Room-temperature thermoelectric materials: Challenges and a new paradigm

Room-temperature thermoelectric materials provide promising solutions for energy harvesting from the environment,and deliver a maintenance-free power supply for the internet-of-things(IoTs).The currently available Bi_(2)Te_(3) family discovered in the 1950s,still dominates industrial applications,however,it has serious disadvantages of brittleness and the resource shortage of tellurium(1×10^(-3) ppm in the earth's crust).The novel Mg_(3)Sb_(2) family has received increasing attention as a promising alternative for room-temperature thermoelectric materials.In this review,the development timeline and fabrication strategies of the Mg 3 Sb 2 family are depicted.Moreover,an insightful comparison between the crystal-linity and band structures of Mg_(3)Sb_(2) and Bi_(2)Te_(3) is drawn.An outlook is presented to discuss challenges and new paradigms in designing room-temperature thermoelectric materials.

Luminescence and energy transfer of color-tunable Lu2MgAl4SiO12:Eu^2+,Ce^3+,Mn^2+ phosphors

Energy transfer among the co-doped activators is an efficient route to achieve color-tunable emission in inorganic phosphors.Herein,photoluminescence tuning from blue to cyan has been achieved in the Lu2MgAl4 SiO12;Eu^2+,Ce^3+phosphors by varying the Ce^3+concentration with a fixed Eu^2+content.With the further introduction of a Mn^2+-Si^4+couple into the host lattice,the emission color can be tuned to red through the energy transfer of Eu^2+and Mn^2+.The luminescence properties and the energy transfer mechanism were studied in detail.The energy transfer from Eu^2+to Ce^3+is certified as a dipolequadrupole interaction with the energy transfer efficiency of 41.4%and Eu^2+to Mn^2+belongs to a dipole-dipole interaction with the energy transfer efficiency of 94.3%.The results imply that this singlephased Lu2MgAl4 SiO12:Eu^2+,Ce^3+,Mn^2+phosphor has a potential prospect for application in near-UV chip pumped white light emitting diodes.

Deciphering the leakage conduction mechanism of BiFeO_(3)–BaTiO_(3)lead-free piezoelectric ceramics

BiFeO_(3)–BaTiO_(3)(BF–BT)based piezoelectric ceramics are a kind of high-temperature lead-free piezoelectric ceramics with great development prospects due to their high Curie temperature(TC)and excellent electrical properties.However,large leakage current limits their performance improvement and practical applications.In this work,direct current(DC)test,alternating current(AC)impedance,and Hall tests were used to investigate conduction mechanisms of 0.75BiFeO_(3)–0.25BaTiO_(3)ceramics over a wide temperature range.In the range of room temperature(RT)−150℃,ohmic conduction plays a predominant effect,and the main carriers are p-type holes with the activation energy(Ea)of 0.51 eV.When T>200℃,the Ea value calculated from the AC impedance and Hall data is 1.03 eV with oxygen vacancies as a cause of high conductivity.The diffusion behavior of thermally activated oxygen vacancies is affected by crystal symmetry,oxygen vacancy concentration,and distribution,dominating internal conduction mechanism.Deciphering the conduction mechanisms over the three temperature ranges would pave the way for further improving the insulation and electrical properties of BiFeO_(3)–BaTiO_(3)ceramics.

Moisture-induced degradation of the narrow-band red-emitting SrLiAl3N4:Eu2+phosphor

The degradation of materials plays an important role in their application and service process. In this work, the moisture-induced degradation of SrLiAl3 N4：Eu2＋（SLAN）, a very promising narrow-band redemitting phosphor, was comprehensively investigated by treating it in two different moisture conditions in order to reveal the potential mechanism and optimize the luminescence properties. The degradation rate gradually slows down with the decreasing environmental humidity indicating that water plays a key role in the degradation. Moreover, we take the other option with 100% humidity at different temperatures for rapid degradation. In the rapid degradation, the luminescence of SLAN is quenched quickly and the phase and micro structure change obviously, with the phosphor being bleached. The host turns into NH_3, Al_2 O_3, Sr_3 Al_2（OH）_（12） and LiAl_2（OH）_7 finally. It is further confirmed that the rapid degradation occurs with the help of water and the phosphor is oxidized during this process.

Enhanced thermoelectric properties of Mn_(x)Cu_(1.8)S via tuning band structure and scattering multiscale phonons

Digenite(Cu_(1.8)S)as a potential p-type thermoelectric(TE)material has attracted extensive attention due to its environmental benign,abundant resources and low cost of component elements.In this study,the TE properties of MnxCu_(1.8)S bulk samples prepared by mechanical alloying(MA)combined with spark plasma sintering(SPS)were investigated.Doping Mn would initially substitute Cu and tune the band structure of Cu1.8S with an enlarged band gap Eg.However,if Mn content is beyond the solubility limit of x=0.01 in Cu1.8S will cause the formation of MnS,which contributes to the formation of Cu-rich phases at 0.02 ≤x≤ 0.08.Benefiting from the synergetic scattering effect of point defects(Mn Cu,V_(S))and MnS,Cu1.96S,Cu1.97S,Cu2S phases,the lowest thermal conductivity k value of 0.75 W m^(-1) K^(-1) was obtained at 773 K for Mn0.08Cu1.8S.Along with the decreased k,the highest figure of merit ZT value of 0.92 at 773 K achieved in Mn0.08Cu1.8S bulk sample.A maximum engineering ZTeng of 0.3 and its efficiency hmax of about 6%were obtained at 323e773 K,which is almost 3 times than that of the pristine Cu1.8S(ηmax=2.2%).Introducing Mn in Cu1.8S is an effective and convenient strategy to improve TE performance.

Development of integrated two-stage thermoelectric generators for large temperature difference

Multi-stage thermoelectric(TE) modules can withstand a large temperature difference and can be used to obtain a high conversion efficiency. In this study, two-stage PbTe/Bi2Te3 TE modules were developed with an enhanced efficiency through a comprehensive study of device structure design, module fabrication, and performance evaluation. PbTe-based AgPbmSbTem+2(abbreviated as LAST) is a typically high ZT material, while the corresponding TE module was rarely reported so far. How to utilize LAST to fabricate high efficiency TE modules therefore remains a central problem. Finite element simulation indicates that the temperature stability of the two-stage module for LAST is better than that of two-segmented module. Compared to Cu,Ni, and Ni-Fe alloys, Co-Fe alloy is an effective metallization layer for PbTe due to its low contact resistance and thin diffusion layer. By sintering a slice of Cu on TE legs, pure tinfoil can be used as a common welding method for mid-temperature TE modules. A maximum efficiency(ηmax) of 9.5% was achieved in the range of 303 to 923 K in an optimized PbTe/Bi2Te3 based two-stage module, which was almost twice that of a commercial TE module.

Illumination-dependent free carrier screening effect on the performance evolution of ZnO piezotronic strain sensor

Performance modulation of ZnO optoelectronic devices in the presence of proper piezoelectric polarization charges has been widely reported, whereas relatively less work has been performed about the influence of photoexcitation on piezotronics. In this stud~ we experimentally investigated the performance evolution of ZnO piezotronic strain sensor under various 365 nm UV irradiation densities. The device demonstrated a response ratio of -200 under no illumination and under -0.53% compressive strain, and the response time is approximately 0.3 s. However, tremendous performance degradation was observed with the increase in the illumination densi~, which is attributed to the W-modulated change in the free electron concentration and Schottky barrier height. It was observed that increased carrier density intensifies the screening effect and thus, the modulation ability of piezo-polarization charges weakens. Meanwhile, the deterioration of rectifying behavior at the interface under UV illumination also jeopardizes the device performance.

Selecting nitride host for Yb^(3+) toward near-infrared emission with low-energy charge transfer band

Yb^(3+)-doped phosphors have characteristic near-infrared(NIR)emissions,but their applications in phosphor-converted light-emitting-diodes(pc-LEDs)and Si solar cells are limited due to their mismatching excitation spectra.Here,we selected nitride La3 Si6 N11(LSN)as host material to achieve Yb^(3+)NIR emission upon low-energy charge transfer(CT)excitation.The obtained phosphor LSN:Yb^(3+)has a broad CT excitation band ranging from 250 to 500 nm and narrowband NIR emissions ranging from 950 to 1100 nm centered at 983 nm.On the basis of spectral data,the vacuum referred binding energies(VRBE)schemes are constructed to locate energy levels of all lanthanide ions in LSN.We also fabricated NIR pc-LED device using 395 nm LED chip to demonstrate the potential applications of LSN:Yb^(3+)phosphors.

Enhanced piezoelectricity in 0.7BiFeO_(3)-0.3BaTiO_(3)lead-free ceramics:Distinct effect of poling engineering

BiFeO_(3)-BaTiO_(3) based ceramics are considered to be the most promising lead-free piezoelectric ceramics due to their large piezoelectric response and high Curie temperature.Since the piezoelectric response of piezoelectric ceramics just appears after poling engineering,in this work,the domain evolution and microscopic piezoresponse were observed in-situ using piezoresponse force microscopy(PFM)and switching spectroscopy piezoresponse force microscopy(SS-PFM),which can effectively study the local switching characteristics of ferroelectric materials especially at the nanoscale.The new domain nucleation preferentially forms at the boundary of the relative polarization region and expands laterally with the increase of bias voltage and temperature.The maximum piezoresponse(Rs),remnant piezoresponse(Rrem),maximum displacement(Dmax)and negative displacement(Dneg)at 45 V and 120C reach 122,69,127 pm and 75 pm,respectively.Due to the distinct effect of poling engineering in full domain switching,the corresponding d33 at 50 kV/cm and 120C reaches a maximum of 205 pC/N,which is nearly twice as high as that at room temperature.Studying the evolution of ferroelectric domains in the poling engineering of BiFeO_(3)-BaTiO_(3)ceramics provides an insight into the relationship between domain structure and piezoelectric response,which has implications for other piezoelectric ceramics as well.

Emerging ultra-narrow-band cyan-emitting phosphor for white LEDs with enhanced color rendition

Phosphor-converted white LEDs rely on combining a blue-emitting InGaN chip with yellow and red-emitting luminescent materials.The discovery of cyan-emitting(470-500 nm)phosphors is a challenge to compensate for the spectral gap and produce full-spectrum white light.Na_(0.5)K_(0.5)Li_(3)SiO_(4):Eu^(2+)(NKLSO:Eu^(2+))phosphor was developed with impressive properties,providing cyan emission at 486 nm with a narrow full width at half maximum(FWHM)of only 20.7 nm,and good thermal stability with an integrated emission loss of only 7% at 150℃.The ultra-narrow-band cyan emission results from the high-symmetry cation sites,leading to almost ideal cubic coordination for UCr_(4)C_(4)-type compounds.NKLSO:Eu^(2+) phosphor allows the valley between the blue and yellow emission peaks in the white LED device to be filled,and the color-rendering index can be enhanced from 86 to 95.2,suggesting great applications in full-spectrum white LEDs.

Highly Textured N-Type SnSe Polycrystals with Enhanced Thermoelectric Performance

Thermoelectric materials,which directly convert heat into electricity based on the Seebeck effects,have long been investigated for use in semiconductor refrigeration or waste heat recovery.Among them,SnSe has attracted significant attention due to its promising performance in both p-type and n-type crystals;in particular,a higher out-of-plane ZT value could be achieved in ntype SnSe due to its 3D charge and 2D phonon transports.In this work,the thermoelectric transport properties of n-type polycrystalline SnSe were investigated with an emphasis on the out-of-plane transport through producing textural microstructure.The textures were fabricated using mechanical alloying and repeated spark plasma sintering(SPS),as a kind of hot pressing,aimed at producing strong anisotropic transports in n-type polycrystalline SnSe as that in crystalline SnSe.Results show that the lowest thermal conductivity of 0.36 Wm^(-1) K^(-1) was obtained at 783 K in perpendicular to texture direction.Interestingly,the electrical transport properties are less anisotropic and even nearly isotropic,and the power factors reach 681.3μWm^(-1) K^(-2) at 783 K along both parallel and perpendicular directions.The combination of large isotropic power factor and low anisotropic thermal conductivity leads to a maximum ZT of 1.5 at 783 K.The high performance elucidates the outstanding electrical and thermal transport behaviors in n-type polycrystalline SnSe,and a higher thermoelectric performance can be expected with future optimizing texture in n-type polycrystalline SnSe.

Lattice Doping of Lanthanide Ions in Cs_(2)AgInCl_(6)Nanocrystals Enabling Tunable Photoluminescence

Lead-free halide double perovskite Cs_(2)AgInCl_(6 )has become the research hotspot in the optoelectronic fields.It is a challenge to utilize the lattice doping by different lanthanide ions with rich and unique photoluminescence(PL)emissions for emerging photonic applications.Here,we successfully incorporated Dy^(3+),Sm^(3+),and Tb3+ions into Cs_(2)AgInCl_(6) nanocrystals(NCs)by the hot-injection method,bringing diverse PL emissions of yellowish,orange,and green light in Cs_(2)AgInCl_(6):Ln^(3+)(Ln^(3+)=Dy^(3+),Sm^(3+),Tb^(3+)).Moreover,benefiting from the energy transfer process,Sm^(3+)and Tb^(3+)ion-codoped Cs_(2)AgInCl_(6) NCs achieved tunable emission from green to yellow orange and a fluorescent pattern from the as-prepared NC-hexane inks by spray coating was made to show its potential application in fluorescent signs and anticounterfeiting technology.This work indicates that lanthanide ions could endow Cs_(2)AgInCl_(6) NCs the unique and tunable PL properties and stimulate the development of lead-free halide perovskite materials for new optoelectronic applications.

Structural Engineering of Eu^2+-Doped Silicates Phosphors for LED Applications

CONSPECTUS:Phosphor-converted light-emitting diodes(pc-LEDs)are of great importance for their applications in solid-state lighting,backlit display,and near-infrared detection light source.Herein,the main challenges for these emergent pc-LEDs are to achieve full-spectrum lighting,wide color gamut display and broadband high efficiency near-infrared emission,respectively,which depends on the luminescence properties of phosphors used.Owing to the unique 4f-5d transition,Eu^2+is one of the most commonly used activators in luminescent materials for pc-LEDs,and Eu^2+-doped earth-abundant silicates phosphors exhibit outstanding luminescence properties,including multicolor emission,adjustable bandwidth,excellent thermal stability as well as high luminescence efficiency.These attributes motivate scientists to find Eu^2+-doped silicates phosphors that can practically meet the various LED application requirements.Since the traditional trial and error exploration is time-consuming and not necessarily successful,it is necessary to find reliable structural engineering strategies to discover new phosphor systems and also realize purposeful photoluminescence tuning.The adjustable 4f-5d electronic transitions of Eu^2+,the variable crystal structures of the silicate hosts and their coupling effect simultaneously account for the targeted luminescence behaviors and their precise emission color tuning.Thus,we aim at developing Eu^2+-doped silicate phosphors that can solve the application challenges through a comprehensive understanding of Eu^2+photoluminescence mechanism and the structure−property relationships.In this Account,we first illustrate the luminescence theory of Eu^2+in inorganic solids and summarize the research results of the effect originated from centroid shift,crystal field splitting,Stokes shift,and emission bandwidth.On the basis of the factors dominating the variation of luminescence characteristics,several structural strategies to manipulate Eu^2+emission in silicates are proposed,including(1)modify the chemical composition and crystal structure by various substitutions,(2)choose or change a suitable crystallographic site for Eu^2+and(3)control crystalline phase transition by external factors.Meanwhile,we briefly introduce the photoluminescence behaviors of Eu^2+in different silicates controlled by these structural engineering strategies.Second,we outline our recent research progress on blue LED pumped Eu^2+-doped silicate phosphors with emphasis on the design principle and the relationship between the structure and luminescence.The state-of-the-art LED application including full spectrum solid-state lighting,wide color gamut display and near-infrared night-vision technologies are introduced.Finally,we proposed the future research opportunities and challenges.The development of these Eu^2+-doped silicate phosphors exhibiting excellent luminescence performance is highly inspiring,and we expect this Account can be helpful for controlling the photoluminescence by theory-structure−property relationships and guide scientists discover the next generation of Eu^2+-doped phosphors for emerging applications.

近紫外芯片激发的白光LED用黄色荧光粉Sr_9Mg Li(PO_4)_7:Eu^(2+)(英文)

发现新型白光LED用荧光粉一直是材料科学领域一项重要的挑战.采用本课题组提出的矿物结构模型,高效设计了稀土发光材料新物相,并解析、确定新物相的晶体结构.本论文用黄色荧光粉Sr29Mg Li(PO4)7:Eu+设计合成了一种新型白光LED,并对其结构和发光特性进行了分析表征.研究发现:其物相结构是源于β-Ca3(PO4)2矿物模型.在365 nm近紫外光激发下,Sr_9Mg Li(PO_4)_7:Eu^(2+)呈现出一个从450 nm到700 nm的宽带发射.通过把Sr_9Mg Li(PO_4)_7:Eu^(2+)黄粉与商用蓝粉Ba Mg Al10O17:Eu2+混合涂敷在蓝光芯片表面,制作得到了白光LED器件.测试结果显示,LED的Ra,CCT,CIE值分别为83,5612 K,(0.324,0.358),表明Sr_9Mg Li(PO_4)_7:Eu^(2+)可作为白光LED用黄光荧光粉.

高效宽带近红外发光材料Sr_(2)ScSbO_(6):Cr^(3+)及其在类太阳光源中的应用

类太阳光光源包含可见光和近红外(NIR)光,在健康照明等领域具有重要的应用.然而众多应用却由于目前缺乏具有高效率和高发光热稳定的900 nm附近的NIR发光材料而受到限制.在此,我们报道了一种具有双钙钛矿型结构的宽带NIR荧光粉Sr_(2)ScSbO_(6):Cr^(3+),其具有高发光效率和良好的发光热稳定性.在550 nm光激发下,该材料具有以~890 nm为中心的宽带NIR发射,发光内/外量子效率分别为82.0%/35.7%.此外,材料显示出优良的发光热稳定性,在430 K下的发光积分强度保持初始强度的66.4%.结合465 nm蓝光芯片和其他商用荧光粉,我们成功制作了荧光粉转换发光二极管器件,呈现出从蓝色到近红外光的相对完整的类太阳光谱,有望用于日常照明和类太阳光谱发光器件.