Advanced humidity sensing properties of CuO ceramics

This research explores the capacitive humidity sensing properties of CuO ceramic, selected for its simplicity as an oxide and ease of fabrication, in addition to its remarkable dielectric properties. The CuO sample was fabricated by sintering at 980 °C for 5 h. A microstructure with a relative density of 88.9% was obtained. X-ray diffraction confirmed the formation of a pure CuO phase. Broadband dielectric spectroscopy revealed that the observed giant dielectric properties at room temperature (RT) were attributed to extrinsic effects, including the internal barrier layer capacitor and sample-electrode contact effects. A key focus of this study was to examine the giant dielectric properties of CuO ceramic as a function of relative humidity (RH) at RT and frequencies of 102 and 103 Hz. It was observed that the capacitance of CuO continuously increased with rising RH levels, ranging from 30 to 95%. Notably, the maximum hysteresis errors were constrained to 2.3 and 3.3% at 102 and 103 Hz, respectively. Additionally, the CuO ceramic demonstrated very fast response and recovery times, approximately 2.8 and 0.95 min, respectively. The repeatability of the humidity response of the capacitance was also established. Overall, this research highlights the high potential of CuO as a giant dielectric material for application in humidity sensors.

Significantly improved giant dielectric properties and enhanced nonlinear coefficient of Ni2+ doped CaCu3Ti4O12/CaTiO3 composites

CaCu_3-xNi_xTi₄O₁₂/CaTiO₃ ceramic composites were fabricated using initial Ca₂Cu_2-xNi_xTi₄O₁₂ compositions (x = 0, 0.05, 0.10, and 0.20) to improve the dielectric properties (DPs) of the CaCu₃Ti₄O₁₂ ceramics. CaCu₃Ti₄O₁₂ and CaTiO₃ phases were confirmed. Microstructural analysis and Rietveld refinement showed that the Ni²⁺ dopant might substitute the Cu²⁺ sites of the CaCu₃Ti₄O₁₂ structure. The average grain sizes of CaCu₃Ti₄O₁₂ (4.1-5.6 μm) and CaTiO₃ (1.2-1.4 μm) changed slightly with the Ni²⁺ doping concentration. The best DPs were obtained for the CaCu_3-xNi_xTi₄O₁₂/CaTiO₃ with x = 0.2. The loss tangent was significantly reduced by an order of magnitude compared to that of the undoped composite, from tanδ∼0.161 to ∼0.016 at 1 kHz, while the dielectric permittivity slightly decreased from ε'∼5.7 × 10³ to ∼4.0 × 10³. Furthermore, the temperature dependence of ε' could be improved by doping with Ni²⁺. The improved DPs were caused by the enhanced electrical responses of the internal interfaces, which resulted in enhanced non-Ohmic properties. The largest nonlinear coefficient (α∼7.6) was obtained for the CaCu_3-xNi_xTi₄O₁₂/CaTiO₃ with x = 0.05. Impedance spectroscopy showed that the CaCu_3-xNi_xTi₄O₁₂/CaTiO₃ composites consisted of semiconducting and insulating components. The DPs of CaCu_3-xNi_xTi₄O₁₂/CaTiO₃ were explained based on the space-charge polarization at the active-interfaces.

Characterizations and Significantly Enhanced Dielectric Properties of PVDF Polymer Nanocomposites by Incorporating Gold Nanoparticles Deposited on BaTiO3 Nanoparticles

Poly(vinylidene fluoride) (PVDF) nanocomposites were fabricated by incorporating BaTiO3 nanoparticles (particle size of ~100 nm, nBT), which were deposited by Au nanoparticles (nAu) with an average particle size of 17.8 ± 4.0 nm using a modified Turkevich method. Systematic characterizations on the synthesized nAu-nBT hybrid nanoparticles and nAu-nBT/PVDF nanocomposites with different contents of a filler were performed. The formation of nAu-nBT hybrid nanoparticles was confirmed with the calculated nAu:nBT ratio of 0.5:99.5 wt.%. The homogeneous dispersion of nAu and nBT in the PVDF polymer was obtained due to the interaction between the negative surface charge of the nAu-nBT filler (compared to that of the nBT) and polar β-PVDF phase, which was confirmed by the zeta potential measurement and Fourier-transform infrared spectroscopy, respectively. A significantly increased dielectric permittivity (ε' ~ 120 at 103 Hz) with a slight temperature-dependent of <±15% ranging from -20 to 140 °C was obtained. Notably, a low loss tangent (tanδ < 0.08) was obtained even at a high temperature of 140 °C. Therefore, incorporating a PVDF polymer with nAu-nBT hybrid nanoparticles is an attractive method to improve the dielectric properties of a PVDF polymer for dielectrics applications.

Gold-Nanoparticle-Deposited TiO2 Nanorod/Poly(Vinylidene Fluoride) Composites with Enhanced Dielectric Performance

Flexible dielectric polymer composites have been of great interest as embedded capacitor materials in the electronic industry. However, a polymer composite has a low relative dielectric permittivity (ε' < 100), while its dielectric loss tangent is generally large (tanδ > 0.1). In this study, we fabricate a novel, high-permittivity polymer nanocomposite system with a low tanδ. The nanocomposite system comprises poly(vinylidene fluoride) (PVDF) co-filled with Au nanoparticles and semiconducting TiO2 nanorods (TNRs) that contain Ti3+ ions. To homogeneously disperse the conductive Au phase, the TNR surface was decorated with Au-NPs ~10-20 nm in size (Au-TNRs) using a modified Turkevich method. The polar β-PVDF phase was enhanced by the incorporation of the Au nanoparticles, partially contributing to the enhanced ε' value. The introduction of the Au-TNRs in the PVDF matrix provided three-phase Au-TNR/PVDF nanocomposites with excellent dielectric properties (i.e., high ε' ≈ 157 and low tanδ ≈ 0.05 at 1.8 vol% of Au and 47.4 vol% of TNRs). The ε' of the three-phase Au-TNR/PVDF composite is ~2.4-times higher than that of the two-phase TNR/PVDF composite, clearly highlighting the primary contribution of the Au nanoparticles at similar filler loadings. The volume fraction dependence of ε' is in close agreement with the effective medium percolation theory model. The significant enhancement in ε' was primarily caused by interfacial polarization at the PVDF-conducting Au nanoparticle and PVDF-semiconducting TNR interfaces, as well as by the induced β-PVDF phase. A low tanδ was achieved due to the inhibited conducting pathway formed by direct Au nanoparticle contact.

Significantly Enhanced Dielectric Properties of Ag-Deposited (In1/2Nb1/2)0.1Ti0.9O2/PVDF Polymer Composites

The enhanced dielectric permittivity (ε') while retaining a low loss tangent (tanδ) in silver nanoparticle-(In1/2Nb1/2)0.1Ti0.9O2/poly(vinylidene fluoride) (Ag-INTO/PVDF) composites with different volume fractions of a filler (fAg-INTO) was investigated. The hybrid particles were fabricated by coating Ag nanoparticles onto the surface of INTO particles, as confirmed by X-ray diffraction. The ε' of the Ag-INTO/PVDF composites could be significantly enhanced to ~86 at 1 kHz with a low tanδ of ~0.044. The enhanced ε' value was approximately >8-fold higher than that of the pure PVDF polymer for the composite with fAg-INTO = 0.5. Furthermore, ε' was nearly independent of frequency in the range of 102-106 Hz. Therefore, filling Ag-INTO hybrid particles into a PVDF matrix is an effective way to increase ε' while retaining a low tanδ of polymer composites. The effective medium percolation theory model can be used to fit the experimental ε' values with various fAg-INTO values. The greatly increased ε' primarily originated from interfacial polarization at the conducting Ag nanoparticle-PVDF and Ag-INTO interfaces, and it was partially contributed by the high ε' of INTO particles. A low tanδ was obtained because the formation of the conducting network in the polymer was inhibited by preventing the direct contact of Ag nanoparticles.

Suppressing loss tangent with significantly enhanced dielectric permittivity of poly(vinylidene fluoride) by filling with Au-Na1/2Y1/2Cu3Ti4O12 hybrid particles

Three-phase gold nanoparticle–Na_1/2Y_1/2Cu₃Ti₄O₁₂ (Au–NYCTO)/poly(vinylidene fluoride) (PVDF) composites with 0.095–0.487 hybrid particle volume fractions (f) were fabricated. Au nanoparticles with a diameter of ∼10 nm were decorated on the surfaces of high-permittivity NYCTO particles using a modified Turkevich's method. The polar β-PVDF phase was confirmed to exist in the composites. Significantly enhanced dielectric permittivity of ∼98 (at 1 kHz) was obtained in the Au–NYCTO/PVDF composite with f_Au–NYCTO = 0.487, while the loss tangent was suppressed to 0.09. Abrupt changes in the dielectric and electrical properties, which signified percolation behavior, were not observed even when f_Au–NYCTO = 0.487. Using the effective medium percolation theory model, the percolation threshold (f_c) was predicted to be at f_Au–NYCTO = 0.69, at which f_Au was estimated to ∼0.19 and close to the theoretical f_c value for the conductor–insulator composites (f_c = 0.16). A largely enhanced dielectric response in the Au–NYCTO/PVDF composites was contributed by the interfacial polarization effect and a high permittivity of the NYCTO ceramic filler. Au nanoparticles can produce the local electric field in the composites, making the dipole moments in the β-PVDF phase and NYCTO particles align with the direction of the electric field.

Three-phase gold nanoparticle–Na_1/2Y_1/2Cu₃Ti₄O₁₂ (Au–NYCTO)/poly(vinylidene fluoride) (PVDF) composites with 0.095–0.487 hybrid particle volume fractions (f) were fabricated.

Green synthesis of silver nanoparticles in aloe vera plant extract prepared by a hydrothermal method and their synergistic antibacterial activity

Background

There is worldwide interest in silver nanoparticles (AgNPs) synthesized by various chemical reactions for use in applications exploiting their antibacterial activity, even though these processes exhibit a broad range of toxicity in vertebrates and invertebrates alike. To avoid the chemical toxicity, biosynthesis (green synthesis) of metal nanoparticles is proposed as a cost-effective and environmental friendly alternative. Aloe vera leaf extract is a medicinal agent with multiple properties including an antibacterial effect. Moreover the constituents of aloe vera leaves include lignin, hemicellulose, and pectins which can be used in the reduction of silver ions to produce as AgNPs@aloe vera (AgNPs@AV) with antibacterial activity.

Methods

AgNPs were prepared by an eco-friendly hydrothermal method using an aloe vera plant extract solution as both a reducing and stabilizing agent. AgNPs@AV were characterized using XRD and SEM. Additionally, an agar well diffusion method was used to screen for antimicrobial activity. MIC and MBC were used to correlate the concentration of AgNPs@AV its bactericidal effect. SEM was used to investigate bacterial inactivation. Then the toxicity with human cells was investigated using an MTT assay.

Results

The synthesized AgNPs were crystalline with sizes of 70.70 ± 22-192.02 ± 53 nm as revealed using XRD and SEM. The sizes of AgNPs can be varied through alteration of times and temperatures used in their synthesis. These AgNPs were investigated for potential use as an antibacterial agent to inhibit pathogenic bacteria. Their antibacterial activity was tested on S. epidermidis and P. aeruginosa. The results showed that AgNPs had a high antibacterial which depended on their synthesis conditions, particularly when processed at 100 ^oC for 6 h and 200 ^oC for 12 h. The cytotoxicity of AgNPs was determined using human PBMCs revealing no obvious cytotoxicity. These results indicated that AgNPs@AV can be effectively utilized in pharmaceutical, biotechnological and biomedical applications.

Discussion

Aloe vera extract was processed using a green and facile method. This was a hydrothermal method to reduce silver nitrate to AgNPs@AV. Varying the hydrothermal temperature provided the fine spherical shaped nanoparticles. The size of the nanomaterial was affected by its thermal preparation. The particle size of AgNPs could be tuned by varying both time and temperature. A process using a pure AG phase could go to completion in 6 h at 200 ^oC, whereas reactions at lower temperatures required longer times. Moreover, the antibacterial effect of this hybrid nanomaterial was sufficient that it could be used to inhibit pathogenic bacteria since silver release was dependent upon its particle size. The high activity of the largest AgNPs might have resulted from a high concentration of aloe vera compounds incorporated into the AgNPs during hydrothermal synthesis.