GHz Microwave Irradiation Research Articles

Phase selectivity of microwave heating evidenced by Raman spectroscopy

Phase-selective temperature evolution of silicon-diamond powder mixtures under 2.45GHz microwave irradiation is followed using a Raman microscope fitted onto a microwave cavity. This spectroscopic method is shown to enable the collection of thermal information for both the silicon and the diamond phase selectively. An initial difference up to 100°C is found between the thermal trajectories extracted from silicon and diamond. A thermal equilibration is observed, subsequently suggesting that heat is flowing from hot silicon towards the colder diamonds. The existence of microscopic thermal gradients reflects the specific interaction of each of the two components with the applied electromagnetic field, governed here by the free carrier concentration. In addition to phase-specific temperature measurements, monitoring of chemical changes during microwave heating is also illustrated for magnetite powder. Raman spectroscopy turns out to be a powerful contact-free, noninterfering tool to follow and analyze on-line microwave heating processing of heterogeneous solid mixtures.

Surface properties of activated carbon treated by cold plasma heating

To modify the surface properties of activated carbon powders, we have applied the cold plasma treatment method. The cold plasma was used to be generated in the evacuated reactor vessel by 2.45 GHz microwave irradiation. In this paper, changes of surface properties such as distribution of acidic functional groups and roughness morphology were examined. By the cold plasma treatment, activated carbons with large specific surface area of ca. 2000 m 2/g or more could be prepared in a minute. The amount of every gaseous organic compound adsorbed on the unit gram of treated activated carbons was more increased that on the unit gram of untreated carbons. Especially, the adsorbed amount of carbon disulfide was remarkably increased even if it was compared by the amount per unit surface area. These results suggest that the surface property of the sample was modified by the plasma treatment. It became apparent by observing SEM photographs that dust and impure particles in macropores of activated carbons were far more reduced by the plasma treatment than by the conventional heating in an electric furnace under vacuum. In addition, a bubble-like surface morphology of the sample was observed by AEM measurement. The amount of acidic functional groups at the surface was determined by using the Boehm's titration method. Consequently, the increase of lactone groups and the decrease of carboxyl groups were also observed.

Electrical Properties and Microstructures of Sol-Gel-Deposited Lead Zirconate Titanate Thin Films Crystallized by 28 GHz Microwave Irradiation

Pb(ZrxTi1-x)O3 (PZT) thin films were coated on Pt/Ti/SiO2/Si substrates by a sol-gel method and then crystallized by 28 GHz microwave irradiation. The crystalline phases and microstructures as well as the electrical properties of the microwave-irradiated PZT films were investigated as a function of the elevated temperature generated by microwave irradiation. X-ray diffraction analysis indicated that the PZT films crystallized well into the perovskite phase at an elevated temperature of 480°C by microwave irradiation. Scanning electron microscopy images showed that the films had a granular grain structure and most of the grains were approximately 1.5 µm in size. With increasing elevated temperature from 480°C to 600°C by microwave irradiation, the breadth of grain boundaries of the films became narrow and the remanent polarization of the films increased slightly. It is clear that microwave irradiation is effective for obtaining well-crystallized PZT films with good properties at low temperatures in a short time.

Low-temperature growth of high-quality lead zirconate titanate thin films by 28 GHz microwave irradiation

Pb ( Zr x Ti 1 − x ) O 3 (PZT) thin films were coated on Pt/Ti/SiO2/Si substrates by a sol-gel method and then crystallized by 28 GHz microwave irradiation. The elevated temperature generated by microwave irradiation to obtain the perovskite phase is only 480 °C, which is significantly lower than that of conventional thermal processing. X-ray diffraction analysis indicated that the PZT films crystallized well in the perovskite phase. A scanning electron microscopy image showed that the film has a spherulite grain structure and most of the grains are approximately 2 μm in size. The average values of the remanent polarization, coercive field, dielectric constant, and loss of the PZT films are 40μC∕cm2, 50 kV/cm, 1100, and 004, respectively. It is clear that microwave irradiation is effective for obtaining well-crystallized PZT films with good properties at low temperatures.

Electrochemical properties of microwave irradiated synthesis of poly(3,4-ethylenedioxythiophene)/V 2O 5 nanocomposites as cathode materials for rechargeable lithium batteries

A series of poly(3,4-ethylenedioxythiophene) (PEDOT)/V 2O 5 nanocomposites are prepared via the redox intercalative polymerization reaction of 3,4-ethylenedioxythiophene (EDOT) monomer and crystalline V 2O 5 within 10 min by using rapid 2.45 GHz microwave irradiation with full power (800 W). The unique properties of the resultant nanocomposites are investigated by various characterization techniques using powder XRD, TGA/DTA and four-point probe conductivity analysis supports the intercalation of polymer nanosheet between V 2O 5 layers leading to enhanced bi-dimensionality. X-ray photoelectron spectroscopy analysis clearly shows the presence of mixed valent V 4+/V 5+ in the V 2O 5 framework after the redox intercalative polymerization which also confirms charge transfer from the polymer to the V 2O 5 framework. The application potential of these composites as cathode materials in rechargeable lithium batteries is also demonstrated by the electrochemical intercalation of lithium into the PEDOT/V 2O 5 nanocomposites, where an enhancement in the discharge capacity (370 mAh/g) is observed compared to that of crystalline V 2O 5.

Blood-forming system in rats after whole-body microwave exposure; reference to the lymphocytes

The influence of 2.45 GHz microwave (RF/MW) irradiation on blood-forming cells after whole-body irradiation of rats was investigated. The exposures were conducted with a field power density of 5–10 mW/cm 2, and whole-body average specific absorption rate (SAR) of 1–2 W/kg. Four experimental subgroups were created and irradiated 2, 8, 15 or 30 days, for 2 h a day, 7 days a week. Concurrent sham-exposed rats were also included in the study. The cell response was assessed by number and type of the bone marrow nuclear cells and peripheral blood white cells using standard laboratory methods. Significant decrease in lymphoblast count was obtained at 15 and 30th experimental day ( P < 0.05), whereas other examined parameters did not significantly differed in comparison to the sham-exposed controls. The findings point out at stress response in blood-forming system in rats after selected microwave exposure, which could be considered rather as sign of adaptation than malfunction.

Flexible dye-sensitized solar cells by 28GHz microwave irradiation

Microwave preparation of TiO2 nanocrystalline electrode for use in dye-sensitized solar cells is examined. A multi-mode microwave heating system operating at a frequency of 28 GHz is used to produce rapid synthesis. Well-sintered TiO2 nanocrystalline thin film is successfully fabricated on transparent conductive PET–ITO electrode. Photoelectron energy conversion efficiency of 2.16% is achieved in an electrode prepared by 28 GHz microwave irradiation at 1.0 kW for 5 min.

Flexible dye-sensitized solar cells by 28 GHz microwave irradiation

Microwave preparation of TiO 2 nanocrystalline electrode for use in dye-sensitized solar cells is examined. A multi-mode microwave heating system operating at a frequency of 28 GHz is used to produce rapid synthesis. Well-sintered TiO 2 nanocrystalline thin film is successfully fabricated on transparent conductive PET–ITO electrode. Photoelectron energy conversion efficiency of 2.16% is achieved in an electrode prepared by 28 GHz microwave irradiation at 1.0 kW for 5 min.

Preparation of TiO2 nanocrystalline electrode for dye-sensitized solar cells by 28GHz microwave irradiation

Microwave preparation of TiO2 nanocrystalline electrode for use in dye-sensitized solar cells is examined. A multi-mode microwave heating system operating at a frequency of 28 GHz is used to produce rapid processing. Well-sintered TiO2 nanocrystalline thin film is successfully fabricated on transparent conductive FTO glass electrode. Photoelectron energy conversion efficiency of 5.51% is achieved in an electrode prepared by 28 GHz microwave irradiation at 0.7 kW for 5 min.

Synthesis of Inorganic Materials by 28 GHz microwave irradiation

Synthesis of Inorganic Materials by 28 GHz microwave irradiation

Josephson effect in MgB/sub 2//Ag/MgB/sub 2/ step-edge junctions

We have made MgB/sub 2//Ag/MgB/sub 2/ junctions using the steps formed on Al/sub 2/O/sub 3/(0001) substrates. The current-voltage characteristics of these superconductor-normal-superconductor (SNS) type Josephson junctions have shown nonhysteretic behavior with some excess current. The measured I/sub c/R/sub n/ values were 1.2 mV at 4.2 K and 0.24 mV at 20 K. We have observed the modulation of critical current under dc magnetic field as well as Shapiro steps under 20 GHz microwave irradiation.

Low temperature preparation of perovskite oxide films by ECR sputtering assisted with microwave treatment

SrTiO 3 (STO) and (Ba,Sr)TiO 3 (BST) films were prepared on unheated substrates by mirror-confinement-type electron cyclotron resonance (MCECR) plasma sputtering. STO films deposited on Si substrates by Ar plasma were amorphous, whereas those on Pt/Ti/SiO 2/Si substrates by Ar/O 2 plasma were sufficiently crystallized in spite of a low substrate temperature below 450 K. Annealing by 28 GHz microwave irradiation was applied to amorphous STO films on Si substrates and amorphous BST films on Pt/Ti/SiO 2/Si substrates to crystallize these films. With microwave annealing, these films were crystallized at lower temperatures than with electric furnace annealing. The dielectric constant of STO films on Pt/Ti/SiO 2/Si substrates annealed by microwave irradiation at 573 K was approximately 260, which was nearly equal to the bulk value of STO. Microwave irradiation to these films is considered to be useful since it can reduce the annealing temperatures and improve the electrical property.

Characteristics of NdBa2Cu3O7-δ/PrBa2Cu3O7-δ/NdBa2Cu3O7-δ Planar Josephson Junctions

We report on the well-characterized properties of a-axis oriented trilayer Josephson junctions made of NdBa2Cu3O7-δ and PrBa2(Cu,Co)3O7-δ films, and c-axis oriented trilayer Josephson junctions made of NdBa2Cu3O7-δ and PrBa2Cu3O7-δ films with smooth surface morphology and good crystallinity. a-Axis oriented NdBa2Cu3O7-δ/PrBa2(Cu,Co)3O7-δ/NdBa2Cu3O7-δ trilayers and c-axis oriented NdBa2Cu3O7-δ/PrBa2Cu3O7-δ/NdBa2Cu3O7-δ trilayers were prepared on (100) SrTiO3 substrates to obtain planar junctions. Their current–voltage characteristics are of the resistively-shunted-junction type at 4.2 K. They showed well-developed Shapiro steps corresponding to 5–20 GHz microwave irradiation. Moreover, the power dependences of the maximum amplitudes of the Shapiro steps were clearly obtained and consistent with the theoretical values expressed as Jn(2eV/hf). The magnetic field dependence of the Josephson current for a-axis oriented trilayer Josephson junctions showed a Fraunhofer-like pattern, suggesting the spatial uniformity of the critical current flowing over the PrBa2(Cu,Co)3O7-δ barrier layer.

The influence of microwave irradiation on the pronounced subgap structures in intrinsic Josephson junctions of (Bi 1− xPb x) 2Sr 2CaCu 2O y

We have studied the behavior of pronounced subgap structures in the current–voltage ( I– V) characteristics of intrinsic Josephson junctions in (Bi 1− x Pb x ) 2Sr 2CaCu 2O y (with x=∼0.15, ∼0.2) mesas at 4.2 K without and with 2–20 GHz microwave irradiation. Without microwave irradiation, the I– V characteristics exhibited a set of superconducting and resistive quasiparticle branches corresponding to the constituent intrinsic Josephson junctions, with accompanying small subgap structures on the quasiparticle branches. Under irradiation of relatively high power of microwaves, the pronounced hysteretic subgap structures reduced, and new fine structures appeared beside them on the quasiparticle branches. The appearance of both original and new fine subgap structures was interpreted with regard to the interference of Raman-optical phonons with ac Josephson current in the intrinsic Josephson junctions.

Synthesis of Ferrites by 28 GHz Microwave Irradiation

Synthesis of Ferrites by 28 GHz Microwave Irradiation

Microwave synthesis of Fe-doped β-rhombohedral boron

Iron-doped β-rhombohedral boron was synthesized by 28 GHz microwave irradiation on a powder mixture of iron and β-boron. β-Boron strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to promote a reaction with iron dopant. The powder mixture was heated to 1800°C within 2 min by microwave irradiation, resulting in the formation of β-rhombohedral boron interstitially doped with iron. The reaction proceeded rapidly without accompanying grain growth. The XRD analysis and the electrical conductivity measurements revealed successful incorporation of iron into two doping sites of β-boron.

Dynamics of microwave-induced fluxons in HgI 2-intercalated Bi 2Sr 2CaCu 2O 8+ δ Josephson stacks

The microwave response of intrinsic Josephson junctions (IJJs) in mesa structures formed on HgI 2-intercalated Bi 2Sr 2CaCu 2O 8+ δ single crystals was studied in a range of microwave frequencies above the junction plasma frequency. Under 73–76 GHz microwave irradiation, the supercurrent branch becomes resistive above a certain onset microwave power. At a low current bias, the current–voltage characteristics show linear behavior, while at a high current bias, the resistive branch splits into multiple sub-branches. The voltage spacing between neighboring sub-branches increases with the microwave power and the total number of sub-branches is almost identical to the number of IJJs in the mesa. All the experimental results suggest that each sub-branch represents a specific mode of collective motion of Josephson vortices generated by the microwave irradiation in a stack of IJJs.

Weak ferromagnetism of bixbyite-type (In0.67Fe0.33)2O3 synthesized by 28 GHz microwave irradiation

Weak ferromagnetism of bixbyite-type (In0.67Fe0.33)2O3 synthesized by 28 GHz microwave irradiation

Microwave Synthesis of Yttrium Aluminum Iron Garnet Powder

The solid solution of yttrium aluminum garnet-yttrium iron garnet [YAIG, Y3(Al,Fe)5O12] was synthesized from the component oxides by 28 GHz-microwave irradiation. The compositions of the resulting garnets were not the same as the nominal compositions. This difference could be explained by selective coupling of chemical species with microwaves, predictable from the temperature–time profiles of each raw material under microwave irradiation. The diffusion process under microwave irradiation was discussed.

Rapid Formation and Growth of Bixbyite‐Type (In0.67Fe0.33)2O3 by 28 GHz Microwave Irradiation

(In0.67Fe0.33)2O3 with the bixbyite structure was synthesized via 28 GHz microwave irradiation, using multimode microwave heating equipment. Indium sesquioxide strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to drive a reaction with iron sesquioxide. A mixture of In2O3 and α‐Fe2O3 powders (In:Fe ratio of 2:1) was irradiated with microwaves at a frequency of 28 GHz. The mixture was heated to 1400°C during the microwave irradiation. The formation of a solid solution was completed within a minute, which indicated a drastic enhancement of the reaction rate. Scanning electron microscopy revealed remarkable grain growth under microwave irradiation.