Synthesis and Investigation of (La,Ce)B6 Films
Synthesis and Investigation of (La,Ce)B6 Films with High Seebeck Coefficients for Application in Thermoelectric Detectors
Tech Area / Field
- MAT-SYN/Materials Synthesis and Processing/Materials
- PHY-SSP/Solid State Physics/Physics
3 Approved without Funding
Institute for Physical Research, Armenia, Ashtarak-2
- Bryansk State University, Russia, Bryansk reg., Bryansk
- Department of the Navy / Naval Research Laboratory, USA, DC, Washington
Project summary10.1. Introduction and Overview.
Thermoelectric effect has multiple applications in science and technology. The task of obtaining new highly thermoelectric materials is quite compelling as they may improve the performance of the known thermoelectric devices, as well as serve as a basis for developing new types of devices. The performance of thermoelectric devices depends on the figure of merit (ZT) of the material, given by the expression ZT=S2T/rKT, where S, T, r and KT are the Seebeck coefficient, absolute temperature, electrical resistivity and total thermal conductivity, respectively. Although most of research in the field of thermoelectric materials has focused on room temperatures , there is also interest towards thermoelectric materials for low temperatures. For one, many prospective tasks of modern science require fast-acting detectors capable of registering a single photon. The U.S. Navy Research Lab has been conducting research aimed at devising and implementing a new fast-acting microcalorimetry based single-photon detector . Sensitive element of a thermoelectric detector consists of three parts: an absorber, a heat sink and a connecting bridge made of thermoelectric material. As photon is being absorbed, the absorber’s temperature exceeds that of the heat sink, whereby thermopower is generated onto the thermoelectric strip that connects the absorber with the heat sink. Such sensor does not require either a separate power unit or a bias voltage. Therefore, it does not need additional leads for electronic circuitry either, and a matrix detector built on them will have a very simple engineering and electronic structure. Preliminary testing has demonstrated that the heat flow through a thermoelectric strip is fast enough to secure a count frequency above 1 Mcps . A thermoelectric device’s sensitivity to separate photons is defined by the signal/noise ratio, consideration of which gives acceptable energy resolution. Thus, to provide energy resolution of 1 eV at single-photon absorption, thermoelectric material must have Seebeck coefficient ~ 100 mV/K. Materials with higher Seebeck coefficient are abundant, but the trick is that in order to achieve the necessary signal/noise ratio, the detector must operate at below 1 K temperatures. One of the well-known low temperature thermoelectric materials is gold with iron impurity . However, in our opinion, lanthanum hexaboride with cerium impurity (La1-xCex)B6 holds more promise. Bulk samples of (La1-xCex)B6 containing 1% of Ce display Kondo minimum at 20 K with a subsequent increase of the resistivity with a saturation tendency at the 2-4 mW×cm level at temperature decline to 100 mK . Simultaneously, the Seebeck coefficient rises up to 90 mV/K . Should it be possible to obtain thin film samples of (La1-xCex)B6 with similar properties, the task of preparation a thermoelectric detector with record-braking parameters will be attained. The existing experience in obtaining lanthanum hexaboride films could serve as a basis for the development of (La1-xCex)B6 film technology.
10.2. Aim of the Project.
The present project plans to conduct experimental research on obtaining and exploring the physical properties of (La,Ce)B6 films and multilayered structures, including layers of hexaboride and metals to be applied in low temperature thermopower devices. The main objective of the project is to obtain (La,Ce)B6 films with high Seebek coefficient at temperatures below 1K. The intermediate target is to obtain multilayered structures and investigation of the conditions of their lithography with 2 m resolution. The ultimate goal of the project is to manufacture a prototype of a thermoelectric detector and to test its parameters.
10.3. Impact of the Project on the Progress in the Related Field.
In the process of creating a thermoelectric detector sensor a number of research challenges of obtaining (La,Ce)B6 films need to be addressed. Also, several tasks related to creating multilayered structures and their lithography must be attained. The execution of the project will contribute to the advancement in the field of study of thermoelectric materials for low temperatures. Consequently, conditions for developing effective low temperature thermoelectric devices will be created. Fast-acting high energy resolution detectors are required in modern celestial X-ray astronomy. The project will lay down the foundation for developing and preparation a single-photon thermoelectric detector.
10.4. Expected Results and their Application.
The project falls into the applied research category. During the execution of the project the influence of synthesis conditions, substrate material and the concentration of cerium on microstructure, phase composition and physical properties of (La1-xCex)B6 films will be investigated. We are hoping to acquire (La1-xCex)B6 films with Seebeck coefficient of ~80 mV/K at temperatures below 1K. Comparative analysis of the existing theories describing the Kondo effect with the obtained experimental data will be performed. Of big importance in achieving the ultimate goal of the project will be investigation of the conditions of obtaining multilayered structures and elaboration of the lithography process both for (La1-xCex)B6 film and multilayers. Prototype of thermoelectric detector sensor will be fabricated and tested.
The obtained results will permit, upon the completion of the project, to commence high-energy resolution thermoelectric detector development stage. The research results may be also used for creating other thermoelectric devices, such as solid state refrigerators designed to cool below the liquid helium boiling temperature . Beside thermoelectric applications, (La1-xCex)B6 films can be used in optical industry as selective mirrors and filters with high reflection in infrared region of the spectrum and at the same time, high transmission in the visible range. The development of a simple yet durable technology for obtaining quality hexaboride coatings may become the basis for a production of thermionic emission cathodes.
10.5. Competence of the Project Team and Role of Foreign Collaborators.
The tasks for the present project have been set by NRL scientists. The research planned for this project is integral part of investigation currently underway at NRL aimed at creating a thermoelectric detector. The high scientific potential and power of the NRL research group speak for themselves. The research level can be assessed by comments in one of major scientific journals, which claim that the NRL research group has put forward the first truly original idea of a single-photon detector in the last 20 years . The idea of a thermoelectric detector to be developed in this project is the second breakthrough idea introduced very recently by NRL researchers. IPR scientists have traditionally been recognized in the field of material science. In particular, the HTSC Lab under Dr. A.S. Kuzanyan, has conducted a wide spectrum of research on obtaining and exploring the physical properties of high temperature superconductors and thermoelectric materials, developing new methods of laser deposition. Also noteworthy is that the members of the lab have participated in research aimed at furthering the concept of thermoelectric detector. Dielectric Coating Group of the IPR Solid State Generators Lab under Dr. R.B. Kostanyan has an extensive experience both in obtaining film through electron-beam deposition and in research of their optical properties. The both groups have actively participated in the first experiments of (La,Ce)B6 thin film synthesis . The researchers of Bryansk State University have great experience in low temperature research. The BSU Solid State Thermodynamics Lab has in recent years conducted a complex investigation of the physical properties of rare earth’s hexaborides within the temperature interval from liquid helium to room.
Evidently, the present project is an ideal combination of the possibilities of theoretical analysis and the NRL’s subtle (precision) experiments, the Armenian group’s experience and its ability to conduct material science research, with high qualification of the Russian group. The IPR and NRL groups have worked together before and have had several joint publications. The addition of Russian scientists will reinforce this team. We are confident the collaboration will be successful. The summation of efforts of the involved groups will permit to successfully solve the tasks of the project. The participants will jointly study synthesized samples, test the obtained results, exchange and share information, publish their findings in scientific journals and provide each other technical assistance.
We also plan to conduct joint seminars and participate in international conferences. We are looking forward to productive and promising collaboration within the framework of the present project as well as continuous future cooperation.
The NRL group role is essential in this project. They have proposed the concept of a thermoelectric detector, formulated the tasks and outlined the ways of achieving the final objective of the project. In addition, the NRL group will be directly involved in the project by conducting theoretical analysis of the experimentally obtained results and measuring the properties of the most interesting samples at temperatures below 3 K. Besides, collaboration with the NRL group will include:
a) information exchange during the course of the project,
b) comments to technical reports to be submitted to ISTC,
c) joint use of samples,
d) testing of the results obtained during the execution of the project,
e) joint seminars and telecons.
10.6. Scope of Activities.
The project consists of four main tasks:
1. Synthesis and investigation of the properties of (La1-xCex)B6 films.
2. Fabrication and investigation of the properties of multilayered structures, including layers of metals and (La1-xCex)B6.
3. Investigation of the lithography process of (La1-xCex)B6 films and multilayered structures.
4. Fabrication and investigation of a prototype of a thermoelectric detector sensor.
The first task aims at obtaining (La1-xCex)B6 films with high thermoelectric coefficient at temperatures below 1 K. This task is imperative in achieving the final goal of the project. It includes three milestones, within which we will examine the influence of the conditions of synthesis, cerium concentration and the type of a substrate on the microstructure, phase composition and the physical properties of the film obtained through electron-beam and laser deposition. Modernization of the laser deposition setup will be performed to create an opportunity of conducting ultra-high vacuum deposition. Comparison of the physical properties of film obtained by the two methods with each other as well as with the Kondo theory will also be made. The objective of the second task is to examine the influence of buffer layers and coatings on the properties of (La1-xCex)B6, films, to obtain low-Ohm metal-hexaboride contacts. This problem consists of two milestones during which technology of obtaining substrate/buffer layer/ (La1-xCex)B6 and substrate/buffer layer/(La1-xCex)B6/adsorber structures will be developed. The last multilayered structure may already be used to produce a thermoelectric detector sensor prototype.
The third task seeks to fabricate bridge structures for (La1-xCex)B6 films. It incorporates two milestones. Photo and laser lithography methods for (La1-xCex)B6 films with line resolution of 2 m will be developed. A search for selective etchants will be conducted and a photolithography method for buffer layers and absorber will be developed. All groups participating in the project will be involved in the first three tasks. The IPR researchers will perform film synthesis, examine their phase and element composition, microstructure, temperature dependencies of electrical resistivity and thermoelectric coefficient in temperature region of 3-300 K, as well as their optical properties. The researchers from BSU will investigate temperature dependence of the heat conductivity coefficient and conduct fine X-Ray research. Most interesting samples will be examined at the NRL at temperatures below 3 K. The NRL group will also conduct theoretical analysis of the obtained experimental data. Successful execution of the first three assignments will allow for the commencement of the forth task, which aims at fabrication and testing of a thermoelectric detector sensor prototype. The fabrication of the sensor prototype and the investigation of the microstructure will be done at the IPR. The testing of detecting properties will be performed largely at the NRL.
10.7. Meeting ISTC Goals and Objectives.
In accordance with the objectives of ISTC, the proposed project will help organize a new group of scientists and engineers to conduct a peaceful research. Within the framework of this project, collaboration between the US (NRL) and Armenian (IPR) research groups will continue. The project will make it possible to include Russian scientists (BSU) in the research and will facilitate both Russian and Armenian groups’ integration into the international scientific community. As it was noted above, (La,Ce)B6 films may have various applications. Technologies developed during the execution of the project can be applied to solve various technical tasks and may serve as a basis for launching small size companies, which will certainly make Armenia and Russia’s transition to market economy much smoother.
10.8. Technical Approach and Methodology.
The electron-beam, laser thermal deposition methods will be applied. Investigation of phase and element composition, crystal perfection and microstructure will be performed by X-Ray analysis, scanning electron microscopy and X-ray microanalysis. In order to determine the thermoelectric parameter of the film’s figure of merit, temperature dependencies of the resistivity, Seebeck coefficient and heat conductivity will be investigated. For r(T) measurement standard four-contact method will be applied. S(T) will be measured with stationary method with two equivalent heaters. Film’s heat conductivity will be determined by means of two methods: method of axial heat flow through the film on the plate with subtraction of plate heat conductivity and comparative method of consequent heat flow through film on the plate and through the plate.
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