Y. Q. Ma

Anhui University, Luchow, Anhui Sheng, China

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Publications (73)177.63 Total impact

  • S. Huang · Y.Q. Ma · S.T. Xu
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    ABSTRACT: Well-dispersed uniform cobalt ferrite nanoparticles were synthesized by thermal decomposition of a metal-organic salt in organic solvent with a high boiling point, and characterized by XRD, TEM and detailed magnetic measurements. The moments of CoFe2O4 nanoparticles consist of the core and canted surface moments below 200 K, and the exchange-coupling between the surface and core spins enhanced the remanence (Mr) to saturation (Ms) magnetization ratio (Mr/Ms) at the temperature of 10 K. Interestingly, the anomalous memory effect was observed in a broad temperature range which can be attributed to the relaxation of surface spins below 200 K and the one from the moments of magnetically ordered entity larger than one particle above 200 K.
    No preview · Article · Dec 2015 · Solid State Communications
  • X. Sun · Y.Q. Ma · Y.F. Xu · S.T. Xu · B.Q. Geng · Z.X. Dai · G.H. Zheng
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    ABSTRACT: The dispersive and uniform CoFe2O4 nanoparticles were synthesized by thermal decomposition of metal organic salt in organic solvent with high boiling point, and subsequently reduced in H2 ambient for 1.5, 4 and 8 h to obtain the CoFe2O4/CoFe2 composite. The M(H) loops of all composites were measured at 10, 50, 100, 150, 200, 250, 300 and 390 K, respectively. The jumps around the zero magnetic field in M(H) loops below 200 K signify that no exchange-coupling takes place between soft and hard species. At 10 K, the maximum Hc and Mr/Ms ratio are 16 kOe and 0.81 for the sample reduced for 1.5 h, the much larger value than that previously observed. When the measuring temperature increases, the Hc value monotonously decreases to 1.8, 1.7, 1.2 kOe, and correspondingly the Mr/Ms ratio decreases to 0.55, 0.45 and 0.40 at T = 300 K for samples reduced for 1.5, 4 and 8 h, respectively; these higher values at room temperature has not been obtained previously in CoFe2O4/CoFe2 composites as far as we know. We suggest that it is important to keep high anisotropy and weak dipolar interaction for improving Hc and Mr/Ms and that the Mr/Ms will be further improved if the exchange coupling was really realized between soft and hard magnets. Further investigations are in process.
    No preview · Article · Oct 2015 · Journal of Alloys and Compounds
  • X. Sun · Y.Q. Ma · S.T. Xu · Y.F. Xu · B.Q. Geng
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    ABSTRACT: Abstract Well-dispersed uniform NiFe2O4 nanoparticles (NPs) with an average particle size of 15.4 nm were synthesized by thermal decomposition of a metal-organic salt, and then were diluted in a SiO2 matrix via a sol-gel method with different concentration. The magnetization (M) dependence of NiFe2O4/SiO2 on the temperature (T) and on the applied magnetic field (H) was systematically characterized by the Quantum Design superconducting quantum interference device (SQUID) PPMS system. The results of M ~ H/T divide the magnetic properties between 10 K and 300 K into two regions: the low temperature blocked-particle regime below the blocking temperature TB and the interacting superparamagnetic (ISP) regime above TB. In the ISP regime, all samples deviate from the ideal Langevin superparamagnetic behavior due to the effective anisotropy induced by the stress, surface spins and interparticle dipolar interaction. The Raman spectra indicate that the stress in all samples exhibits the vibration behavior, which leads to the effective anisotropy and hence coercivity vibration.
    No preview · Article · Sep 2015 · Materials Characterization
  • B.Q. Geng · Y.Q. Ma · S.T. Xu · Y.F. Xu · X. Sun · Z.X. Dai · G.H. Zheng
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    ABSTRACT: Well-dispersed uniform CoFe2O4 nanoparticles were synthesized and then some nanoparticles were reduced in a H2/N2 gas mixture followed by oxidation in a pure O2 ambient to prepare CoFe2/oxide composite. The CoFe2O4 nanoparticles exhibited a high H c value of 15,731Oe and an approximately theoretical M r /M s ratio of 0.83 at T=10K as a consequence of the strong surface anisotropy, which have seldom been obtained previously. The CoFe2/oxide composites exhibited the novel phenomena in their temperature derivative curves of the difference between the field cooling (FC) and zero field cooling (ZFC) magnetization, which presented two peaks at a low temperature T 1 and at a high temperature T 2. The low-temperature region below T 1 corresponds to the completely frozen state. In such completely frozen temperature regime, the virgin magnetization curves exhibited an anomalous behavior that the magnetization decreases with increasing the applied field in the high field region due to the enhanced pinning action of disordered moments in oxides on the moments of CoFe2 alloy. These results revealed that the surface anisotropy and dipolar interaction as well as the pinning effect play a crucial rule in the magnetic properties of CoFe2O4 nanoparticles and CoFe2/oxide composite, and further investigations are in process.
    No preview · Article · Sep 2015
  • B.Q. Geng · Y.Q. Ma · Y.F. Xu · S.T. Xu · X. Sun · G.H. Zheng · Z.X. Dai
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    ABSTRACT: In order to clarify the intrinsic reason for the smaller remanence (Mr)-to-saturation (Ms) magnetization ratio Mr/Ms than that expected by the Stoner–Wohlfarth model in CoFe2O4/CoFe2 nanoparticles in the previous report, we first prepared well-dispersed CoFe2O4 nanoparticles, and then they were diluted in the SiO2 matrix followed by reduction in H2 as far as possible to exclude or reduce disadvantageous variables (such as the growth and aggregation of particles and the exchange coupling between soft magnetic particles in the process of reducing) affecting magnetic properties. Such an idea has not been taken into account before to our knowledge. The analyses on the magnetic results indicate that the CoFe2O4/CoFe2 nanoparticles herein reported are a pure dipolar system, in which the coercivity (Hc) and Mr/Ms ratio are very sensitive to the anisotropy and the strength of dipolar interaction. These results signify that it is important to maintain the CoFe2O4/CoFe2 nanoparticles with higher anisotropy and weaker dipolar interaction for improving Mr/Ms and Hc. This suggestion was further confirmed by our another result wherein an Mr/Ms value of 0.64 was obtained even though no exchange coupling was observed in the CoFe2O4/CoFe2 nanoparticles, and further work is in process. Graphical abstract: Numerous efforts have devoted to improve the values of Ms and Mr/Ms by compositing hard CoFe2O4 (CFO) ferrite with soft CoFe2 (CF) alloy, which unfortunately give the low Mr/Ms value (<0.5) even in presence of the exchange coupling. Key issues involve the preparation of CFO/CF composite. Previously the preparation of CFO/CF undergoes the synthesis of CFO and the subsequent reducing in the H2 ambient, as shown in Figure (a), while in this work well dispersed CFO nanoparticles were first prepared , and then diluted in the SiO2 matrix followed by reducing in H2 to exclude or reduce disadvantageous variables, such as the growth and aggregation of particles and the exchange coupling between soft magnetic particles in the process of reducing, as shown in Figure (b). Our results suggest that higher anisotropy and weaker dipolar interaction favor the larger Mr/Ms value, as shown in Figure (c).[InlineMediaObject not available: see fulltext.]
    No preview · Article · Jul 2015 · Journal of Nanoparticle Research
  • S T Xu · Y Q Ma · G H Zheng · Z X Dai
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    ABSTRACT: Well-dispersed uniform cobalt ferrite nanoparticles were synthesized by thermal decomposition of a metal-organic salt in organic solvent with a high boiling point. Some of the nanoparticles were diluted in a SiO2 matrix and then the undiluted and diluted samples were characterized and their magnetic behavior explored. The undiluted and diluted samples exhibited maximum coercivity Hc of 23 817 and 15 056 Oe at 10 K, respectively, which are the highest values reported to date, and the corresponding ratios of remanence (Mr) to saturation (Ms) magnetization (Mr/Ms) were as high as 0.85 and 0.76, respectively. Interestingly, the magnetic properties of the samples changed at 200 K, which was observed in magnetic hysteresis M(H) loops and zero-field cooling curves as well as the temperature dependence of Hc, Mr/Ms, anisotropy, dipolar field, and the magnetic grain size. Below 200 K, both samples have large effective anisotropy, which arises from the surface spins, resulting in large Hc and Mr/Ms. Above 200 K, the effective anisotropy decreases because there is no contribution from surface spins, while the dipolar interaction increases, resulting in small Hc and Mr/Ms. Our results indicate that strong anisotropy and weak dipolar interaction tend to increase Hc and Mr/Ms, and also clarify that the jumps around H = 0 in M(H) loops can be attributed to the reorientation of surface spins. This work exposes the underlying mechanism in nanoscale magnetic systems, which should lead to improved magnetic performance.
    No preview · Article · Mar 2015 · Nanoscale
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    ABSTRACT: By controlling the phase distribution and interfacial conditions, the composites of (soft)Ni0.6Zn0.4Fe2O4/(hard) SrFe12O19 ferrites were prepared with different mass ratios fs of soft magnet. The coercivity mechanism was analyzed according to the theory for melt-spun and sintered magnets. Using the modified Brown׳s equation, we derive the exchange coupling coefficient αex that can be quantificationally expressed as αex=α0 exp(−fs/f0) with α0 and f0 being constants, which has not been reported before as far as we know.
    No preview · Article · Sep 2014 · Materials Letters
  • Y.F. Xu · Y.Q. Ma · S.T. Xu · F.L. Zan · G.H. Zheng · Z.X. Dai
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    ABSTRACT: All samples were synthesized via supercritical (390 degrees C) and low-temperature (190 degrees C) hydrothermal methods followed by annealing at 1000 degrees C, and contained both SrFe12O19 and alpha-Fe2O3 phases as reported in literature. The magnetic properties sensitively depended on the relative content of each phase, which was generally attributed to phase homogeneity and microstructural characteristics in literature as far as we know. In this paper, we observed a double-peak behavior in the curve of irreversible susceptibilities chi(irr) versus an external magnetic field H, which was assigned to the cation vacancies and their distribution. The coercivity of the sample almost changed linearly with the average value of magnetic fields at which the chi(irr) (H) peak located. Therefore, we conclude that the magnetic parameters, at least the coercivity, are mainly determined by cation vacancies which have usually been neglected before in SrFe12O19.
    No preview · Article · Sep 2014 · Materials Research Bulletin
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    ABSTRACT: We investigated the effects of Ag added Sr0.9La0.1TiO3 ceramics prepared via a hydrothermal method and sintering. The results indicate that Ag addition is an efficient way to enhance its thermoelectric properties. The maximum thermoelectric figure-of-merit, ZT, is 0.20 at 955 K for x=0.15 sample.
    No preview · Article · Feb 2014 · Journal of Low Temperature Physics
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    ABSTRACT: Eu3+-doped yttrium borate samples were prepared via a hydrothermal method using borate or tributyl borate as B3+ sources, with and without carbon nanotubes as templates. The morphology of the samples is easily altered by varying the carbon nanotube template, B3+ source and post-annealing treatment. Photoluminescence measurements indicate that the color purity of samples is clearly improved by template-assisted synthesis. Furthermore, with tributyl borate as the B3+ source, CIE coordinates of (0.67, 0.33), very similar to the (0.665, 0.334) coordinates for the commercial red phosphor Y2O2S:Eu3+, can be obtained. Finally, an extra Eu3+-O-2 excitation band is observed for samples made by template-assisted synthesis. The underlying mechanism is discussed in detail. (c) 2013 Elsevier B.V. All rights reserved.
    No preview · Article · Jan 2014 · Journal of Alloys and Compounds
  • L. Zhou · X. Yang · A.L. Feng · H.B. Tang · Z.L. Ding · Y.Q. Ma · M.Z. Wu · S.W. Jin · G. Li
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    ABSTRACT: Chalcopyrite flower-like CuInS2 were successfully synthesized by a one-step solvothermal method and has been applied as a counter electrode for efficient dye-sensitized solar cells. The as-synthesized flower-like CuInS 2 were composed of nanoplates. The prepared CuInS2 powders occupy energy band gaps of 1.38 eV, which is considerably approaching to ideal band gap for optimum solar energy conversion via the photovoltaic effect. The dye-sensitized solar cells with flower-like CuInS2 as a counter electrode can yield 4.5 % photoelectric conversion efficiency.
    No preview · Article · Jan 2014
  • F. L. Zan · Y.Q. Ma · Q. Ma · Y.F. Xu · Z. X. Dai · G. H. Zheng
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    ABSTRACT: Precursor powders, containing mainly CoFe2O4 and Co(OH)(2), have been hydrothermally synthesized and then annealed in either air to obtain a ferrimagnetic CoFe2O4/antiferromagnetic Co3O4 composite, or H-2 ambient to reduce the precursors to Co/Fe alloy. Giant exchange bias (H-E = 2233 Oe) and significant coercivity enhancement (H-C = 10153 Oe) were observed below the Neel temperature of Co3O4 in samples annealed in air, resulting from the exchange coupling between ferrimagnetic CoFe2O4 and antiferromagnetic Co3O4 phases. By reducing precursor powders in H-2 ambient, a solid solution of Fe and Co (Co3Fe7) was obtained; the average atomic moment for Co3Fe7 is 2.4 mu(B), larger than that of separated Fe or Co. We also show that the magnetic parameters can be sensitively tuned by varying processing conditions, which may be useful for obtaining the desired magnetic performance. (c) 2013 Elsevier B.V. All rights reserved.
    No preview · Article · Dec 2013 · Journal of Alloys and Compounds
  • D. Zou · Y.Q. Ma · S.B. Qian · G.H. Zheng · Z.X. Dai · G. Li · M.Z. Wu
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    ABSTRACT: YBO3:Eu3+ phosphors with different morphologies such as flower-like, spherical-like and cake-like have been hydrothermally synthesized without any additives by simply adjusting the B3+ sources, stirring speeds and initial solution pH values. During the hydrothermal reaction, stirring with suitable speed can reduce particle size and obtain more uniform size distribution. The emission intensity can be distinctly enhanced by stirring and post-annealing due to the micro-structural improvement and better crystallization. Interestingly, the weak acidity (pH = 6) in the initial solution or post-annealing enhances the emission in the wavelength range of visible light, and weakens the emission around 700 nm due to weakened distortion of the outside surroundings of Eu3+, which is useful in Plasma Display Panel (PDP) application.
    No preview · Article · Oct 2013 · Journal of Alloys and Compounds
  • Source
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    ABSTRACT: Ruddlesden-Popper (RP) phase (Sr0.9La0.1)3Ti2O7/xAg (x=0, 0.05, 0.10, 0.15) are systematically investigated with regard to their phase composition and thermoelectric transport properties. The XRD results show that all oxide samples are of two phases and they have layered microstructure. The electrical conductivity is found to be increased simultaneously. And the absolute Seebeck coefficient decreases firstly and then increases Ag addition, which leads to (Sr0.9La0.1)3Ti2O7/xAg (x=0.10) sample possesses optimum power factor. The total thermal conductivity lowers with Ag addition. The dimensionless figure of merit, ZT, reaches 0.12 for x=0.10. These results suggest that a proper of metal element Ag addition would be an effective way improving thermoelectric performance of (Sr0.9La0.1)3Ti2O7 system.
    Preview · Article · Jun 2013 · Journal of Low Temperature Physics
  • F.L. Zan · Y.Q. Ma · Q. Ma · G.H. Zheng · Z.X. Dai · M.Z. Wu · G. Li · Z.Q. Sun · X.S. Chen
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    ABSTRACT: CoFe2O4/Co0.7Fe0.3 nanocomposite permanent magnets have been synthesized in one step by a hydrothermal method at reaction temperatures of 80, 120, 140, 160 and 180 °C and characterized by XRD, SQUID, SEM, (HR)TEM, and SAED. All samples consisted of octahedral CoFe2O4 particles and spherical-like Co0.7Fe0.3 particles. The maximum magnetization and coercivity were 191 emu/g and 1311 Oe, respectively, values not previously observed for the CoFe2O4/Co0.7Fe0.3 system. This maximum magnetization was attributable to the larger mass ratio of Co0.7Fe0.3 to CoFe2O4, which were in intimate contact. Magnetic dipolar interaction plays a crucial role in magnetic properties and leads to the reduction of the magnetization and the Mr/Ms ratio. The coercivity of all samples exhibited complex variation with reaction temperature and its mechanism may deserve further investigation.
    No preview · Article · Apr 2013 · ChemInform
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    ABSTRACT: The title nanocomposite permanent magnets are hydrothermally synthesized from a mixture of CoCl2, (NH4)2Fe(SO4)2, sodium citrate, NaOH, and H2O (autoclave, 80—180 °C, 24 h).
    No preview · Article · Apr 2013 · ChemInform
  • G. H. Zheng · Z. X. Dai · Y. Q. Dong · F. L. Zan · D. Zou · Y. Q. Ma · G. Li

    No preview · Article · Nov 2012 · Material Research Innovations
  • G.-H. Zheng · Z.-X. Dai · Y.-Q. Dong · Y.-Q. Ma · G. Li
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    ABSTRACT: The effects of Al doping at Mn sites in manganites La 0·9Te0·1MnO3 have been studied by preparing the series La0·9Te0·1Mn 1-xAlxO3 (0·05≤x≤0·20). All samples can be indexed by rhombohedral lattice with the space group R3̄C. Upon Al doping, both the Curie temperature TC and magnetisation M are suppressed. The resistivity measurements indicate that there exists a weak metal-insulator transition for the sample with x=0·05, with an increase in the doping level, the metal-insulator transition disappears and the resistivity increases. Thermopower S(T) exhibits a maximum at TS, which is close to TC, for all samples. By fitting the S(T) and ρ(T) curves, it is found that the temperature dependences of both S(T) and ρ(T) in the high temperature paramagnetic region follow the variable range hopping mechanism for all samples. Thermal conductivity k(T) values decrease with the increasing Al doping level. The results are discussed according to the destruction of the Mn2+-O-Mn3+ double exchange interaction and the disorder effect due to the Al doping.
    No preview · Article · Nov 2012 · Material Research Innovations
  • G.-H. Zheng · Z.-X. Dai · Y.-Q. Dong · Y.-Q. Ma · G. Li
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    ABSTRACT: The effects of Al doping at Mn sites in manganites La0.9Te0.1MnO3 have been studied by preparing the series La 0.9Te0.1Mn1-xAlxO3 (0.05#x#0.20). All samples can be indexed by rhombohedral lattice with the space group R-3C. Upon Al doping, both the Curie temperature T C and magnetisation M are suppressed. The resistivity measurements indicate that there exists a weak metal-insulator transition for the sample with x50.05, with an increase in the doping level, the metal-insulator transition disappears and the resistivity increases. Thermopower S(T) exhibits a maximum at TS, which is close to T C, for all samples. By fitting the S(T) and ρ(T) curves, it is found that the temperature dependences of both S(T) and ρ (T) in the high temperature paramagnetic region follow the variable range hopping mechanism for all samples. Thermal conductivity k(T) values decrease with the increasing Al doping level. The results are discussed according to the destruction of the Mn2+ -O-Mn3+ double exchange interaction and the disorder effect due to the Al doping.
    No preview · Article · Jan 2012 · Energy Materials Materials Science and Engineering for Energy Systems
  • G.H. Zheng · Z.X. Dai · Y.Q. Dong · F.L. Zan · D. Zou · Y.Q. Ma · G. Li
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    ABSTRACT: N type thermoelectric materials Sr1-xLax TiO 3 (x50?02, 0?06 and 0?10) have been obtained by hydrothermal method and sintering at 900 and 1100°C. The thermoelectric properties have been measured, and all of the samples in the measured temperature range have been found to be insulating. Very low thermal conductivity k50.75 W m-1 K-1 is found at 500 K for Sr0.9 La0.1TiO 3 after sintering at 900°C. With increasing doping level x, the thermal conductivity k decreases. With increasing sintering temperature, the grain size of samples increases, and the thermal conductivity increases. This phenomenon is ascribed to the strong suppression of the electron and phonon conductivities due to scattering by the numerous grain boundary scattering.
    No preview · Article · Jan 2012 · Energy Materials Materials Science and Engineering for Energy Systems

Publication Stats

913 Citations
177.63 Total Impact Points

Institutions

  • 2006-2015
    • Anhui University
      Luchow, Anhui Sheng, China
    • Chinese Academy of Sciences
      • Key Laboratory of Organic Solids
      Peping, Beijing, China
  • 2004-2006
    • Hefei Institute of Physical Sciences, Chinese Academy of Sciences
      Luchow, Anhui Sheng, China
  • 1999-2004
    • Sichuan Agricultural University
      Hua-yang, Sichuan, China