentration and spin-entropy of Co^(4+) originating from the Sr doping effects in[Ca_(2)CoO_(3)]sublayer,which are evidenced by the scanning electron microscope(SEM),Raman,Hall,and X-ray photoelectron spectroscopy(XPS)analysis.Furthermore,the reduced thermal conductivity is attributed to the improved phonon scattering from heavier Sr doped Ca site in[Ca_(2)CoO_(3)]sublayer.Our findings demonstrate that doping at Ca sites of[Ca_(2)CoO_(3)]layer is a feasible pathway to boost TE performance of CCO material through promoting the electronic conductivity and the Seebeck coefficient,and reducing the thermal conductivity simultaneously.This work provides a deep understanding of the current limited ZT enhancement on CCO material and provides an approach to enhance the TE performance of other layered structure materials.展开更多
Secondary phase Bi_(2)O_(3),which could promote the sintering densification of ceramics,was used to prepare(Ca_(0.85)Ag_(0.1)La_(0.05))_(3)Co_(4)O_(9) thermoelectric ceramics.The mechanism of liquidphase sintering pr...Secondary phase Bi_(2)O_(3),which could promote the sintering densification of ceramics,was used to prepare(Ca_(0.85)Ag_(0.1)La_(0.05))_(3)Co_(4)O_(9) thermoelectric ceramics.The mechanism of liquidphase sintering process revealed that the diffusion rate of particles originated from the dissolvedeprecipitated processing for samples and suppressed the coarsening of grain growth.The effects of Bi_(2)O_(3) on the microstructure and thermoelectric properties were investigated.The results showed that the relative density of samples increased from 92.1% to 95.5% through the liquidphase sintering mechanism.The band gaps were tuned and it had the profound impact on the transport of charge carriers.Electrical resistivity decreased while Seebeck coefficient increased from 110 mV/K to 190 mV/K with increasing Bi_(2)O_(3).Furthermore,the peak ZT value of 0.25 for 6 wt% Bi_(2)O_(3) sample at 1073 K was obtained,resulting from low thermal conductivity of 0.92 W/(m·K).It suggests that Bi_(2)O_(3) additive can dramatically improve the thermoelectric properties of Ca_(3)Co_(4)O_(9).展开更多
Highly textured dense Ca3-xYxCo4 O9+δ(0≤x≤0.3)samples were fabricated by combining sol-gel process with spark plasma sintering(SPS).Y^3+substitution for Ca^2+simultaneously increased the Seebeck coefficient and red...Highly textured dense Ca3-xYxCo4 O9+δ(0≤x≤0.3)samples were fabricated by combining sol-gel process with spark plasma sintering(SPS).Y^3+substitution for Ca^2+simultaneously increased the Seebeck coefficient and reduced the thermal conductivity.The latter was attributed to the increase in lattice anharmonicity,structural distortion,and grain boundary area,which enhanced the phonon scattering.Ca2.7Y0.3Co4 O9+δshowed the largest dimensionless figure-of-merit(ZT=0.194)at 1073 K because it had the largest Seebeck coefficient and the lowest thermal conductivity.This ZT value was 55%larger than that of undoped Ca3 Co4 O9(0.125 at 1073 K).Therefore,Y^3+substitution,sol-gel powder synthesis,and SPS are highly effective for enhancing the thermoelectric prope rties of Ca_(3)Co_(4)O_(9).展开更多
基金supported by the National Natural Science Foundation of China(Grant No.51836009)。
文摘Increasing the phonon scattering center by adding nanoparticles to thermoelectric materials is an effective method of regulating the thermal conductivity.In this study,a series of Ca3 Co_(4)O_(9)/x wt.%CNTs(x=0,3,5,7,10)polycrystalline ceramic thermoelectric materials by adding carbon nanotubes(CNTs)were prepared with sol-gel method and cold-pressing sintering technology.The results of x-ray diffraction and field emission scanning electron microscopy show that the materials have a single-phase structure with high orientation and sheet like microstructure.The effect of adding carbon nanotubes to the thermoelectric properties of Ca_(3)Co_(4)O_(9)was systematically measured.The test results of thermoelectric properties show that the addition of carbon nanotubes reduces the electrical conductivity and Seebeck coefficient of the material.Nevertheless,the thermal conductivity of the samples with carbon nanotubes addition is lower than that of the samples without carbon nanotubes.At 625 K,the thermal conductivity of Ca3 Co_(4)O_(9)/10 wt.%CNTs sample is reduced to0.408 W·m^(-1)·K^(-1),which is about 73%lower than that of the original sample.When the three parameters are coupled,the figure of merit of Ca_(3)Co_(4)O_(9)/3 wt.%CNTs sample reaches 0.052,which is 29%higher than that of the original sample.This shows that an appropriate amount of carbon nanotubes addition can reduce the thermal conductivity of Ca_(3)Co_(4)O_(9)ceramic samples and improve their thermoelectric properties.
基金Project supported by the National Key R&D Program of China(Grant No.2016YFB0601101)the National Natural Science Foundation of China(Grant No.51476173)
文摘The tuning of electron and phonon by ion doping is an effective method of improving the performances of thermoelectric materials.A series of lower concentration K-doped Ca_(3-x)K_(x)Co_(4)O_(9)(x=0,0.05,0.10,0.15)polycrystalline ceramic samples are prepared by combining citrate acid sol-gel method with cold-pressing sintering method.The single-phase compositions and plate-like grain morphologies of all samples are confirmed by x-ray diffraction and field emission scanning electron microscope.The effects of lower concentration K doping on the thermoelectric properties of the material are evaluated systematically at high temperatures(300–1026 K).Low concentration K doping causes electrical conductivity to increase up to 23%with little effect on the Seebeck coefficient.Simultaneously,the thermal conductivity of K-doped sample is lower than that of the undoped sample,and the total thermal conductivity reaches a minimum value of approximately1.30 W·m^(-1)·K^(-1),which may be suppressed mainly by the phonon thermal conduction confinement.The dimensionless figure-of-merit ZT of Ca_(2.95)K_(0.05)Co_(4)O_(9)is close to 0.22 at 1026 K,representing an improvement of about 36%compared with that of Ca_(3)Co_(4)O_(9),suggesting that lower concentration K-doped Ca_(3)Co_(4)O_(9)series materials are promising thermoelectric oxides for high-temperature applications.
基金financially supported by the National Natural Science Foundation of China(Grant No.51802181)the Natural Science Foundation of Shaanxi Province(Grant No.2019JQ-771)the Foundation of Shaanxi University of Science&Technology(Grant No.2017GBJ-03).
文摘Thermoelectric(TE)performance of Ca_(3)Co_(4)O_(9)(CCO)has been investigated extensively via a doping strategy in the past decades.However,the doping sites of different sublayers in CCO and their contributions to the TE performance remain unrevealed because of its strong correlated electronic system.In this work,Sr and Ti are chosen to realize doping at the[Ca_(2)CoO_(3)]and[CoO_(2)]sublayers in CCO.It was found that figure of merit(ZT)at 957 K of Ti-doped CCO was improved 30% than that of undoped CCO whereas 1 at% Sr doping brought about a 150% increase in ZT as compared to undoped CCO.The significant increase in electronic conductivity and the Seebeck coefficient are attributed to the enhanced carrier concentration and spin-entropy of Co^(4+) originating from the Sr doping effects in[Ca_(2)CoO_(3)]sublayer,which are evidenced by the scanning electron microscope(SEM),Raman,Hall,and X-ray photoelectron spectroscopy(XPS)analysis.Furthermore,the reduced thermal conductivity is attributed to the improved phonon scattering from heavier Sr doped Ca site in[Ca_(2)CoO_(3)]sublayer.Our findings demonstrate that doping at Ca sites of[Ca_(2)CoO_(3)]layer is a feasible pathway to boost TE performance of CCO material through promoting the electronic conductivity and the Seebeck coefficient,and reducing the thermal conductivity simultaneously.This work provides a deep understanding of the current limited ZT enhancement on CCO material and provides an approach to enhance the TE performance of other layered structure materials.
基金supported by the National Natural Science Foundation of China(No.51672219,5171101743,51702259)Excellent Youth Fund of the National Natural Science Foundation of China(No.11722219)+3 种基金Foundation of National High Technology Research and Development Program(No.2015AA0172)Fundamental Research Fund for the Central Universities(No.G2016KY0302)the“111”Project(No.B08040)the Research Foundation of the State Key Laboratory of Solidification Processing(NWPU),China(No.137-QP-2015).
文摘Secondary phase Bi_(2)O_(3),which could promote the sintering densification of ceramics,was used to prepare(Ca_(0.85)Ag_(0.1)La_(0.05))_(3)Co_(4)O_(9) thermoelectric ceramics.The mechanism of liquidphase sintering process revealed that the diffusion rate of particles originated from the dissolvedeprecipitated processing for samples and suppressed the coarsening of grain growth.The effects of Bi_(2)O_(3) on the microstructure and thermoelectric properties were investigated.The results showed that the relative density of samples increased from 92.1% to 95.5% through the liquidphase sintering mechanism.The band gaps were tuned and it had the profound impact on the transport of charge carriers.Electrical resistivity decreased while Seebeck coefficient increased from 110 mV/K to 190 mV/K with increasing Bi_(2)O_(3).Furthermore,the peak ZT value of 0.25 for 6 wt% Bi_(2)O_(3) sample at 1073 K was obtained,resulting from low thermal conductivity of 0.92 W/(m·K).It suggests that Bi_(2)O_(3) additive can dramatically improve the thermoelectric properties of Ca_(3)Co_(4)O_(9).
基金This research was supported by the Basic Science Research Program through the National Research Foundation of Korea(NRF)funded by the Ministry of Education(No.2017R1D1A1B03031196)。
文摘Highly textured dense Ca3-xYxCo4 O9+δ(0≤x≤0.3)samples were fabricated by combining sol-gel process with spark plasma sintering(SPS).Y^3+substitution for Ca^2+simultaneously increased the Seebeck coefficient and reduced the thermal conductivity.The latter was attributed to the increase in lattice anharmonicity,structural distortion,and grain boundary area,which enhanced the phonon scattering.Ca2.7Y0.3Co4 O9+δshowed the largest dimensionless figure-of-merit(ZT=0.194)at 1073 K because it had the largest Seebeck coefficient and the lowest thermal conductivity.This ZT value was 55%larger than that of undoped Ca3 Co4 O9(0.125 at 1073 K).Therefore,Y^3+substitution,sol-gel powder synthesis,and SPS are highly effective for enhancing the thermoelectric prope rties of Ca_(3)Co_(4)O_(9).
