Yuping Duan

Novel microwave dielectric response of Ni/Co-doped manganese dioxides and their microwave absorbing properties

A facile redox reaction between KMnO4 and MnSO4 was carried out to investigate the dielectric response and microwave absorbing properties of Ni/Co-doped MnO2. The samples were characterized by X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM), and vector network analysis. The analysis results revealed that the powders were α-MnO2 with one-dimensional nanostructure. The doping of Ni/Co had a certain effect on the dielectric properties: the relative complex permittivity showed a more distinct dielectric response characteristic, and the imaginary part exhibited a great enhancement of 2–18 GHz, which resulted in controllable wave-absorbing properties. The microwave absorbing bandwidth (RL < −10 dB) for Co-doped MnO2 was located at 10.96–16.13 GHz with a thickness of 2 mm. Furthermore, the Debye equation was introduced to explain the novel microwave dielectric response of doped MnO2. Some other properties derived from dielectric performances were also investigated, such as dielectric loss tangent and dielectric conductivity. In particular, first-principles calculations based on density functional theory (DFT) were used to uncover the relationship of electronic structure and dielectric properties on the microscopic scale.

A theoretical study of the dielectric and magnetic responses of Fe-doped α-MnO2 based on quantum mechanical calculations

The microwave electromagnetic properties of Fe-doped α-MnO2 are studied theoretically using quantum mechanical calculations based on density functional theory. The calculations employ the Perdew–Burke–Ernzerhof model, a generalized gradient approximation, to deal with the exchange–correlation interaction. The possible role of Fe doping in modifying the electromagnetic performance is studied utilizing density of states (DOS) and the bond length between the metal and oxygen. Calculations of the bond length in the presence of Fe show a contracted bond length between the metal atoms and O with enhanced bond strength, resulting in increased storage of electric field energy. This explains the experimental observation of a reduced dielectric loss after Fe-doping. The DOS results demonstrate that Fe doping enhances the spin-polarization of MnO2. Therefore, the total magnetic moment is increased after doping, corresponding to the magnetic enhancement of MnO2. The theoretical predictions concluded from the quantum mechanical calculations agree well with the experimental observations. The results provide an early stage exploration of theoretical research on the microwave absorbing properties of doped MnO2.

First-principles calculations of graphene-based polyaniline nano-hybrids for insight of electromagnetic properties and electronic structures

In situ polymerization between aniline and ammonium persulfate (APS) under acidic conditions is described for the preparation of graphene-based polyaniline hybrids with investigation of their electromagnetic properties. Samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR), scanning electron microscopy (SEM), transition electron microscopy (TEM) and vector network analysis. The results revealed that the graphene-based PANI hybrids formed a uniform PANI particle coating on the graphene sheets. Such special structures lead to π–π interactions between graphene and PANI, and thus the electromagnetic absorption properties are significantly improved versus pure conducting polymers. The maximum reflection loss reaches −32.1 dB at 5.45 GHz with a thickness of 4 mm. The bandwidth (RL < −10 dB) reaches an amazing 5.62 GHz with a thickness of 2 mm. In particular, first-principles calculations based on density function theory (DFT) were carried out to uncover the relationship between dielectric properties and electronic structures.

Controlled synthesis and electromagnetic performance of hollow microstructures assembled of tetragonal MnO2 nano-columns

Hollow microstructures assembled of tetragonal MnO2 nano-columns have been synthesized through a facile hydrothermal method with the introduction of iron ions. The obtained samples were characterized by XRD, SEM, and vector network analysis. Results reveal that the presence of Fe3+ ions leads to transformation of phase structure from α-MnO2 to a mixture of ɛ-MnO2 and α-MnO2, and it is essential to include Fe3+ ions for the formation of the hollow MnO2 microstructures. The formation mechanism was investigated and proposed in detail. Fe-doped MnO2 exhibits decreased dielectric loss, but increased magnetic loss compared with the pure one.

Time-dependent density functional theory study on the excited-state hydrogen-bonding characteristics of polyaniline in aqueous environment

A theoretical study was carried out to study the excited-state of hydrogen-bonding characteristics of polyaniline (PANI) in aqueous environment. The hydrogen-bonded PANI-H2O complexes were studied using first-principles calculations based on density functional theory (DFT). The electronic excitation energies and the corresponding oscillator strengths of the low-lying electronically excited states for hydrogen-bonded complexes were calculated by time-dependent density functional theory (TDDFT). The ground-state geometric structures were optimized, and it is observed that the intermolecular hydrogen bonds CN⋯HO and NH⋯OH were formed in PANI-H2O complexes. The formed hydrogen bonds influenced the bond lengths, the charge distribution, as well as the spectral characters of the groups involved. It was concluded that all the hydrogen-bonded PANI-H2O complexes were primarily excited to the S1 states with the largest oscillator strength. In addition, the orbital transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) involved intramolecular charge redistribution resulting to increase the electron density of the quinonoid rings.

Theoretical Investigation of the Protonation Mechanism of Doped Polyaniline

The theoretical calculation of polyaniline tetramers were performed in implicit water solvent medium using the polarizable continuum model with density functional theory. In order to explore the protonation mechanism of polyaniline, the geometry characteristics, charge distribution, frontier molecular orbitals (MOs) and stability of emeraldine base and emeraldine salts in tetramer were investigated by B3LYP/6-31G+(d,p), and the significant increase mechanism of the electrical conductivity of polyaniline upon protonation was researched in detail. It was shown that optimized molecular geometry by protonation doping suggested a tendency towards bipolaron delocalization and a greater π-conjugation proved by bond length alternation and the torsional angles. Furthermore, the NBO of ES3 was distributed equably and should be conducive to the electrical conductivity, the frontier MOs were effectively established, and an electronic transition from HOMO to LUMO+1 is turned out to be a ππ* transition finally. In addition, compared with the computed energies of different emeraldine salt configurations, the bipolaronic lattice was taken for the most stable structure.

Facile synthesis of hierarchical nanocomposites of aligned polyaniline nanorods on reduced graphene oxide nanosheets for microwave absorbing materials

A facile in situ polymerization process was introduced for the preparation of RGO–PANI hybrids with investigation of their microwave absorbing properties. The structural and morphological characterization were performed using Fourier transform infrared (FTIR), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that the hierarchical nanocomposites of aligned polyaniline nanorods grown vertically on the surface of RGO were successfully synthesized. The hybrids with special structure possessed more distinct dielectric response characteristics and better electromagnetic absorption properties than pure PANI. The maximum reflection loss value of the composites is up to −43 dB at 12.4 GHz and the absorption bandwidth exceeding −10 dB is 3.5 GHz with a thickness of 2 mm. The results indicate that hierarchical nanocomposites of aligned polyaniline nanorods on reduced graphene oxide nanosheets are efficient materials for microwave absorption.

The effect of intermolecular hydrogen bonding on the polyaniline water complex

The polyaniline water hydrogen-bonded complex was studied by first-principles calculation. The density functional theory method was used to calculate the structure characters, natural bond orbital charge distribution, infrared spectra and the frontier molecular orbital. Results showed that the H–O···H–N and C–N···H–O type intermolecular hydrogen bonds were formed. The bonds involved in the intermolecular H-bond were all influenced by the hydrogen bonding interaction. During the hydrogen bond formation, the polymer chains in the complexes were all charged, which can be an important factor contributing to the increase of electrical conductivity. The N1–H vibration was strongly influenced, and the locations as well as the intensities of N1–H absorption bands were all changed in the complexes. In the orbital transition of HOMO to LUMO, the electron density transferred from benzenoid ring to quinoid ring.

The effects of annealing treatment on the morphology and microwave absorption properties of α-MnO2

Chemically synthesized α-MnO2 powder was annealed at different temperatures with (NH4)2S2O8 as the oxidant and MnSO4 as the manganese source. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and vector network analysis. Results indicated that the structures and morphologies of the products did not change remarkably, mainly consisting of α-MnO2 with a sea urchin-like shape, when the annealing temperature was below 500u2009°C. However, it was observed that the powder changed from α-MnO2 to Mn2O3 at 500u2009°C and 600u2009°C. The scanning electron microscopy images showed that the morphologies of the resulted Mn2O3 powder were microspheres in various sizes. Microwave absorption properties of the as-synthesized samples were also discussed, which showed that the dielectric loss tangent, magnetic loss tangent and reflection loss could be improved significantly by increasing the annealing temperature appropriately (<500u2009°C).