A Novel Composite from Nanodispersed Silica and an Organic Ferroelectric of Diisopropylammonium Bromide: Preparation, Characterization and Dielectric Properties
2019 Volume 60 Issue 10 Pages 2132-2136
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2019 Volume 60 Issue 10 Pages 2132-2136
A novel composite consisting of silica nanoparticles (SiO2) and a typical organic ferroelectric of diisopropylammonium bromide (C6H16NBr, DIPAB) was prepared at different composition mass ratios. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques were utilized for characterization of the composite. Experiments for testing ferroelectricity was conducted from room temperature to 165°C under a weak electric field (5 V·cm−1) at a low frequency of 1 kHz. The results revealed a reduction of phase transition temperature with increasing SiO2 content. The Landau theories developed for bonded and isolated ferroelectric particles were used to explain the obtained anomalies.
Phase transition in SiO2+DIPAB composite.
Organic ferroelectrics are multifunctional candidates for future organic electronics owing to numerous advantages over inorganic ones as low cost, light weight, mechanical flexibility, low processing temperature and nontoxicity to environment.1–3) Many applications of organic ferroelectrics have been known as long-missing nonvolatile memory elements in organic optical devices, organic ferroelectric diodes, flexible nonvolatile transistors, etc. Diisopropylammonium bromide (DIPAB) is one of the most promising molecular organic ferroelectrics. DIPAB has a strong ferroelectricity with high polarization value (23 µC/cm2) comparable to BaTiO3, high Curie point (Tc = 152°C), low dielectric loss and strong piezoelectric response.4–7)
Nowadays, practical applications of primitive ferroelectrics are limited because of modern technological requirements. For DIPAB, low mechanic strength and difficulties in preparation of big single crystals are a huge drawback. The best solution for this problem is combining it with reinforcing dielectric materials. As reported in literature, DIPAB crystals grown in epoxy resin (EP)8) and polyvinylidene fluoride (PVDF)9) showed significant improvement of its mechanical strength, thermal stability, dielectric performance and ferroelectricity. In general cases for ferroelectrics, several ferroelectric characteristics as phase transition, domain-wall structure, spontaneous polarization, dielectric susceptibility, conductivity, etc. can be adjusted under the influence of dielectric inclusion.8–19) For instance, the combination of ferroelectrics with cellulose,10–15) silicon,16) glass and alumina17–19) led to the shift of Curie point toward lower or higher temperatures.
Silicon dioxide (SiO2) takes part in electronics technology in the role of insulating material. In the area of nanoelectronics, SiO2 is commonly used as a reinforcing component to synthesize advanced electronics materials having several promising properties for designing electronics devices with more compact, thinner and lighter high-performance appliances.20–22) Known that SiO2 nanoparticles with the large specific area and high hydrophilicity play an important role in adjusting properties of ferroelectric filler.23) In the present work, silicon dioxides (SiO2) nanoparticles was utilized to synthesize a composite with DIPAB inclusion at different composition mass ratios. The structure and phase transition of SiO2/DIPAB composite are investigated. The obtained results will be discussed thoroughly.
Before preparation of SiO2/DIPAB composite, SiO2 nanoparticles and DIPAB crystals were prepared. Silica nanoparticles were synthesized by sol-gel method and stored in the form of nanodispersed hydrosol24) with the particle size ranged in 20–200 nm (Fig. 1(a)). The particle size distribution of SiO2 was examined using a Zetasizer analyzer with a detection range of 0.3 nm–10 µm (Malvern Instruments Limited, Malvern, UK). For measuring SiO2 particle size distribution, 100 µl of silica hydrosol was dispersed in 4 ml distilled water. DIPAB crystals were grown using a wet chemical method according the procedure as described in detail in literature as follows.25) Firstly, diisopropylamine reacted with a 48% aqueous solution of HBr (molar ratio of 1:1). As a result, a colorless solution was obtained and evaporated after a few days to get a transparent solid. DIPAB crystals were then formed by subsequent recrystallization in a mixture of methanol and ethanol (molar ratio of 1:1) at room temperature. The obtained DIPAB was heated at 152°C at 30 min to remove residual water and to remove the unstable phase with P212121 symmetry which could exist in fresh DIPAB samples.24) Reliability of the synthesized DIPAB was confirmed by DSC signal (Fig. 2(a)) and XRD pattern (Fig. 2(b)). The DSC scanning rate was maintained at 2 K/min using a DSC-250 (TA Instruments, USA), while XRD pattern was taken at room temperature with the wavelength of 1.5406 Å and a voltage of 40 kV on a Rigaku Ultima IV X-ray diffractometer (Rigaku Americas Corporation, USA). As seen in Fig. 2(a), an endothermic peak appears at 152°C which refers to the transition from monoclinic P21 to nonpolar P21/m phase.25,26) In addition, its XRD pattern corresponds to the monoclinic P21 ferroelectric phase at room temperature in the presence of main peaks at 2θ = 12.5° (001), 16.7° (110), 17.3° $(11\bar{1})$, 21.4° (101), 21.98° (020), 22.67° $(20\bar{1})$, 24° (111), 25.36° (002), 26.65° $(20\bar{2})$, 27.5° (012), 28.87° $(21\bar{2})$, 30.78° (121), 31.77° $(22\bar{1})$, 33° (201), 36.9° $(21\bar{3})$, 42.18° (230) and 47.6° (131).25,26) Based on the obtained results, the starting materials for preparation of SiO2/DIPAB composite were ready.
(a) Size distribution and (b) XRD pattern for nanodispersed SiO2 hydrosol.
(a) DSC signal and (b) XRD pattern for the synthesized DIPAB.
The scheme for preparation of SiO2/DIPAB composite is presented in Fig. 3. Firstly, a saturated solution of DIPAB in methanol was prepared. Then, the DIPAB solution and nanodispersed SiO2 hydrosol were mixed together by taking each part from them out at different SiO2:DIPAB mass ratios of 0.2:1, 0.5:1, 1:1, 4:1 and 7:1 using a magnetic stirrer. Stirring was kept for 6 h in a closed bottle. Next, all the mixtures were put in drying oven for about 48 h to form a glue which was then heated at 120°C for 6 h to remove residual water. The fresh samples were crushed in motar and compressed into tablets of 6 mm in diameter and 1 mm in thickness. Noted that the DIPAB was also compressed into tablets and utilized for experiments for comparison. As for pure DIPAB, the synthesized composite was preheated at 152°C for 30 min before conducting further experiments in this study.
Scheme for preparation of SiO2/DIPAB composite.
The information of functional groups for SiO2, DIPAB and SiO2/DIPAB composite was taken on a Bruker Tensor 37 spectrophotometer (USA). The phase transition in the composite was measured on a model GW Instek LCR-821 meter at 1 kHz. The setup for measuring the third harmonic coefficient for the composite included a harmonic oscillator with an operating frequency of 1 kHz. The signal was taken from a resistor connected in series with the sample, then fed to a digital spectrum analyzer consisting of a computer with a 24-bit ZET 230 analog-digital converter and ZETView software. During the measurement amplitudes of the third harmonic and the main signal were recorded. The ferroelectric hysteresis P-E loops were taken using a Precision LC tester (Radiant Technology, Korea) at temperature of 147°C, frequency of 50 Hz and maximum field amplitude of 40 kV/cm. The relative measurement error did not exceed 0.1%. The heating speed was maintained at 2 K/min. The temperature for all experiments was stabilized with an accuracy of 0.1 K. The relative measurement error did not exceed 0.1%.
Figure 4(a) shows XRD patterns for SiO2/DIPAB composite at different composition mass ratios at room temperature. The crystalline region centered at 2θ = 23.3° for SiO2 component appeared for all composite samples and became more pronounced with increasing SiO2 content, while characteristic peaks of DIPAB – less pronounced. Besides, the XRD peaks of DIPAB component were slightly shifted toward higher diffraction angles, probably, due to the strong interaction between DIPAB and SiO2 through hydrogen bonds.10) This is reasonable because DIPAB is a hydrogen-containing ferroelectric and SiO2 is a hydrophilic material from which water can be completely removed only at higher than 500°C.27) The higher the SiO2 content was, the higher the isolation of DIPAB crystals in the composite and therefore the stronger the interaction could be.
(a) XRD patterns and (b) DSC signal for SiO2/DIPAB composite at different composition mass ratios at room temperature. The results for DIPAB and SiO2 are added for comparison.
For SiO2/DIPAB composite, the DSC signal has a typical shape as for pure DIPAB with one endothermic peak detected in the entire temperature range from room temperature to 165°C (Fig. 4(b)). To clearly present the endothermic peaks, the DSC results from room temperature to 130°C is not shown. Interestingly, the indicated peaks shifted toward lower temperatures with increasing SiO2 content.25,27) The nature of this anomaly will be thoroughly discussed in further experimental results.
The study results for phase transition of SiO2/DIPAB at different composition mass ratios are shown in Fig. 5(a) in the form of the temperature dependences of dielectric constant ε(T). It is revealed that the phase transition temperatures for all composite samples were lower than those for the bulk DIPAB (Tc = 252°C). It is worth to notice that the shape of ε(T) and the Curie point Tc = 252°C detected for the synthesized DIPAB are in good agreement with literature data.6,28) With increasing the SiO2 content, the phase transition temperature To decreased and the transition peaks became more blurred. At relatively small DIPAB content, dependences of ε(T) demonstrate an almost monotonic increase in dielectric constant upon heating in the presence of an unclear transition peak. The increase in dielectric constant was apparently due to the contribution of the Maxwell-Wagner polarization which was related to redistribution of the charge density at interfaces between the SiO2 nanoparticles and DIPAB inclusion. The similar results for the influence of SiO2 nanoparticles on phase transition of ferroelectrics were reported in literature.23) However, the phase transition for such ferroelectrics shifted toward higher temperatures as compared to the bulk ones.
Temperature dependences of (a) dielectric constant, (b) the third harmonic coefficient and (c) P-E hysteresis loops for SiO2/DIPAB composite at different composition mass ratios. The results for DIPAB and SiO2 are added for comparison.
Due to strong diffusion in phase transition, the nonlinear dielectric properties of nanocomposites in the term of temperature dependences of the third harmonic coefficient γ3ω(T) were also investigated to establish the temperature interval in which the ferroelectric phase occurred (Fig. 5(b)). Upon heating, the third harmonic coefficient in the vicinity of phase transition point decreased rapidly to minimum values, and then changes insignificantly with further increasing temperature. The values of phase transition temperature in the composite at different composition mass ratios are listed in Table 1. It can be seen in Table 1 that the Curie point of DIPAB decreased by 1–12°C with increasing SiO2 content.
The presence of ferroelectricity in synthesized composite was confirmed by hysteresis P-E loops as shown in Fig. 5(c). The result for the used polycrystalline DIPAB is also added for comparison with a typical shape of P-E curve.7) As indicated that the P-E ferroelectric loops were detected for all composite samples at different mass ratios. With decreasing DIPAB content, the remnant polarization decreased while the coercive field increased. This anomaly can be explained by intensifying the interaction between SiO2 and DIPAB component, inhibiting the repolarization process as discussed below in this study. Besides, the decrease in DIPAB content led also to the change of P-E loop from the characteristic DIPAB shape into the near-ellipse ones with a slight sphere due to some leakage conductance.
Despite the fact that both DIPAB and SiO2 are hydrogen-containing materials and the interaction between them through hydrogen bonds could exist, the observed reduction of phase transition temperature couldn’t be related to this interaction due to the following arguments. Firstly, the SiO2/DIPAB interaction leads to the fixation of polarization in DIPAB and results in the increase in phase transition temperature.10) This conflicts with the reduction of phase transition temperature for SiO2/DIPAB composite as observed above. Secondly, from our point of view, the hydrogen bonds were probably broken or too weak at high temperatures of 140–152°C, at which the phase transition in the composite occurred. In this regard, the influence of dielectric/ferroelectric interaction on phase transition in SiO2/DIPAB composite can be negligible.
Another factor which must be considered is ferroelectric/ferroelectric interaction. Indeed, based on the Landau-Ginzburg-Devonshire theory,15) the phase transition temperature To of a heterogeneous system from bounded particles can be determined by the energy of dipole-dipole interaction. Depending on the dipole orientation, this energy could be positive and results in the reduction of phase transition temperature.15)
The final contribution to the reduction of phase transition temperature could be the size effects. The influence of size effects was probably significant at relatively high SiO2 content because the DIPAB will be naturally distributed in the composite as isolated particles with smaller size. Indeed, the Landau phenomenological theory developed for isolated particles also predicted that the temperature of structural phase transition should decrease with decreasing particle size if the order parameter or the value of the exchange integral at the particle boundaries is less than those of the volume samples.29,30)
In summary, the study results showed successful synthesis of a nanocomposite from SiO2 nanoparticles and an organic ferroelectric of diisopropylammonium bromide at different composition mass ratios. Under the influence of SiO2 nanoparticles, the phase transition temperature in the nanocomposite was lower than those in volume diisopropylammonium bromide. Besides, the transition point shifted toward lower temperatures with increasing SiO2 content. The size effects and dipole-dipole interaction between ferroelectric particles were responsible for this anomaly. The dependence of phase transition temperature in diisopropylammonium bromide on SiO2 content is valuable for improving operating parameters of electronics devices. Overall, the obtained results suggest a new method of using SiO2 to adjust properties of organic ferroelectrics.
