Comparative performance of the nanocomposite thermoelectric AZO-PMMA films.
In comparison with metallic thermoelectric films, oxide films with artificial nanodefects have been seldom studied. And there has been no report on the incorporation of island-shaped organic nanoparticles. We describe a new approach to introduce nanometer-sized phonon scatterers in aluminum-doped ZnO (AZO) thermoelectric thin films–concurrent multi-beam multi-target-pulsed laser deposition and the matrix-assisted pulsed laser evaporation (MBMT-PLD/MAPLE). The approach was used to make nanocomposite thin films of AZO matrix with evenly dispersed poly(methyl methacrylate) (PMMA) nanoparticles. The introduction of the nanoparticles enhanced phonon scattering with consequent decrease of thermal conductivity by 20%. The electrical conductivity did not decrease after the addition of the second phase, as it would be predicted by Wiedemann-Franz law, but improved by 350% over pure AZO film. The thermoelectric figure of merit of the nanocomposite film became twice that of the pure AZO film. Taking advantage of room-temperature deposition, optimized AZO nanocomposite films are expected to be used in real applications, such as thin film modules deposited on flexible polymeric substrates for ubiquitous harvesting of the waste heat.
- laser ablation
- pulsed laser deposition
- matrix-assisted pulsed laser evaporation
- nanocomposite films
- polymer nanoparticles
- thermos-electric energy harvesters
ZnO is a well-known
In contrast to metallic thermoelectric films, oxide films with artificial nanodefects have been seldom studied: SrTiO3/Nb-SrTiO3 , AZO/hydroquinone , and AZO/Y2O3 . There is still no report on oxide films with nanodefects in the form of polymer nanoparticles. This chapter describes nanocomposite thin films made of AZO matrix with evenly dispersed poly(methyl methacrylate) (PMMA) nanoparticles produced by the concurrent multi-beam multi-target pulsed laser deposition of AZO and matrix-assisted pulsed laser evaporation of the polymer (MBMT-PLD/MAPLE) [10–13].
A wide variety of nanocomposite materials based on polymers and inorganic substances can be classified as “polymer nanocomposites,” where the polymer serves as a host for inorganic nanoparticles [14, 15]. In case, when the volume fraction of the polymer decreased below 90% (and the interaction between the inorganic inclusions became stronger, as illustrated in Figures 1 and 2), the resulting materials were often called as “organic-inorganic hybrid composites” . In some occasions, one polymer was used as a host for a nanoparticles made of another polymer . Making a uniformly mixed nanocomposite films with low fractional volume (<10%) of the polymer and at least one inorganic phase prevailing and serving as a host for the polymer nanoparticles (Figures 1 and 2) has remained an unachievable goal because the list of suitable technologies was limited mainly to the sol-gel process . Hence, the two novelty features of this work are: (i) the use of MBMT-PLD/MAPLE to produce an oxide-polymer nanocomposite film at room temperature with the oxide acting as a host to the polymer nanoparticles and (ii) the use of the polymer nanoparticles as phonon scatterers. The description given below demonstrates how the approach improves the thermoelectric properties of AZO films.
2. Methods and materials
The MBMT-PLD/MAPLE system at Dillard University is schematically presented in Figure 3. Two pulsed laser beams ablated concurrently two targets and ejected the target materials in the plumes that propagated toward the surface of a substrate where they mixed and formed a nanocomposite film. Linear actuators tilted the targets and changed the directions of the plumes in order to secure uniform mixing of the target materials in the film. Plume 1 is formed by laser beam 1 ablating a conventional solid inorganic PLD target. The second target (to the right) was a polymer solution frozen by circulating liquid nitrogen (LN). Accordingly, the process of polymer deposition was the matrix-assisted pulsed laser evaporation (MAPLE) [10–13]. All the components of the deposition system were placed in a vacuum chamber (not shown) with optical windows for the laser beams. The inorganic, AZO target was a pellet of Zn0.98Al0.02O where the aluminum fraction was 2% of the total by weight as compared with zinc, not counting the oxygen. The AZO pellet had 20 mm in diameter and 3 mm in thickness. The pellet was prepared by the spark plasma sintering method as described elsewhere . The MAPLE target was a solution of PMMA in chlorobenzene at a proportion of 1 g solids per 10 mL liquids filtered with 0.2 μm filter. The solution was poured in a copper cup of the MAPLE target assembly (Figure 3) and frozen in liquid nitrogen (LN). The laser source was a Spectra Physics Quanta Ray Nd:YAG Q-switched Pro-250-50 laser with a pulse repetition rate of 50 Hz, 750 mJ energy per pulse at the 1064 nm fundamental wavelength, and 400 mJ energy per pulse at the 532 nm second harmonic. The AZO target was ablated with the 532 nm beam. The fluence was tuned up between 0.8 and 1.0 J/cm2 per pulse. The MAPLE target was evaporated with the 1064 nm beam. The fluence was ranging from 0.84 to 2.4 J/cm2 per pulse to maintain a volume fraction of PMMA in the AZO matrix of about 5%. The deposition time was 3.0 min. The thickness of the deposited films was approximately 150 nm as measured with an atomic force microscope. The films were deposited on Al2O3 (sapphire) (100) single crystal substrates at room temperature in vacuum. The inorganic target was rotated during the irradiation of laser beam. The substrate-target distance was maintained about 35 mm. Deposition of pure AZO films in the same experimental conditions was carried out in order to have reference samples.
3. Results and discussion
X-ray diffraction (XRD) analysis of the as-grown AZO-PMMA films conducted with a Bruker D2 Phaser diffractometer (Figure 4) revealed the presence of polycrystalline AZO, while peaks of PMMA were absent. Polycrystallinity of AZO is due to the fact that the ablation in MBMT-PLD/MAPLE system was performed at room temperature to solidify PMMA after its condensation on the substrate (PMMA has been reported to melt at about 430 K ). As widely observed in literature, AZO films deposited at room temperature by conventional PLD are commonly polycrystalline [20–22].
Figure 5 (a) and (b) present TEM images of the film samples that were peeled off from the Al2O3 substrate. Typically, polymer nanoparticles-nanoclusters of a size of 10–50 nm are seen embedded in the AZO matrix. The formation of the nanoclusters is due to the heavy entanglement of the polymer molecules ruled by chemical interactions during their condensation on the substrates and chemical interaction with the ceramic host. Some polymer molecules could also self-assemble in individual fibers creating networks across the AZO matrix.
The electrical conductivity versus temperature (
Electro-conductive and thermo-electrical characteristics of the AZO-PMMA nanocomposite films in the range of 300–600 K are presented in Figures 6 through 11 and Table 1. Performance of pure AZO films deposited on the same substrate is also reported for comparison.
|Sample||Electrical conductivity |
|Seebeck coefficient |
|Power factor |
|Thermal conductivity |
|Pure AZO||AZO + PMMA||Pure AZO||AZO + PMMA||Pure AZO||AZO + PMMA||Pure AZO||AZO + PMMA||Pure AZO||AZO + PMMA|
|Film on Al2O3||433/539||1382/1630||−15/−30||−9/−20||0.01/0.05||0.01/0.07||7.4 ± 0.2||5.9 ± 0.3||0.005/0.04||0.0055/0.07|
Seebeck coefficient versus temperature plot is presented in Figure 7. The negative sign of Seebeck coefficient indicates electrons as the electric charge carriers and correspondingly the hybrid composite material being an
Electrical conduction is governed by thermally activated hopping mechanism as is indicated by linear plot of ln(
The electrical conductivity
As was mentioned above, one of the mechanisms of significant (three times) increase of the electrical conductivity of AZO + PMMA films with respect to pure AZO could be the carbonization of the polymer phase. Carbonization occurs as a pyrolysis–a thermochemical decomposition of organic material at elevated temperatures without oxygen (or any halogen). It involves the simultaneous change of chemical composition and physical phase. Pyrolysis is a type of thermolysis, and is most commonly observed in organic materials exposed to elevated temperatures. It is one of the processes involved in charring wood, starting at 200–300°C (390–570°F). It also occurs in fires where solid fuels are burning or when vegetation comes into contact with lava in volcanic eruptions. In general, pyrolysis of organic substances produces gas and liquid products and leaves a solid residue richer in carbon content, char. Pyrolysis leading to carbonization of polymers, such as PMMA, poly(vinylidene fluoride)—PVDF, poly (acrylonitrile)—PAN, and poly-N-vinylformamide—PNVF, during laser ablation and heat treatment has been reported before and used to modify the electrical conductivity of polymer coatings and fibers [34–36]. In case of AZO + PMMA films, carbonization could turn the network of the polymer nanoclusters and nanofibers into an additional efficient passage for the electric current (Figure 12) contributing to the overall increase of the electrical conductivity of the nanocomposite films and eventually to the increase of
The new concurrent MBMT-PLD/MAPLE deposition method has been successfully used to produce nanocomposite oxide-polymer thermoelectric films composed of AZO matrix with uniformly dispersed PMMA nanoparticles. The deposition was conducted at room temperature and on the cold (kept also at room temperature) substrate with no buffer gas. The volume fraction of the polymer material in the AZO matrix was chosen to be around 5%. These deposition conditions affected favorably the enhancement of the thermoelectric effect because the long-chain polymer molecules had a better chance to reach the substrate and mix with the inorganic matrix material without being decomposed. The chosen polymer fraction was small enough not to compromise substantially the electrical conductivity of the matrix. The PMMA nanoparticles allowed enhancing the phonon scattering with consequent decrease of thermal conductivity as compared to pure AZO film. The electrical conductivity did not drop after the addition of the second phase, as it would be expected from Wiedemann-Franz law, but grew three times higher over pure AZO film. This was assumingly due to the increase of the number of oxygen vacancies in AZO caused by PMMA or/and by the polymer carbonization at elevated temperatures. The thermoelectric figure of merit was improved by a factor of two. Further experiments would be necessary to establish the optimal proportion of the polymer nanoadditive in AZO matrix and other deposition parameters that maximize the thermoelectric performance of the nanocomposite films. Taking advantage of room-temperature deposition, optimized AZO nanocomposite films are expected to be used in real applications such as thin film modules deposited on flexible polymer substrates for ubiquitous harvesting of the waste heat.
A.D. appreciates the financial support from US Army Grant No. W911NF-15-1-0446.