Normal-state resistivity anisotropy in underdoped RBa_2Cu_3O_{6+x} crystals

Ferroelectric and piezoelectric properties of tungsten substituted SrBi 2Ta 2O 9ferroelectric ceramicsIndrani Coondoo *,S.K.Agarwal a ,A.K.Jha ba Superconductivity and Cryogenics Division,National Physical Laboratory,Dr K.S.Krishnan Road,New Delhi 110012,India bDepartment of Applied Physics,Delhi College of Engineering,Bawana Road,Delhi 110042,India1.IntroductionDefects in crystals significantly influence physical and various other properties of materials [1].For instance,as it is well known,doping by other elements leads to significant changes in the electrical properties of silicon.Historically,‘‘defect engineering’’has been developed in the field of semiconducting materials such as compound semiconductors as well as in diamond,Si and Ge [2–4].Subsequently,the concept of defect engineering has been applied to other functional materials,and the significant improve-ment in material properties have been achieved in high transition-temperature superconductors [5],amorphous SiO 2[6],photonic crystals [7]and also in the field of ferroelectrics,such as BaTiO 3,Pb(Ti,Zr)O 3(PZT),etc.[8,9].Various structural and electrical properties of bismuth layer-structured ferroelectrics (BLSF)are also strongly affected on deviation from stoichiometric composi-tions and defects have been recognized as a crucially important factor [10–13].It has been found that in BLSF small changes in chemical composition result in significantly altered dielectric and ferroelectric properties including dielectric constant and remanent polarization.In SrBi 2Ta 2O 9(SBT)and SrBi 2Nb 2O 9(SBN),orthor-hombic structural distortions with non-centrosymmetric spacegroup A 21am cause spontaneous ferroelectric polarization (P s )along a axis [14,15].SBT,a member of the BLSF family,has occupied an important position among the Pb-free ferroelectric memory materials [16–18].Tungsten (W 6+)has recently been investigated as a dopant for bismuth titanates and lanthanum doped bismuth titanates,in which the remanent polarization was observed to enhance when a small amount of Ti 4+was substituted by W 6+[19,20].With the objective to improve structural,dielectric and ferroelectric proper-ties,the hexavalent tungsten (W 6+)was chosen as a donor cation for partial replacement of the pentavalent tantalum (Ta 5+)SBT.In this report,the effect of tungsten substitution in SBT (SBTW),on the microstructural,ferroelectric and piezoelectric properties is reported.The results including the improvement in polarization properties have been discussed.2.ExperimentalSamples of compositions SrBi 2(W x Ta 1Àx )2O 9(SBWT),with x =0.0,0.025,0.050,0.075,0.10and 0.20were synthesized by solid-state reaction method taking SrCO 3,Bi 2O 3,Ta 2O 5and WO 3(all from Aldrich)in their stoichiometric proportions.The powder mixtures were thoroughly ground and passed through sieve of appropriate size and then calcined at 9008C in air for 2h.The calcined mixtures were ground and admixed with about 1–1.5wt%polyvinyl alcohol (Aldrich)as a binder and then pressed at $300MPa into disk shaped pellets.The pellets were sintered at 12008C for 2h in air.Materials Research Bulletin 44(2009)1288–1292A R T I C L E I N F O Article history:Received 3October 2008Received in revised form 5December 2008Accepted 6January 2009Available online 15January 2009Keywords:A.CeramicsC.X-ray diffractionD.FerroelectricityA B S T R A C TTungsten substituted samples of compositions SrBi 2(W x Ta 1Àx )2O 9(x =0.0,0.025,0.050,0.075,0.10and 0.20)were synthesized by solid-state reaction method and studied for their microstructural,electrical conductivity,ferroelectric and piezoelectric properties.The X-ray diffractograms confirm the formation of single phase layered perovskite structure in the samples with x up to 0.05.The temperaturedependence of dc conductivity vis-a`-vis tungsten content shows a decrease in conductivity,which is attributed to the suppression of oxygen vacancies.The ferroelectric and piezoelectric studies of the W-substituted SBT ceramics show that the remanent polarization and d 33values increases with increasing concentration of tungsten up to x 0.05.Such compositions with low conductivity and high P r values should be excellent materials for highly stable ferroelectric memory devices.ß2009Elsevier Ltd.All rights reserved.*Corresponding author.Present address:Liquid Crystal Group,National Physical Laboratory,Dr K.S.Krishnan Road,New Delhi 110012,India.Tel.:+919810361727;fax:+911125170387.E-mail address:indrani_coondoo@ (I.Coondoo).Contents lists available at ScienceDirectMaterials Research Bulletinj o ur n a l h o m e p a g e :w w w.e l se v i e r.c om /l oc a t e /m a t r e sb u0025-5408/$–see front matter ß2009Elsevier Ltd.All rights reserved.doi:10.1016/j.materresbull.2009.01.001X-ray diffractograms of the sintered samples were recorded using a Bruker diffractometer in the range 108 2u 708with CuK a radiation.The sintered pellets were polished to a thickness of 1mm and coated with silver paste on both sides for use as electrodes and cured at 5508C for half an hour.Electrical conductivity was performed using Keithley’s 6517A Electrometer.The polarization–electric field (P –E )hysteresis measurements were done at room temperature using an automatic P –E loop tracer based on Sawyer–Tower circuit.Piezoelectric charge co-efficient d 33was measured using a Berlincourt d 33meter after poling the samples in silicone–oil bath at 2008C for half an hour under a dc electric field of 60–70kV/cm.3.Results and discussion3.1.Structural and micro-structural studiesThe phase formation and crystal structure of the ceramics were examined by X-ray diffraction (XRD),which is shown in Fig.1.The XRD patterns of the samples show the characteristic peaks of SBT.The peaks have been indexed with the help of a computer program–POWDIN [21]and the refined lattice parameters are given in Table 1.It is observed that a single phase layered perovskite structure is maintained in the range 0.0 x 0.05.Owing to the same co-ordination number i.e.6and the smallerionic radius of W (0.60A˚)in comparison to Ta (0.64A ˚),there is a high possibility of tungsten occupying the tantalum site.The observance of unidentified peak of very low intensity in the compositions with x >0.05indicates the solubility limit of W concentration in SBT.The unidentified peak is possibly due to tungsten not occupying the Ta sites in the structure as the intensity of this peak is observed to increase with tungsten content.Composition and sintering temperature influences the micro-structure such as grain growth and densification of the specimen,which in turn control other properties of the material [11,13].The effects of W substitution on the microstructure have been examined by SEM and the obtained micrographs are shown in Fig.2.It shows the microstructure of the fractured surface of the studied samples.It is clearly observed that W substitution has pronounced effect on the average grain size and homogeneity of the grains.Randomly oriented and anisotropic plate-like grains are observed in all the samples.It is also observed that the average grain size increases gradually with increasing W content.The average grain size in the sample with x =0.0is $2–3m m while that in the sample with x =0.20the size increases to $5–7m m.3.2.Electrical studiesThe electrical conductivity of ceramic materials encompasses a wide range of values.In insulators,the defects w.r.t.the perfect crystalline structure act as charge carriers and the consideration of charge transport leads necessarily to the consideration of point defects and their migration [22].Many mechanisms were put forward to explain the conductivity mechanism in ceramics.Most of them are approximately divided into three groups:electronic conduction,oxygen vacancies ionic conduction,and ionic and p-type mixed conduction [22].Intrinsic conductivity results from the movement of the component ions,whereas conduction resulting from the impurity ions present in the lattice is known as extrinsic conductivity.At low temperature region (ferroelectric phase),the conduction is dominated by the extrinsic conduction,whereas the conduction at the high-temperature paraelectric phase ($300–7008C)is dominated by the intrinsic ionic conduction [23,25].Fig.3shows the temperature dependence of dc conductivity (s dc )for the undoped and doped SBT samples.The curves show that the conductivity increases with temperature.This is indicative of negative temperature coefficient of resistance (NTCR)behavior,a characteristic of dielectrics [22].It is observed in Fig.3that throughout the temperature range,the dc conductivity of the doped samples are nearly two to three orders lower than that of the undoped sample.Two predominant conduction mechanisms indicated by slope changes in the two different temperature regions are observed in Fig.3.Such changes in the slope in the vicinity of the ferro-paraelectric transition region have been observed in other ferroelectric materials as well [23,24].In addition,it is also observed (Table 2)that the activation energy calculated using the Arrhenius equation [22]in the paraelectric phase increase from $0.80eV for the undoped sample to $2eV for the doped samples.The X-ray photoemission spectroscopic study has confirmed that when Bi 2O 3evaporates during high-temperature processing,vacancy complexes are formed in the (Bi 2O 2)2+layers [26].As a result,defective (Bi 2O 2)2+layers are inherently present in SBT.The undoped SBT shows n-type conductivity,since when oxygen vacancies are created,it leaves behind two trapped electrons [27]:O o !12O 2"þV o þ2e 0(1)where O o is an oxygen ion on an oxygen site,V o is a oxygen vacant site and e 0represents electron.The conductivity in the perovskites can be described as an ordered diffusion of oxygen vacancies [28].Their motion is manifested by enhanced ionic conductivity associated with an activation energy value of $1eV [26].These oxygen vacancies can be suppressed by addition of donors,since the donor oxide contains more oxygen per cation than the host oxide it replaces [29].It has been reported that conductivity in Bi 4Ti 3O 12(BIT)can be significantly decreased,up to three orders of magnitude with the addition of donors,such as Nb 5+and Ta 5+at the Ti 4+sites [23,30].A few other studies on layered perovskites have also reported a decrease inconductivityFig.1.XRD patterns of SrBi 2(W x Ta 1Àx )2O 9samples sintered at 12008C.Table 1Lattice parameters of SrBi 2(W x Ta 1Àx )2O 9samples.Concentration of W a (A ˚)b (A ˚)c (A ˚)0.0 5.5212 5.513924.92230.025 5.5214 5.520225.10790.05 5.5217 5.519925.05850.075 5.5191 5.504525.05670.10 5.5142 5.506125.0850.205.51335.493925.0861I.Coondoo et al./Materials Research Bulletin 44(2009)1288–12921289with addition of donors [23,24,31].In the present study,the Ta 5+-site substitution by W 6+in SBT can be formulated using a defect chemistry expression as WO 3þV o!Ta 2O 512W Ta þ3O o (2)It shows that the oxygen vacancies are reduced upon the substitution of donor W 6+ions for Ta 5+ions.Hence,it is reasonable to believe that the conductivity in SBT is suppressed by donor addition.As per the above discussion,the high s dc observed in the undoped SBT (Fig.3)can be attributed to the motion of oxygen vacancies.As already discussed,the doped samples show reduced conductivity because the transport phenomena involving oxygen vacancies are greatly reduced.The high E a value of $1.75–2eVcorresponding to the high-temperature region in the doped ceramics is consistent with the fact that in the donor-doped materials,the ionic conduction reduces [32].The activation energy E a in the low temperature ferroelectric region (Table 2)corre-sponds to extrinsic conduction.At lower temperatures the extrinsic conductivity results from the migration of impurity ions in the lattice.Some of these impurities may also be associated with lattice defects.Pure SBT has large number of Schottky defects (oxygen vacancies)in addition to impurity ions whereas in the doped samples,due to charge neutrality,there is relatively less content of oxygen vacancies.Thus,in the doped samples the conductivity in the low temperature region is largely due to the impurity ions only.This explains the high activation energy in pure SBT in the low temperature region compared to doped samples (Table 2).In the high-temperature region,the value of E a in the doped samples is observed to increase with W concentration up to x =0.05but beyond that,it decreases (Table 2).The decrease in the activation energy for samples with x >0.05suggests an increase in the concentration of mobile charge carriers [33].This observation can be ascribed to the existence of multiple valence states of tungsten.Since tungsten is a transitional metal element,the valence state of W ions in a solid solution most likely varies from W 6+to W 4+depending on the surrounding chemical environment [34].When W 4+are substituted for the Ta 5+sites,oxygen vacancies would be created,i.e.one oxygen vacancy would be created for every two tetravalent W ions entering the crystal structure,whichFig.3.Variation of dc conductivity with temperature in SrBi 2(W x Ta 1Àx )2O 9samples.Fig.2.SEM micrographs of fractured surfaces of SrBi 2(W x Ta 1Àx )2O 9samples with (a)x =0.0,(b)x =0.025,(c)x =0.050,(d)x =0.075,(e)x =0.10and (f)x =0.20Table 2Activation energy (E a )in the high-temperature paraelectric region and low temperature ferroelectric region;Curie temperature (T c )in SrBi 2(W x Ta 1Àx )2O 9samples.Concentration of W E a (high temp.)(eV)E a (low temp.)(eV)T c (8C)0.00.790.893110.025 1.920.593080.05 1.960.543250.075 1.940.543380.10 1.860.573680.201.740.54390I.Coondoo et al./Materials Research Bulletin 44(2009)1288–12921290explains the increase in the concentration of mobile charge carriers which ultimately results in an decrease in the E a beyond x>0.05. Hence it is reasonable to conclude that W ions in the SBWT exists as a varying valency state,i.e.at lower doping concentration they exist in hexavalent state(W6+)and at a higher doping concentra-tion,they tend to exist in lower valency states[8].The P–E loops of SrBi2(Ta1Àx W x)2O9are shown in Fig.4.It is observed that W-doping results in formation of well-defined hysteresis loops.Fig.5shows the compositional dependence of remanent polarization(2P r)and the coercivefield(2E c)of SrBi2(Ta1Àx W x)2O9samples.Both the parameters depend on W content of the samples.It is observed that2P rfirst increases with x and then decreases while2E cfirst decreases with x and then increases(Fig.5).The optimum tungsten content for maximum2P r ($25m C/cm2)is observed to be x=0.075.It is known that ferroelectric properties are affected by compositional modification,microstructural variation and lattice defects like oxygen vacancies[10,35,36].In hard ferroelectrics, with lower valent substituents,the associated oxide vacancies are likely to assemble in the vicinity of domain walls[37,38].These domains are locked by the defects and their polarization switching is difficult,leading to an increase in E c and decrease in P r[38]. On the other hand,in soft ferroelectrics,with higher valent substituents,the defects are cation vacancies whose generation in the structure generally increases P r.Similar observations have been made in many reports[38–41].Watanabe et al.[42]reported a remarkable improvement in ferroelectric properties in the Bi4Ti3O12ceramic by adding higher valent cation,V5+at the Ti4+ site.It has also been reported that cation vacancies generated by donor doping make domain motion easier and enhance the ferroelectric properties[43].Further,it is known that domain walls are relatively free in large grains and are inhibited in their movement as the grain size decreases[44].In the larger grains, domain motion is easier which results in larger P r.Also for the SBT-based system,it is known that with increase in the grain size the remanent polarization also increases[45,46].Based on the obtained results and above discussion,it can be understood that in the undoped SBT,the oxygen vacancies assemble at sites near domain boundaries leading to a strong domain pinning.Hence,as observed,well-saturated P–E loop for pure SBT is not obtained.But in the doped samples,the suppression of the oxygen vacancies reduces the pinning effect on the domain walls,leading to enhanced remanent polarization and lower coercivefield.Also,the increase in grain size in tungsten added SBT,as observed in SEM micrographs(Fig.2)contribute to the increase in polarization values.In the present study,the grain size is observed to increase with increasing W concentration.However, the2P r values do not monotonously increase and neither the E c decreases continuously with increasing W concentration(Fig.5). The variation of P r and E c beyond x>0.05,seems possibly affected by the presence of secondary phases(observed in XRD diffracto-grams),which hampers the switching process of polarization [47–50].Also,beyond x>0.05the increase in the number of charge carriers in the form of oxygen vacancies leads to pinning of domain walls and thus a reduction in the values of P r and increase in E c is observed.Fig.6shows the variation of piezoelectric charge coefficient d33 with x in the SrBi2(Ta1Àx W x)2O9.The d33values increases with increase in W content up to x=0.05.A decrease in d33values is observed in the samples with x!0.075.The piezoelectric coefficient,d33,increases from13pC/N in the sample with x=0.0to23pC/N in the sample with x=0.05.It is known that the major drawback of SBT is its relatively higher conductivity,which hinders proper poling[51].High resistivity is therefore important for maintenance of poling efficiency at high-temperature[52,53].The W-doped SBT samples show an electrical conductivity value up to three orders of magnitude lower than that of undoped sample(Fig.3).The positional variation of2P r and2E c in SrBi2(W x Ta1Àx)2O9samples.Fig.6.Variation of d33in SrBi2(W x Ta1Àx)2O9samples.Fig. 4.P–E hysteresis loops in SrBi2(W x Ta1Àx)2O9samples recorded at roomtemperature.I.Coondoo et al./Materials Research Bulletin44(2009)1288–12921291decrease in conductivity upon donor doping improve the poling efficiency resulting in the observed higher d33values.Moreover, since the grain size increases with W content in SBT,it is reasonable to believe that the increase in grain size will also contribute to the increase in d33values[54].The decrease in the value of d33for samples with x!0.075is possibly due to the presence of secondary phases as observed in diffractograms[1,51,55]and the increase in oxygen vacancies for samples with x>0.05.4.ConclusionsX-ray diffractograms of the samples reveal that the single phase layered perovskite structure is maintained in the samples with tungsten content x0.05.SEM micrographs reveal that the average grain size increases with increase in W concentration. The temperature dependence of the electrical conductivity shows that tungsten doping results in the decrease of conductivity by up to three order of magnitude compared to W free SBT.All the tungsten-doped ceramics have higher2P r than that of the undoped sample.The maximum2P r($25m C/cm2)is obtained in the composition with x=0.075.The reduced conductivity allows high-temperature poling of the doped samples.Such compositions with low loss and high P r values should be excellent materials for highly stable ferroelectric memory devices.The d33value is observed to increase with increasing W content up to x0.05.The value of d33 in the composition with x=0.05is$23pC/N as compared to$13 pC/N in the undoped sample.AcknowledgmentsThe authors sincerely thank Prof.P.B.Sharma,Dean,Delhi College of Engineering,India for his generous support and providing ample research infrastructure to carry out the research work.The authors are thankful to Dr.S.K.Singhal,Scientist, National Physical 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二硼化钛陶瓷在不同温度下的氧化行为黄飞，傅正义，王为民，王皓，王玉成，张金咏，张清杰(武汉理工大学，复合材料新技术国家重点实验室，武汉 430070)摘要：采用静态氧化法对不同温度下TiB2陶瓷的氧化行为进行研究，利用X射线衍射仪、扫描电镜、X射线光电子能谱仪对氧化前后的样品进行表征。

结果表明：低温下TiB2陶瓷氧化动力学满足抛物线规律，并在表面形成液相B2O3，阻止氧化反应的进一步进行，冷却后B2O3以玻璃态覆盖在表面。

高温下TiB2氧化反应在4h前满足抛物线规律，表面形成一层TiO2多孔结构；氧化4h后，随着氧扩散距离的延长，扩散阻力加大，从而使氧化速率降低，氧化反应不再满足抛物线规律。

关键词：二硼化钛；氧化动力学；微观结构中图分类号：TF123；TB332 文献标识码：A 文章编号：0454–5648(2008)05–0584–04OXIDATION BEHA VIOR OF TITANIUM DIBORIDE CERAMIC AT DIFFERENT TEMPERATURES HUANG Fei，FU Zhengyi，W ANG W eimin，W ANG Hao，W ANG Yucheng，ZHANG Jinyong，ZHANG Qinjie(State key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University ofTechnology, Wuhan 430070, China)Abstract: The oxidation behavior of TiB2 ceramics at different temperatures was investigated using the static oxidation kinetic method. The samples before and after oxidation have been characterized by X-ray diffractometer, scanning electron microscope and X-ray photoelectron spectrometer. The results show that the oxidation kinetics appear the parabolic law at low temperature. A liquid B2O3 coating on the surface of TiB2 ceramic could prevent from further oxidation. After the ceramic samples were cooled, their sur-faces were covered with glassy B2O3. At high temperature, the oxidation reaction of TiB2 ceramics showed the parabolic law only before 4h. Porous rutile TiO2 formed on the surface. But the oxidation behavior with the parabolic law for the TiB2 ceramics was not observed after oxidation for 4h because of the long path of diffusion, strong diffusion resistance and low reaction rate.Key words: titanium diboride; oxidation kinetics; microstructureTitanium diboride with P6/mmm structure is a uniquely stable compound of the boron element and tita-nium element.[1] TiB2 based materials have received wide attention because of their high hardness and elastic modulus, good abrasion resistance and superior thermal and electrical conductivity.[2–3] Potential applications in-clude high temperature structural materials, cutting tools, armor, electrodes in metal smelting and wear parts. De-spite its useful properties, the application of monolithic TiB2 is limited by poor sinterability, exaggerated grain growth at high temperature and poor oxidation resistance above 800.℃[4–5]The starting temperature to oxidize TiB2 ceramics is about 400℃ and oxidation kinetics is controlled by outward diffusion of interstitial titanium ions and inner diffusion of oxygen ions.[5–6] But there are conflicting viewpoints about the detailed oxidation process, for ex-ample, about the oxidation products and oxidation mechanism. Koh et al.[7] investigated the oxidation be-havior of dense TiB2 specimens with 2.5% in mass (the same below) Si3N4 and found that TiB2 exhibited two distinct oxidation behaviors depending on the tempera-ture. At temperatures below 1000℃, the oxidation layer comprised two layers: an inner layer of crystalline TiO2 and an outer layer mainly composed of B2O3. When the oxidation temperatures were higher than 1000℃, the收稿日期：2007–09–23。

第50卷第3期2021年5月内蒙古师范大学学报(自然科学版)Journal of Inner Mongolia Normal University(Natural Science Edition)Vol.50No.3May2021六硼化钇纳米粒子超导及光吸收性能研究王军】，包黎红】，潮洛蒙2（1.内蒙古师范大学物理与电子信息学院，内蒙古呼和浩特010022；2.内蒙古科技大学理学院，内蒙古包头014010）摘要：采用固相烧结法成功制备出了六硼化钇（YB&）纳米粒子，首次系统研究了该纳米粉末超导及光吸收性能.结果表明，当超导转变温度T=2.75K时由正常态转变为超导态，其临界磁场为H c2=0.18T.为进一步研究YB6纳米粒子电-声子相互作用机理，采用拉曼光谱对声子振动频率进行了测量，结合McMillan公式计算出YB6纳米粒子电-声子作用常数为A=0.63,该值远小于单晶块体YB6的1.01.为进一步解释其原因，采用高分辨透射电镜对晶体缺陷进行了详细表征.结果发现，晶体缺陷导致其声子振动频率的改变，从而降低了纳米YB6电-声子相互作用常数.光吸收结果表明YB6纳米粒子吸收谷波长为785nm,对可见光具有很强的穿透性.关键词：超导性；光吸收；YB6中图分类号：O511+.3文献标志码：A文章编号：1001—8735（2021）03—0204—06doi：10.3969/j.issn.1001—8735.2021.03.003众所周知，材料的宏观物理化学性能与微观结构密切相关[12].特别是当材料晶粒尺寸减小到纳米尺度后，纳米晶材料不仅具有亚稳态的特点，而且相比于粗晶材料展现出许多新奇的物理化学性能.与此同时,材料的纳米化会改变材料电子态密度及电-声子相互等物理量，从而会对超导及光学性能有很大的影响[34].因此，如何将纳米材料微观结构与宏观性能之间进行有效关联，将对材料新性能的发现和研究具有重要作用.在众多金属硼化物中，由于六硼化钇YB6具有第二高超导转变温度T c=&4K,故其超导性能受到广泛关注.目前关于这方面的研究主要集中于单晶YB6块体材料上[8],而对于YB6纳米离子超导性能研究未见报道.研究者们为了解释单晶块体超导机理及提高临界转变温度，系统研究了压强对YB6单晶块体晶体结构和声子振动的影响[912].结果发现，当压强从0增加至40GPa时，电子-声子相互作用常数从1.44减小到0.44.与此同时，相对应的超导转变温度也从&9K减小到1.4K,表明压强对电-声子作用具有很大影响.这项研究工作的一个重要提示是,YB6的纳米化是否会改变费米能级周围的电子态密度及电子-声子相互作用，从而展现出一些新的超导性能，这是本文的重要研究内容之一.此外，虽然YB6与LaB6具有相同立方晶体结构[13],但它是否同样具有对可见光的高穿透特性，是本论文另一个重要研究内容.目前国内外对纳米YB6超导性能及光吸收实验方面的系统研究未见报道.本文首次系统研究了YB6纳米粒子超导及光吸收性能.为进一步揭示材料微观结构与宏观性能之间的内在关联，采用高分辨透射电镜和拉曼光谱等测量手段对微观结构进行了有效表征，并对超导及光吸收机理进行探讨.收稿日期：2020-11-30基金项目：国家自然科学基金资助项目（51662034）；内蒙古自治区自然科学基金资助项目（2019LH05001）；内蒙古自治区留学人员创新创业启动基金资助项目.作者简介：王军（1992-），男，内蒙古阿拉善左旗人，在读硕士研究生，主要从事纳米稀土六硼化物光吸收及热发射性能研究.通讯作者：包黎红（1983—）,男，内蒙古兴安盟人，教授，博士，主要从事纳米金属硼化物物理性能研究,E-mail:baolihong@.第3期王军等：六硼化钇纳米粒子超导及光吸收性能研究-205-1材料与方法将无水氯化钇(YCl,纯度99.95%)和硼氢化钠(NaBH4,纯度98%)粉末在空气中按摩尔比为1：&8混合研磨10〜15min.将混合均匀的粉末放入压机中，在压强为10GPa下预压成块，将其装入石英管中进行真空烧结.反应温度为1100C,保温2h.由于固相反应后产物中有YBO s的杂相，故对烧结后产物分别使用稀盐酸，蒸馏水，无水乙醇等溶液进行多次清洗.采用场发射扫描电子显微镜(日立SU-8010)和X射线衍射仪(飞利浦PW1830,CuKa)对YB6纳米粒子的物相及形貌进行表征.采用PPMS测量仪对纳米YB6交流磁化率和临界磁场进行了测量，最低温度为1.8K.采用透射电子显微镜(FEITecnai F20S-Twin200kv)观察微观结构.拉曼散射由拉曼光谱仪(LabRamHR,波长：514.5nm,激光源:Ar+)进行测量.采用分光光度计(UH4150)在光源波长350〜2500nm范围内测量其光吸收。

铁基超导超导是物理世界中最奇妙的现象之一。

正常情况下，电子在金属中运动时，会因为金属晶格的不完整性（如缺陷或杂质等）而发生弹跳损耗能量，即有电阻。

而超导状态下，电子能毫无羁绊地前行。

这是因为当低于某个特定温度时，电子即成对，这时金属要想阻碍电子运动，就需要先拆散电子对，而低于某个温度时，能量就会不足以拆散电子对，因此电子对就能流畅运动。

传统的解释常规超导体的超导电性的微观理论预言，超导体的最高温度不会超过麦克米兰极限的39K。

在以往的研究中，只有1987年发现的铜氧化合物超导体打破了这一极限，被称为高温超导体。

最近，在铁基磷族化合物中发现的超导电性其超导临界温度可达55K，同样突破了传统理论预言的麦克米兰极限。

这是第一个非铜基的高温超导体，掀起了高温超导研究的又一次热潮。

铁基超导的研究进入了一个空前发展的阶段，各国都在进行这一新材料的研究，铁基超导体薄膜研究进展与铁基超导体大同位素效应就是其中的热点。

从2008年新的铁基高温超导体发现以来，铁基超导薄膜的研究进展相对缓慢。

这是因为较难精确控制人们所需要的亚稳相中的多元素配比、以及多种热力学相之间的互相竞争。

由于元素配比和不同热力学相竞争所导致的较少量的杂质，在块状材料的合成中有时可以接受，但对低维的薄膜材料却不能允许。

迄今已发现四种主要晶体结构的铁基超导体，包括含砷或磷（chalcogens）的1111相、122相、111相，以及含氧硫族元素（pnictogens）的11相。

它们都具有超导的Fe-X （X为As、P、Se、S或Te等）层，且前三类超导体中这些层由La-O等隔离层隔开，而超导的11相FeSe、Fe(Se,Te)只有Fe-X层，晶体结构最简单。

目前人们只得到了11相的单相、外延、超导薄膜。

而对含砷的铁基超导体而言，经过近两年的探索，仍未能得到单相的超导薄膜。

中国科学院物理研究所/北京凝聚态物理国家实验室(筹)超导实验室的曹立新副研究员带领博士生韩烨、李位勇，与相关科研人员合作，在国际上率先制备出单相的外延FeSe超导薄膜（第十届全国超导薄膜和超导电子器件学术研讨会，大连，2008年10月11日-15日），率先发表文章（Journal of Physics: Condensed Matter 21, 235702, 2009），并申请了国家专利。