The single-phase high-entropy alloy film is difficult to meet severe friction conditions due to its low hardness and high friction coefficient.Nano-composite structure film is composed of at least two separated phases...The single-phase high-entropy alloy film is difficult to meet severe friction conditions due to its low hardness and high friction coefficient.Nano-composite structure film is composed of at least two separated phases,showing the properties of strength and toughness integration and excellent wear resistance.The design of nanocomposite structures can effectively improve the mechanical properties and tribological properties of high-entropy alloy films.In this study,the(CuNiTiNbCr)C_(x) nanocomposite high-entropy films(HEFs)integrated with high hardness,high toughness,and self-lubrication were synthesized by the double-target co-sputtering method.The effect of carbon content on microstructure,mechanical properties,and tribological properties of(CuNiTiNbCr)C_(x) films was studied.With the increase of carbon content in the HEFs,the carbon atoms preferentially react with Ti,Nb,and Cr to form a(TiNbCr)C ceramic-reinforced phase,and then the excess carbon atoms precipitate in the form of amorphous carbon(a-C)lubricating phase in the HEFs.The structure of the HEFs changes from an amorphous structure to a nanocomposite structure of amorphous(amorphous CuNiTiNbCr phase+a-C phase)/nanocrystalline(TiN-bCr)C phase.When the carbon content is about 21.2 at.%,the carbide phase in the film reaches saturation and the hardness and modulus of the films are highest,which are 18 GPa and 228 GPa,respectively.The HEFs with a carbon content of 44.0 at.%show the best toughness and tribological properties with a friction coefficient of 0.16 and a wear rate of 2.4×10^(-6) mm^(3)/(N m),which is mainly attributed to the excellent resistance to fatigue crack growth and the interfacial lubricating layer formed in the friction process.The nanocomposite(CuNiTiNbCr)C_(x) HEFs show very promising application prospect in the field of friction protection.展开更多
YBa2Cu3Ox(YBCO) thin films grown on different substrates with and/or without Eu2CuO4(ECO) buffer layer were investigated by X-ray wide angle diffraction,reflection,diffuse scattering and topography.Theresults show tha...YBa2Cu3Ox(YBCO) thin films grown on different substrates with and/or without Eu2CuO4(ECO) buffer layer were investigated by X-ray wide angle diffraction,reflection,diffuse scattering and topography.Theresults show that for the yttria stabilized ZrO2(YSZ) substrate,the presence of an ECO buffer layer improves the crystalline quality of the YBCO film,while a negative effect is observed for the SrTiO3(STO) substrate.The lateral correlation length for a sample grown on a YSZ substrate with ECO buffer Layer is much greater than grown on an STO subetrate.The STO substrate used has mosaic structure.2001 Elsevier Science B.V.All rights reserved.展开更多
Ca_(3)Ti_(2)O_(7) with Ruddlesden-Popper structure exhibits the largest polarization among the known hybrid improper ferroelectrics.However,the high Curie temperature impedes the thorough study of phase transition thr...Ca_(3)Ti_(2)O_(7) with Ruddlesden-Popper structure exhibits the largest polarization among the known hybrid improper ferroelectrics.However,the high Curie temperature impedes the thorough study of phase transition through dielectric characterization.According to the previous theoretical design rule,the Curie temperature can be suppressed by increasing the tolerance factor.So,in the present work,high-quality Ca_(3-x)LaxTi_(2-x)Al_(x)O_(7)(x=0.0,0.1,0.2,0.3)ceramics with increased tolerance factors were successfully prepared.The amplitude of oxygen octahedral tilt mode indeed decreases with increasing tolerance factors,leading to a degeneration of ferroelectric polarization.However,the unexpected rigid rotation mode causes the composition-invariable coercive fields.The Curie temperatures decrease linearly with increasing x and tolerance factors.The variable-temperature dielectric constant confirms first-order improper ferroelectric transitions in Ca_(3)Ti_(2)O_(7)-based ceramics.The results of variable temperature Xray diffraction reveal the coexistence of two-phases below Curie temperature.The present work confidently confirms the first-order improper ferroelectric transition in Ca_(3)Ti_(2)O_(7)-based ceramics by combining results of variable-temperature dielectric response and in-situ X-ray powder diffraction.展开更多
In the present work,the crystal structure,ferroelectric and dielectric properties of the dense single-phase Li_(2)CaTa_(2)O_(7) ceramics with A-site ordered double-layer Ruddlesden-Popper structures have been investig...In the present work,the crystal structure,ferroelectric and dielectric properties of the dense single-phase Li_(2)CaTa_(2)O_(7) ceramics with A-site ordered double-layer Ruddlesden-Popper structures have been investigated by the experiments and first-principles calculations.A polar Pna21 phase was determined by the Rietveld refinement against the X-ray diffraction pattern at room temperature,and it was confirmed by its lowest calculated energy and rigid phonon modes.The ferroelectricity was found by observing the ferroelectric hysteresis loop at room temperature,and its remanent polarization was similar to the value calculated from the Berry phase and Born effective charge approaches.The ferroelectricity is a nonconventional proper one since its polarization is induced from the displacement of oxygen anions instead of tantalum cations based on the individual atomic polarization.A first-order transition from an antiferroelectric to paraelectric phase around 900 K was found by the DSC and variable-temperature dielectric measurements.From the present work,the room-temperature ferroelectricity is experimental confirmed in the present ceramics,inspiring the search for new ferroelectricity in the ceramics with A-site ordered Ruddlesden-Popper structures.展开更多
Two sets of La_(2)NiMnO_(6)ceramics with different B-site ordering degree were prepared and evaluated.The valence states of Ni^(2+)and Mn^(4+)were con-rmed,and the B-site ordering degree was estimated qualitatively us...Two sets of La_(2)NiMnO_(6)ceramics with different B-site ordering degree were prepared and evaluated.The valence states of Ni^(2+)and Mn^(4+)were con-rmed,and the B-site ordering degree was estimated qualitatively using magnetic and Raman analysis.The ordering of Ni^(2+)and Mn^(4+)was the primary origin for the relaxor-like behavior of the present ceramics.The ordering degree of B-site ions greatly affected the dielectric properties,i.e.,more ordered sample indicated higher dielectric constant.Meanwhile,the grain boundary layer capacitor(GBLC)effect was also activated but limited at the high temperatures and low frequencies.展开更多
In this work,Ba1−xCaxTi_(0.88)Zr_(0.12)O_(3)(x=0.00-0.25)ceramics were prepared by a solid-state reaction method,and the variation of dielectric,ferroelectric and piezoelectric characteristics has been investigated to...In this work,Ba1−xCaxTi_(0.88)Zr_(0.12)O_(3)(x=0.00-0.25)ceramics were prepared by a solid-state reaction method,and the variation of dielectric,ferroelectric and piezoelectric characteristics has been investigated together with the structure evolution.The crystal structure varies from rhombohedral to tetragonal at room-temperature and the morphotropic phase boundary(MPB)is determined around x=0.10,where the significantly enhanced ferroelectric and piezoelectric properties are achieved.With the increase of Ca content,the system gradually evolves into a relaxor ferroelectric.This work provides useful guidance for future research on lead-free piezoelectric materials.展开更多
基金supported by the National Natural Science Foundation of China(Grant No.51975564)the Sichuan Science and Technology Project(Grant No.2021YFSY0050)+1 种基金the State Key Laboratory of Acoustics,Chinese Academy of Sciences(SKLA202214)the Fundamental Research Funds for the Central Universities(2682021CX103).
文摘The single-phase high-entropy alloy film is difficult to meet severe friction conditions due to its low hardness and high friction coefficient.Nano-composite structure film is composed of at least two separated phases,showing the properties of strength and toughness integration and excellent wear resistance.The design of nanocomposite structures can effectively improve the mechanical properties and tribological properties of high-entropy alloy films.In this study,the(CuNiTiNbCr)C_(x) nanocomposite high-entropy films(HEFs)integrated with high hardness,high toughness,and self-lubrication were synthesized by the double-target co-sputtering method.The effect of carbon content on microstructure,mechanical properties,and tribological properties of(CuNiTiNbCr)C_(x) films was studied.With the increase of carbon content in the HEFs,the carbon atoms preferentially react with Ti,Nb,and Cr to form a(TiNbCr)C ceramic-reinforced phase,and then the excess carbon atoms precipitate in the form of amorphous carbon(a-C)lubricating phase in the HEFs.The structure of the HEFs changes from an amorphous structure to a nanocomposite structure of amorphous(amorphous CuNiTiNbCr phase+a-C phase)/nanocrystalline(TiN-bCr)C phase.When the carbon content is about 21.2 at.%,the carbide phase in the film reaches saturation and the hardness and modulus of the films are highest,which are 18 GPa and 228 GPa,respectively.The HEFs with a carbon content of 44.0 at.%show the best toughness and tribological properties with a friction coefficient of 0.16 and a wear rate of 2.4×10^(-6) mm^(3)/(N m),which is mainly attributed to the excellent resistance to fatigue crack growth and the interfacial lubricating layer formed in the friction process.The nanocomposite(CuNiTiNbCr)C_(x) HEFs show very promising application prospect in the field of friction protection.
文摘YBa2Cu3Ox(YBCO) thin films grown on different substrates with and/or without Eu2CuO4(ECO) buffer layer were investigated by X-ray wide angle diffraction,reflection,diffuse scattering and topography.Theresults show that for the yttria stabilized ZrO2(YSZ) substrate,the presence of an ECO buffer layer improves the crystalline quality of the YBCO film,while a negative effect is observed for the SrTiO3(STO) substrate.The lateral correlation length for a sample grown on a YSZ substrate with ECO buffer Layer is much greater than grown on an STO subetrate.The STO substrate used has mosaic structure.2001 Elsevier Science B.V.All rights reserved.
基金financially supported by the National Natural Science Foundation of China under Grant Nos.51772266 and 51790493the National Key R&D Program of China under Grant No.2016YFA0300101.
文摘Ca_(3)Ti_(2)O_(7) with Ruddlesden-Popper structure exhibits the largest polarization among the known hybrid improper ferroelectrics.However,the high Curie temperature impedes the thorough study of phase transition through dielectric characterization.According to the previous theoretical design rule,the Curie temperature can be suppressed by increasing the tolerance factor.So,in the present work,high-quality Ca_(3-x)LaxTi_(2-x)Al_(x)O_(7)(x=0.0,0.1,0.2,0.3)ceramics with increased tolerance factors were successfully prepared.The amplitude of oxygen octahedral tilt mode indeed decreases with increasing tolerance factors,leading to a degeneration of ferroelectric polarization.However,the unexpected rigid rotation mode causes the composition-invariable coercive fields.The Curie temperatures decrease linearly with increasing x and tolerance factors.The variable-temperature dielectric constant confirms first-order improper ferroelectric transitions in Ca_(3)Ti_(2)O_(7)-based ceramics.The results of variable temperature Xray diffraction reveal the coexistence of two-phases below Curie temperature.The present work confidently confirms the first-order improper ferroelectric transition in Ca_(3)Ti_(2)O_(7)-based ceramics by combining results of variable-temperature dielectric response and in-situ X-ray powder diffraction.
基金financially supported by the National Natural Science Foundation of China under Grant Nos.51772266,51790493 and 51961145105the Natural Science Foundation of Zhejiang Province under Grand No.LY20E020012+1 种基金the National Key R&D Program of China under Grant No.2016YFA0300101the Scientific Research Fund of Zhejiang Provincial Education Department under Grant No.Y201941595.
文摘In the present work,the crystal structure,ferroelectric and dielectric properties of the dense single-phase Li_(2)CaTa_(2)O_(7) ceramics with A-site ordered double-layer Ruddlesden-Popper structures have been investigated by the experiments and first-principles calculations.A polar Pna21 phase was determined by the Rietveld refinement against the X-ray diffraction pattern at room temperature,and it was confirmed by its lowest calculated energy and rigid phonon modes.The ferroelectricity was found by observing the ferroelectric hysteresis loop at room temperature,and its remanent polarization was similar to the value calculated from the Berry phase and Born effective charge approaches.The ferroelectricity is a nonconventional proper one since its polarization is induced from the displacement of oxygen anions instead of tantalum cations based on the individual atomic polarization.A first-order transition from an antiferroelectric to paraelectric phase around 900 K was found by the DSC and variable-temperature dielectric measurements.From the present work,the room-temperature ferroelectricity is experimental confirmed in the present ceramics,inspiring the search for new ferroelectricity in the ceramics with A-site ordered Ruddlesden-Popper structures.
基金supported by National Sci-ence Foundation of China under Grant numbers 50832005 and 50702049.
文摘Two sets of La_(2)NiMnO_(6)ceramics with different B-site ordering degree were prepared and evaluated.The valence states of Ni^(2+)and Mn^(4+)were con-rmed,and the B-site ordering degree was estimated qualitatively using magnetic and Raman analysis.The ordering of Ni^(2+)and Mn^(4+)was the primary origin for the relaxor-like behavior of the present ceramics.The ordering degree of B-site ions greatly affected the dielectric properties,i.e.,more ordered sample indicated higher dielectric constant.Meanwhile,the grain boundary layer capacitor(GBLC)effect was also activated but limited at the high temperatures and low frequencies.
基金This work was financially supported by National Natural Science Foundation of China under Grant No.51790493.
文摘In this work,Ba1−xCaxTi_(0.88)Zr_(0.12)O_(3)(x=0.00-0.25)ceramics were prepared by a solid-state reaction method,and the variation of dielectric,ferroelectric and piezoelectric characteristics has been investigated together with the structure evolution.The crystal structure varies from rhombohedral to tetragonal at room-temperature and the morphotropic phase boundary(MPB)is determined around x=0.10,where the significantly enhanced ferroelectric and piezoelectric properties are achieved.With the increase of Ca content,the system gradually evolves into a relaxor ferroelectric.This work provides useful guidance for future research on lead-free piezoelectric materials.
