The dielectric capacitor has been widely used in advanced electronic and electrical power systems due to their capability of ultrafast charging-discharging and ultrahigh power density. Nevertheless, their energy density is still limited by the low dielectric constant (≈ 2.2) of the commercial dielectric polypropylene (PP). The conventional enhancement strategy by embedding inorganic fillers in PP matrix is still difficult and challenging due to that PP hardly dissolves in any inorganic/organic solvent. In this work, we develop a new strategy including freeze-drying, surface functionalization and hot-pressing to incorporate Ti0.87O2 monolayers in PP film. A series of uniform composited Ti0.87O2@PP film has been successfully fabricated with Ti0.87O2 content range of 0-15 wt%. The maximum dielectric constant of the as-prepared Ti0.87O2@PP film is 3.27 when the Ti0.87O2 content is 9 wt%, which is about 1.5 times higher than that of pure PP. Our study provides a feasible strategy to embed two-dimensional material into commercial PP thin-film with superior dielectric performance for practical application.
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Freeze-drying and hot-pressing strategy to embed two-dimensional Ti0.87O2 monolayers in commercial polypropylene films with enhanced dielectric properties
https://doi.org/10.21203/rs.3.rs-59737/v3
This work is licensed under a CC BY 4.0 License
Journal Publication
published 04 Feb, 2021
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Ti0.87O2 nanosheets
polypropylene
freeze-drying
surface functionalization
hot pressing
dielectric constant
Recently, electrical energy storage devices have been attracting immense research interest with the worldly growing demand for energy requirement [1-3]. Dielectric capacitors play an important role in ultrafast charge-discharge capability, which are desired for a broad range of application such as hybrid electrical vehicles (HEVs), pulse power weapon and grid systems [4-7]. However, their energy density is still much lower than those of electrochemical devices including batteries and supercapacitors. Known that the commercial dielectric capacitors constructed from biaxially oriented polypropylene (PP) thin-films just show insufficient energy density of 1.2 J/cm at 640 MV/m with the low dielectric constant (εr = 2.2), severely limiting their potential applications on HEVs and grid systems [8, 9].
In principle, the energy density (W) of dielectric capacitors is determined by the applied electric field (E) and dielectric constant (εr) as
Wherein ε0 is the vacuum permittivity (8.85 × 10−12 F/m). Theoretically, improving dielectric constant (εr) is a very effective route to gain high energy density. Note that ceramics show high dielectric constant but low breakdown strength while polymers exhibit high breakdown strength but low dielectric constant. During the past decades, a lot of researches have been carried out on introducing ceramics nanofillers into polymer matrix for polymer/inorganic composites to realize the optimized dielectric properties.
Two-dimensional (2D) oxide nanosheets, which possess atomic or molecular thickness and infinite planar dimensions, have been attracting remarkable interests on energy storage fields due to their ultrahigh specific surface area, excellent mechanical flexibility and even quantum confinement. Typically, Ti0.87O2 atomic monolayer has been considered as a novel high-κ compound with a layered crystallographic structure in which TiO6 octahedra are edge-linked in a lepidocrocite-type 2D lattice. Previous experimental investigations demonstrate that a multilayer thin-film constructed from Ti0.87O2 nanosheets as building blocks on solid-state SrRuO3 substrate exhibit a high dielectric constant of ∼125 when the thicknesses down to 10 nm [10]. Such a dielectric constant is much larger than that of anatase (εr = 30-40) and rutile TiO2 (εr = 80-100) [11]. The excellent high-κ behavior should be originated from the existence of abundant Ti vacancies rather than oxygen vacancies which act as carrier traps and high-leakage paths.
Motivated by these intrinsic merits, incorporation of oxide nanosheets into polymer matrix to form polymer nanocomposites has recently emerged as a very promising strategy to realize the dielectric thin-film with high dielectric constant and enhanced energy density. For example, Wen et al. revealed that Ti0.87O2 nanosheets are desirable inorganic fillers in poly(vinylidene fluoride) (PVDF) for developing flexible thin-film based capacitor [12]. They successfully incorporated Ti0.87O2 nanosheets into the PVDF matrix through a facile solution casting strategy using NMP as the solvent, which delivers an energy density enhancement of 190% over the bare PVDF. Furthermore, Li et al. reported the successful incorporation of Ca2Nb3O10 nanosheets into ferroelectric PVDF matrix to realize a high energy density of 36.2 J/cm [13]. However, compared with commercial PP, PVDF exhibits high dielectric loss as well as the ferroelectric hysteresis resulting in an energy loss at alternating voltage [14, 15], which significantly restricts its application in high frequency circuits. It is highly desired to incorporate monolayer oxide nanosheets in commercial PP system toward practical application. Unfortunately, the conventional liquid-casting strategy is not viable in commercial PP system due to PP hardly dissolves in any inorganic/organic solvent. The main challenge remains the thin-film fabrication of PP-based polymer/inorganic fillers composites.
Our previous study has realized the application of Ti0.87O2 nanosheets on lithium-ion storage, resistive random access memory (RRAM) and CO2 electroreduction, respectively [16-18]. In this work, we develop a new strategy including freeze-drying, surface functionalization and hot-pressing to incorporate Ti0.87O2 monolayers in commercial PP film. High-quality colloidal solution consisted of Ti0.87O2 nanosheets with atomic thickness was obtained by a soft-chemical exfoliation route from its layered, protonic precursor. Subsequently, aerogel-like, solid-state Ti0.87O2 nanosheets was freeze-dried from the corresponding single-layer colloidal solution and then surface-modified by coupling agent KH550. Then such an aerogel was successfully embedded in PP matrix with well controllable Ti0.87O2 nanosheets concentration through a hot-pressing strategy (Fig. 1). The resulting Ti0.87O2@PP composited thin-film displayed a remarkable enhancement of 134% on dielectric constant with an optimized Ti0.87O2 concentration of 9 wt%. Our study provides a feasible strategy to embed two-dimensional material into commercial PP thin-film with superior dielectric performance for practical energy-storage application.
Synthesis of Ti0.87O2 nanosheet colloidal suspension: Ti0.87O2 nanosheets colloidal suspension were prepared via a multistep soft chemical process. In brief, K2CO3, Li2CO3 and TiO2 were firstly mixed in a molar ratio of 2.4 : 0.8 : 10.4, and then heated at 800 ℃ for 30 min to be decarbonated. After cooling, the powder was ground and then calcined at temperatures of 800-1100 ℃ for 20 h to obtain K0.8Ti1.73Li0.27O4. Then this layered alkali metal titanate was mixed with 1 M HCl solution and stirred for several days to be converted into H1.07Ti1.73O4·nH2O. In this step, the solution-to-solid ratio was 100 mL/g, and the acid solution was replaced daily with a fresh one by decantation. A degree of removal of alkali metal ions was studied by chemical analysis. The product was collected by filtration and washed with copious quantities of water. Next, the obtained H1.07Ti1.73O4·nH2O was further ion-exchanged with 0.1 M tetrabutylammonium hydroxide (TBAOH) at a solution-to-solid ratio of 100 g/mL and then shaken for 7 days, which also led to the osmotic swelling. After exfoliation by ultrasonic, well-dispersed unilamellar nanosheets of Ti0.87O2 were obtained as a stable colloidal suspension.
Preparation of freeze-dried aerogel and surface-modified Ti0.87O2 nanosheets: 100 ml Ti0.87O2 nanosheets colloidal solution with a concentration of 4.0 g/L was added in a 300 ml beaker and then freeze-dried in the freeze dryer for 4 days. Afterwards, loose gel-like sample with a cotton-like shape can be directly obtained. The KH550 modification process of freeze-dried Ti0.87O2 was as follows. First, 2 ml silane coupling agent KH550 was dispersed in 5 ml ethanol. Then 3 g freeze-dried Ti0.87O2 nanosheets and KH550/ethanol mixture were added together into 10 ml ethanol. After being stirred for 24 h, the mixture was centrifuged at 10000 rpm for 5min to collect the white slurry at the bottom of the tube and washed by ethanol three times to remove the excess KH550. The product was then dried in the oven at 60 ℃ for 12 h and the white KH550 modified freeze-dried Ti0.87O2 powder can be obtained.
Preparation of Ti0.87O2@PP composite film: PP particles (melt flow rate 3.2 g/10 min) were purchased from Borealis and the xylene (AR) was purchased from Aladdin. To homogeneously blend the as-prepared Ti0.87O2 and PP particles, firstly, PP particles were ball milled into powder form as follows: 10 g PP commercial particles and 100 ml xylene were added in a 400 ml beaker and heated in oil bath at 140 ℃. Until PP particles dissolved completely, the uniform solution was placed at room temperature for 2 days to volatilize xylene. The resulting white PP block by this treatment was cut to small pieces. 3 g PP pieces and grinding medium (steel balls of different diameter, 5, 10, and 20 mm, with a weight ratio of 7:2:1 in sequence) were put into a 50 ml sealed agate jar and milled in a high-energy planetary ball mill system with an autorotation speed of 400 rpm at room temperature for 2 h. Then the ball-milled powders were passed through an 80-mesh sieve. The screened PP powders and as-prepared Ti0.87O2 were mixed in different weight ratio of Ti0.87O2 (0%, 3%, 6%, 9%, 12%, 15%) and milled with an autorotation speed of 600 rpm at room temperature for 4 h. The well mixed powder was heated at 180 ℃ for 15 min without pressure and then kept at 180 ℃ under 12 MPa for 15 min in the press vulcanizer. Upon cooling to room temperature, the Ti0.87O2@PP composite film was removed from the plate for further analysis.
Materials characterization
The phase structure was characterized by a Bruker D8-A25 diffractometer using Cu Kα radiation (λ = 1.5406 Å). Sample morphologies were characterized using a FEI Navo Nano SEM 450 field-emission scanning electron microscope (SEM) and a Bruker Dimension Icon atomic force microscope (AFM). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were performed using a Philips CM 200 FEG field emission microscope at an acceleration voltage of 300 kV. The film thickness test was carried out via a step profiler (SD-LX).
Dielectric measurement
The dielectric properties of the composite samples were measured using LRC digital bridge (HIOKI IM3533) in the frequency range of 10 Hz to 100 kHz. Prior to measurement, a thin cover of the silver paste was coated on the two sides of all samples. The electrical breakdown test was carried out via high voltage DC generator (ZGF-120/2). The generator applies an increasing voltage on the sample until the sample breakdown. Multimeter (MASTECH MY65) is used to measure the voltage of the sample. We carried out the electrical breakdown test three times on each sample and the real data is the average value of three measurements.
Firstly, titanate precursor (K0.8Ti1.73Li0.27O4) was obtained by a solid-state calcination process, and protonic H1.07Ti1.73O4∙nH2O bulk was prepared from K0.8Ti1.73Li0.27O4 by acid exchange as previously reported (Fig. 2a) [19]. Then 2D Ti0.87O2 nanosheets with atomic thickness were obtained by a liquid-exfoliation strategy from the layered H1.07Ti1.73O4∙nH2O precursor with an aqueous solution of tetrabuthylammonium hydroxide (TBAOH) [20]. After the soft-chemical exfoliation, highly stable, transparent colloidal suspension of Ti0.87O2 nanosheets exhibits a clear Tyndall light scattering (Fig. 2b). As demonstrated by the atomic force microscopy (AFM) characterization, the thickness of the Ti0.87O2 nanosheets is approximately 2-3 nm (Fig. 2c-d). The transmission electron microscopy (TEM) images exhibit two-dimensional ultrathin sheets with lateral dimensions up to 4 μm while fragments and folded edges were also observed in small amounts (Fig. 2e-f). The SAED pattern taken from the aggregation of several sheets exhibits polycrystalline diffraction rings (Fig. 2g).
Considering that PP matrix is insoluble in most of organic/inorganic solvent, it is necessary to recover our Ti0.87O2 nanosheets from the colloidal suspension and then embed it into PP through a non-solution based strategy. We used the freeze-drying technique to obtain the aerogel sample of the atomically flat Ti0.87O2 nanosheets. After freeze-dried of the colloidal solution with a concentration of 4.0 g/L in vacuum for 4 days, white, loose aerogel-like sample with a cotton-like shape was obtained as shown in Fig. 3a. A set of sharp diffraction lines indexed as 0k0 (k = 1, 2, 3, 4, 5) diffraction peaks can be detected in the corresponding XRD pattern (Fig. 3b). The basic d-spacing of (010) peak is estimated to be 1.73 nm, which is indicative of a lamellar structure with a gallery height of 1.73 nm. SEM observation further confirms that the freeze-dried product is consisted of numerous thin-flakes with curved edges and a thickness of 20-30 nm (Fig. 3c-d), suggesting that about 10-15 atomically thin nanosheets were restacked along [00l] direction during the freeze-drying process. Note that such a thickness is also drastically smaller than that of pristine layered H1.07Ti1.73O4∙nH2O bulk, leading to a large aspect ratio of more than 100.
Subsequently, the freeze-dried Ti0.87O2 aerogel were used as the inorganic fillers embedded in PP matrix to fabricate composited thin-film through a hot-pressing process. We found that commercial PP particles are hardly ball-milled directly into powder form. However, dissolution PP particles in xylene at 140 ℃ and cooled down to room temperature results in the recrystallization of PP, which can be ball-milled into powder easily. After the hot-pressing of the freeze-dried Ti0.87O2 aerogel and the ball-milled PP powder into a composite thin-film, the characteristic (0k0) peaks of Ti0.87O2 disappeared (Fig. 4a). We considered that it should be attributed to the drastic aggregation of the Ti0.87O2 aerogel in PP matrix. Such a problem has been successfully solved by the surface modification using coupling agent KH550 in ethanol. After such a pretreatment, the Ti0.87O2 nanosheets still maintain a layered crystal structure (Fig. 4b) and (010) peaks can be clearly detected from the composited thin-film as shown in Fig. 4c-d. We further confirm the uniformity of this composited film by recording the XRD patterns from four distinct positions on the surface of this nanocomposite film. As shown in Fig. 4f, these four positions 1-4 show analogous XRD patterns with the appearance of (010) peaks from our Ti0.87O2 fillers. This result demonstrates that Ti0.87O2 fillers are homogeneously dispersed in PP polymer.
Fig. 5 show the characteristic scanning electron microscopy (SEM) images of the resulting Ti0.87O2@PP composite films with various concentration of KH550/Ti0.87O2 nanosheet fillers from 0 wt% to 15 wt%. One can see that KH550/Ti0.87O2 nanosheet fillers are well dispersed in PP matrix without any accumulation until the concentration up to 9% (Fig. 5a-d). However, the agglomeration of Ti0.87O2 flakes with size of 500-1000 nm can be observed when its concentration up to 12-15 wt% (Fig. 5e-f). In addition, the thickness of this series of Ti0.87O2@PP composite films is determined into 230-285 µm by via a step profiler (SD-LX) as listed in Table 1.
Table 1. Thickness of Ti0.87O2@PP composited films with different concentration of Ti0.87O2 fillers.
|
concentration of Ti0.87O2 filler (wt%) |
thickness (μm) |
|
0 |
239.5 |
|
3 |
259.0 |
|
6 |
285.4 |
|
9 |
220.3 |
|
12 |
230.1 |
|
15 |
246.4 |
The dielectric constant (εr) of Ti0.87O2@PP composite films at different frequencies is shown in Fig. 6a. It can be seen a downward trend of dielectric constant in low frequency (<103 Hz) in all of the Ti0.87O2@PP composite films, and such a downward trend generally become stable in high frequency range (>103 Hz). This result implies excellent compatibility between KH550 treated Ti0.87O2 and pure PP matrix. The dielectric constant of the hybrid film with different Ti0.87O2 concentrations at 103 Hz is given in Fig. 6b. The dielectric constant increases with Ti0.87O2 weight fraction to a maximum of 3.27 at 9 wt%, which is 134% higher than pure PP film prepared through the same method. Such a result is attributed to the giant dielectric constant of Ti0.87O2 originated from the existence of abundant Ti vacancies which act as carrier traps and high-leakage paths [10]. However, further increase of the Ti0.87O2 content than this threshold value of 9 wt% leads to a steady decrease in the dielectric constant. This phenomenon is thought to be caused by accumulation of the nanosheets at high weight fraction of Ti0.87O2 filler. Fig. 6c shows the dielectric loss of all Ti0.87O2@PP composites at different frequencies. The dielectric loss tangent increases to the maximum value when the concentration of Ti0.87O2 filler is 9 wt%, well consistent with the result in Fig. 6a. The breakdown field (E) trend of a series of Ti0.87O2@PP composite film in Fig. 6c suggests that the increase of dielectric constant leads to a decrease in dielectric strength of the composited thin-film. An important reason should be the nanosheets incorporated in PP may induce space void at the Ti0.87O2 nanosheet/ PP matrix interface. Note that such a decrease is much lower than that of the thin-film fabricated by solution-based method (E = 33 MV/m) as reported previously [21]. The superior property should benefit from the large aspect ratio of our Ti0.87O2 nanosheets fillers, which may act as barrier for relieving the diffusion of electrical tree, then restraining the avalanche of electric breakdown as a result [22].
In summary, we successfully developed a new strategy including freeze-drying, surface functionalization and hot-pressing to incorporate Ti0.87O2 monolayers in PP film. The Ti0.87O2 nanosheets were firstly freeze-dried into aerogel form and surface modified by KH550 coupling agent, and such an aerogel was successfully embedded in PP matrix with well controllable Ti0.87O2 nanosheets concentration through a hot-pressing strategy. XRD and SEM characterizations of the Ti0.87O2@PP composite film demonstrate that the Ti0.87O2 fillers are uniformly dispersed in the PP matrix. The dielectric constant of the Ti0.87O2@PP composite film exhibits maximum value of 3.27 when the Ti0.87O2 content is 9 wt%, which is about 1.5 times higher than that of pure PP film. Our study provides a feasible approach to embed two-dimensional material into commercial PP thin-film with high dielectric constant for practical energy-storage applications.
Acknowledgements
This work was financially supported by the Researching Program of State Grid Corporation of China (GYW17201800011).
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