Термоэлектрические эффекты в полимерматричных композитах тема диссертации и автореферата по ВАК РФ 01.04.10, кандидат наук Yusupov Khabib Umaralievich

• Yusupov Khabib Umaralievich
• кандидат наук
• 2018

Оглавление диссертации кандидат наук Yusupov Khabib Umaralievich

CONTENT

INTRODUCTION

1. LITERATURE ANALYSIS

1.2. Traditional thermoelectric materials

1.3. Polymer-based thermoelectric materials

1.3.1. Basic understanding of transport properties of conductive polymers

1.3.2. Brief information regarding widely utilized polymer-based TE materials

1.3.2.1. Polyacetylene

1.3.2.2. Polyaniline (PANI)

1.3.2.3. Poly (3-hexylthiophene) (P3HT)

1.3.3. Poly(3,4-ethylene dioxythiophene) polystyrene sulfonate as a base for thermoelectric materials

1.3.3.1. An influence of carbon-based fillers on thermoelectric properties of PEDOT: PSS

1.3.3.2. Chemical modification of polymer to enhance thermoelectric performance

1.3.3.3. Influence of inorganic filler on thermoelectric properties of PEDOT-based composites

1.4. Conclusions based on the analysis

2. METHODOLOGY

2.1. The manufacturing process of samples

2.1.1. CNTF-based samples with various structures

2.1.2. CNTF-based samples with chemical post-treatment

2.1.3. Samples based on doped AE-SWCNTs with post-treatment

2.2. Equipment used to determine morphology, structure and thermoelectric properties

2.2.1. Scanning electron microscopy (SEM)

2.2.2. RAMAN microscopy

2.2.3. Transmission electron microscopy (TEM)

2.2.4. Four-probe method technique

2.3. Description of the theory and concepts used to perform calculations

2.3.1. The concept of theoretical calculations

2.3.2. The path of the calculation process

2.3.2.1. Kohn-Sham equations

2.3.2.2. The k-points concept

2.3.2.3. Band Structure calculations

2.3.2.4. The concept of Pseudopotentials

2.3.2.5. The concept of supercell

2.3.2.6. Practical aspects of DFT applications

3. RESULTS AND DISCUSSIONS

3.1. The initial application of VA-CNTF as a filler

3.2. Further utilization of VA-CNTFs with chemical post-treatment process

3.3. Influence of AE-SWCNTs, its hybrid and chemical post-treatment

3.4. Conclusions of the conducted work

CONCLUSIONS OF THE WHOLE WORK

LIST OF REFERENCES:

Введение диссертации (часть автореферата) на тему «Термоэлектрические эффекты в полимерматричных композитах»

INTRODUCTION

Thermoelectricity (TE) is the concept, describing various transport effects within materials under certain conditions. There are two main effects in this field that are of high interest: Peltier and Seebeck effects. Peltier effect allows one to manipulate the temperature of the material when the electrical current is applied, whereas Seebeck effect converts heat energy into electric power. The latter is initially of high interest in numerous fields, from electronics to space programs, and secondly is the main topic of current work. The description of the Seebeck coefficient can be expressed as such: when a temperature gradient is created at the two opposite ends of a material, a potential difference is induced, which can sustain an electric current in a closed loop circuit. The efficiency (ZT) of TE materials is determined by their electrical conductivity a, Seebeck coefficient S, thermal conductivity k and operating temperature T. The dependence of ZT from each of the previous parameters is expressed by the following: [1]

ZT = aS7kT

The benchmarking efficiency for the manufacturing of TE systems for common use is 30 % thus far, though the conversion efficiency of the best state-of-the-art TE generators is still below 12%, indicating that a significant gap is still to be filled.[2] Among the best materials are Bi2Te3 alloys, but also, in this case, the conversion efficiency of a TE generator does not exceed 10%.[3-5] However, inorganic semiconductors, though showing the highest efficiency, exhibit some disadvantages, such as high thermal conductivity, high cost, and high operating temperature.[6] For this reason, conductive polymers are considered as alternative materials for TE applications due to their low thermal conductivity, non-toxicity, low operating temperature, and abundance.[7,8] In addition, polymers are flexible and easy to adapt to curved, angled, and/or bendable surfaces, making them the ideal candidates for TE generators in flexible applications [9,10] such as wearable devices or clothing.

Among conductive polymers, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) has been the most used for TE research, due to its good electrical conductivity (achieved sheet resistance 300 Ohm/square with the addition of

conductivity-enhancement agents). However, the low ZT of this pure polymer encouraged strong research efforts to increase its efficiency.[11-13] Significant improvements can be achieved by introducing nanosystems (so-called fillers) into the polymer matrix, as well as chemical treatments [14]. The most successful fillers were carbon nanotubes (CNTs).[15] Chemical treatments modify the insulating part of the polymer, thus leading to an increase in the electrical conductivity.[16,17] Due to their nature and geometry, carbon-based nanomaterials like CNTs, tend to create agglomerates within water-based solutions during the process of integration into polymers, negatively affecting both the electrical conductivity and the Seebeck coefficient of the final composite.[18,19] In addition, the shape of CNTs results in anisotropic thermal and electrical conductivity. [20] Both agglomeration and anisotropic functionalities limit the performance of the material and should be removed (agglomeration) or properly tuned (via preferential orientation and anisotropy).

Another strategy for improvement of TE properties is creating hybrid fillers, i.e., a combination of two and more materials and consideration them as one filler content. In this direction of research, one can consider the work made by Zhang et al. as a graphic example. The report is describing modulation of the electronic band structure of reduced graphene with fluorinated C60, and its following integration to obtain a hybrid system in combination with the conductive polymer (poly(3,4-ethylene dioxythiophene: polystyrene sulfonate).

Current research work is providing three main achievements with respect to the above mentioned:

1. For the first time, a forest of vertically aligned multi-walled carbon nanotubes (VA-CNTF) as a filler for the conductive polymer to increase the TE properties of the compound was studied. VA-CNTF due to the morphology and structure is supposed to enhance TE properties. This approach was chosen due to its simplicity and the novelty of the filler. PEDOT: PSS was chosen as a dispersion medium due to its good electrical conductivity. Benchmarking structures based on the integration of unoriented short length MWCNTs in polymers were also considered, to demonstrate the superior performances of VA-CNTF.

A strategy to improve TE performance is the chemical post-treatment of the compounds. In this work, dimethyl sulfoxide and ethylene glycol were chosen as treating agents for MWCNT-based samples. Non-treated samples even with a very high concentration of fillers show rather low TE properties, while chemical treatment increases the PF in MWCNT-based samples used as a benchmark for the new CNTF compound.

The results show the improvement of the PF in the VA-CNTF-based sample, compared to usual MWCNTs. The highest PF for the CNTF-based material (at 420 K) is found to be 56.5 ^W/mK2, which was achieved without application of any treatment or post-treatment agents for the VA-CNTF samples.

2. For the first time, the combined integration of vertically aligned, highly ordered, carbon nanotubes forest (VA-CNTF) and chemical treatments to improve the TE properties of polymer-based composites was performed. Thanks to their ordered assembly, pulled VA-CNTF create a carpet of well-aligned CNTs, which prevent the formation of agglomerates, and in turn, can provide improved functionalities, compared to disordered CNTs. Such a highly ordered system enables shaping CNT anisotropy in desired directions, which can improve the transport properties of the composite, compared to that of the randomly dispersed CNTs.

Dimethyl sulfoxide (DMSO) and ethylene glycol (EG) were chosen for the post-treatment process, as they showed an improvement to the TE properties in previous studies.[21-23] To investigate the influence of the direction of electric current, relative to CNT orientation on the TE properties, the electric current flowing parallel and normal to the direction of the VA-CNTF layer was measured. It was demonstrated that suitable tuning of the structure, and chemical post-treatment of the composite, induces the improvement of the functional properties, (with the highest achieved Power Factor (PF) equal to 92 ^W/mK2 at 415 K). The present approach applies to fabricating self-supporting layers, which can be easily peeled from the rigid substrate, and then transferred onto curved, angled, and/or bendable surfaces.

3. The investigation of the influence of AE-SWCNTs treated with the AuCl3 solution on the TE properties of a polymer-based system with further tuning of transport properties by chemical post-treatment. AE-SWCNTs creates a carpet of CNTs with

superb morphology for TE application, i.e., even distribution and absence of agglomerates. Treatment with the isopropyl solution of AuCl3 was dictated due to the great electrical properties of Au and its potential positive influence on TE performance. Dimethyl sulfoxide (DMSO) and ethylene glycol (EG) were used as chemicals for post-treatment process. The influence of Au solution was simulated by theoretical calculation (VASP package). The results showed an effect of both possible doping processes, supported by calculation, and post-treatment, the latter revealing as the best technique for manufacturing samples with increased stability in the temperature range from 300 to 410 K.

All three paragraphs allow concluding that the represented work provides a significant development for improving the rapid advancement of highly-efficient, low-cost, flexible polymer TE composites able to operate at low temperatures and are suitable for application in portable and flexible devices.

The authenticity of achieved results

The authenticity of the results is proven by the utilized certified equipment as well as by the applied methods of analysis and tests of functional properties of materials, such as a four-probe method. Scanning electron microscopy, transmission electron microcopy, Raman spectroscopy allow conducting a high quality investigation of the structure of studied materials. Apart from mentioned, the authenticity is supported by the number of published works in the international scientific journals.

Results approbation

Main results of the conducted research were represented by the author on international conferences:

1. 34th annual international conference on thermoelectrics, 13th European conference on thermoelectrics. "Thermoelectric properties of polyacrylonitrile-based nanocomposites," Dresden, Germany, 2015. Poster.

2. 24th International Symposium on Metastable, Amorphous and Nanostructured Materials. "Influence of oriented CNT forest on thermoelectric properties of polymer-based materials", Donostia-San Sebastian, Spain, 2017. Oral.

3. 25th International Symposium on Metastable, Amorphous, and Nanostructured Materials. "Highly efficient and facile PEDOT:PSS-based thermoelectric materials doped with single-walled carbon nanotubes", Rome, Italy, 2018. Oral.

4. 16th Interstate conference "Thermoelectrics and their applications - 2018" , "An influence of combined application of carbon nanotubes and chemical post-treatment on thermoelectric properties of composites based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate", St. Petersburg, Russian Federation, 2018. Oral.

Conceptual issues ported-out for defense.

1) Utilization of VA-CNTF as a filler in TE systems based on the polymer matrix allowing to manufacture the samples with high values of the Seebeck coefficient and electrical conductivity.

2) Manufacturing of polymer matrix composites based on one layer of VA-CNTF is an optimum for thermoelectric properties due to the fact that utilization of more than one-layer leads to decrease of electrical conductivity and values of the Seebeck coefficient. Observed drops of values are due to the dissolving of the polymer layer with the upper layers.

3) Chemical post-treatment of the polymer matrix composites with ethylene glycol and dimethyl sulfoxide leads to a significant improvement of values for electrical conductivity. Such is due to the partial removal of the anions from the polymer matrix.

4) Obtaining mode, which is a process of covering of VA-CNTF with the polymer matrix, allows manufacturing samples with high degree of homogeneity and absence of agglomerates.

5) Results of theoretical calculations predict that doping of SWCNTs with the AuCl4 leads to an improvement of the density of quantum states near the Fermi level, which is confirmed with experimentally observed increase of electrical conductivity of composite materials.

Publications

Based on the performed research work represented in the current Ph.D. thesis 3 scientific papers were published in the international journals:

1. Yusupov K., Stumpf S., You S., et al. Flexible thermoelectric polymer composites based on carbon nanotubes forest // Advanced Functional Materials 28 (2018) 1801246 (IF - 13.325).

2. Yusupov K., Zakhidov A., You S., et al. Influence of oriented CNT forest on thermoelectric properties of polymer-based materials// Journal of Alloys and Compounds 741 (2018) 392 (IF - 3.779).

3. Yusupov K., Khovaylo V., Muratov D., et al. Thermoelectric properties of polyacrylonitrile-based nanocomposites. Journal of Electronic Materials 45 (2016) 3440 (IF - 1.566).

Structure and volume of the work

Ph.D. thesis consists of the introduction, 3 chapters, conclusions of the whole work, and list of references. The thesis is presented on 122 pages, contains 3 tables, and 47 graphs. List of references contains 142 citations.