Nonuniform Microwave Field Research Articles

Improvement of Heating Uniformity by Limiting the Absorption of Hot Areas in Microwave Processing of CFRP Composites.

Carbon fiber reinforced polymer (CFRP) composites are integral to today’s industries. Curing or consolidation are vital processes for manufacturing CFRP components. Microwave processing has many advantages compared with conventional processing technologies using ovens or autoclaves; however, the uneven temperature distribution caused by the non-uniform microwave field has a significant influence on the quality of the cured products. In this study, we propose a new idea to solve this problem, i.e., limiting the absorption of hot areas. Under such circumstances, cold ones can catch up with them more easily. To adjust the absorbing capability of the CFRP laminate, periodically arranged metallic resonance structures supported by a dielectric spacer are introduced on its surface. The dielectric spacer, made of epoxy matrix and strontium titanate particles, is designed to possess a dielectric constant positively related to temperatures. In this situation, the microwave absorption (2.45 GHz) of the metal-dielectric-CFRP configuration is changed from 97.6% at room temperature to 55.9% at 150 °C continuously. As a result, a reduction of 43.1% in maximum temperature difference and 89% in standard deviation has been achieved.

Effect of Vertical and Horizontal Sample Orientations on Uniformity of Microwave Heating Produced by Magnetron and Solid-State Generators.

In this study, the effect of different horizontal and vertical orientations of a model sample (cuboid gellan gel samples containing Maillard reactants) on microwave heat processing was investigated in the solid-state and magnetron microwave systems. To achieve this target, seven orientations inside both microwave cavities were defined. Two of the investigated sample orientations were in a vertical position with and without turntable rotation, and five in a horizontal position. Furthermore, samples at horizontal orientations were put at an angle position without turntable rotation. To analyze the microwave heating patterns, infrared (IR) pictures and photographs of the gellan gel samples were taken after processing to document IR-based thermal and Maillard color changes, respectively. Three main factors for improvement of the heating homogeneity were identified: first, processing samples in the solid-state microwave system; second, position variation of the sample by turntable activated; and third, horizontal orientation. In addition, it was observed that placing the gellan gel samples in a vertical position in the magnetron microwave system resulted in considerably more absorbed power and a more uniform microwave heat processing compared to other horizontal orientations in this system. This indicated a non-uniform microwave field distribution. The results of this study can also confirm the importance of designing suitable food packaging: a vertical shape for more microwave energy absorbance and thus, more energy efficiency, and a horizontal shape for more uniform microwave heat processing.

Strong coupling of magnons in a YIG sphere to photons in a planar superconducting resonator in the quantum limit

We report measurements made at millikelvin temperatures of a superconducting coplanar waveguide resonator (CPWR) coupled to a sphere of yttrium-iron garnet. Systems hybridising collective spin excitations with microwave photons have recently attracted interest for their potential quantum information applications. In this experiment the non-uniform microwave field of the CPWR allows coupling to be achieved to many different magnon modes in the sphere. Calculations of the relative coupling strength of different mode families in the sphere to the CPWR are used to successfully identify the magnon modes and their frequencies. The measurements are extended to the quantum limit by reducing the drive power until, on average, less than one photon is present in the CPWR. Investigating the time-dependent response of the system to square pulses, oscillations in the output signal at the mode splitting frequency are observed. These results demonstrate the feasibility of future experiments combining magnonic elements with planar superconducting quantum devices.

Double-layer particlespout in strong and nonuniform microwave fields

We report a new phenomenon named particlespout in which the particles spout in a specially designed microwave cavity. An ionic crystal of sodium chloride was heated in the microwave applicator. Beyond the melting point, the particles began to evaporate and move upward owing to thermal convection. These particles formed a funnel shape similar to a waterspout but they had two layers. In comparison with conventional heating, only a single but unstable columnar vortex can be observed. A theoretical model is proposed that attributes the observed phenomenon to the rotational kinematics together with the ponderomotive force. These two effects confine the particles to the inner and outer bounds, respectively.

Uniformity Issue in Microwave Drying

Microwave drying is an advanced drying technique in which heat is generated inside the material due to transformation of high-frequency electromagnetic energy so the liquid moisture is intensively evaporated and transported toward the material surface. However, an inherent problem in microwave drying is nonuniform heat generation, which leads to uneven drying. In this article, the factors causing nonuniform microwave heating were identified, and feasible approaches to solving the nonuniformity problem were analyzed in terms of improving the uniformity of the electromagnetic field distribution and random or organized dislocation of the processed material in virtually nonuniform microwave field. The R&D trends in microwave drying toward reducing uneven heating were pointed out and discussed.

Wave function of a microwave-driven Bose-Einstein magnon condensate

It has been observed experimentally that a magnon gas in a film of yttrium-iron garnet at room temperature driven by a microwave field exhibits Bose-Einstein condensation (BEC) when the driving power exceeds a critical value. In a previous paper we presented a model for the dynamics of the magnon system in wave-vector space that provides firm theoretical support for the formation of the BEC. Here we show that the wave function of the magnon condensate in configuration space satisfies a Gross-Pitaevskii equation similarly to other BEC systems. The theory is consistent with the previous model in wave-vector space, and its results are in qualitative agreement with recent measurements of the spatial distribution of the magnon condensate driven by a nonuniform microwave field.

Modeling of the electrode microwave discharge in nitrogen

Self-consistent two-dimensional modeling of a steady microwave discharge initiated at the end of the central electrode in nitrogen is presented. The discharge parameters are calculated at a gas pressure of 1 Torr and incident power of 30 W. The computational model includes the time-dependent Maxwell's equations, the balance equations describing the kinetics of charged and neutral plasma particles and the time-independent homogeneous Boltzmann equation for electrons. The processes involving vibrationally excited ground state molecular nitrogen are taken into account by the well-known analytic expression for the vibrational distribution of molecules in the diffusion approximation. It is shown that the spatially non-uniform microwave field causes the difference in plasma particles kinetics in different parts of the discharge. Results of numerical simulations for space distribution of electronically excited molecules have been found in good qualitative agreement with those taken from spectral measurements of first and second positive systems of nitrogen. Results confirm the concept according to which such a discharge comprises a self-sustained and a non-self-sustained discharge.

Phase locking of Josephson flux-flow oscillators in non-uniform microwave fields

Phase locking phenomena in Josephson flux-flow oscillators induced by non-uniform microwave fields are investigated both numerically and analytically. For low values of the amplitude of the microwave signal, a good agreement is found between analytical and numerical results. For odd phase locking steps, the agreement extends also to high amplitude values of the microwave signal. The discrepancy observed at high microwave amplitudes for even phase locking steps, is also discussed.

Josephson flux-flow oscillators in nonuniform microwave fields

Josephson flux-flow oscillators in nonuniform microwave fields

Analytical solution for the electron paramagnetic resonance signal intensity of a line-like sample of variable length whose centre is situated at an arbitrary position along the common sample–cavity x-axis

An analytical solution of the integral equation, used for the theoretical calculation of the relative EPR signal intensity of a line-like sample with a variable length, which is situated at an arbitrary position along the common sample–cavity x-axis, has been obtained for the case of a non-uniform microwave field and a uniform modulation field, Q-band EPR.

Microwave heating of the continuous flow catalytic reactor in a nonuniform electric field

AbstractTemperature profiles in the continuous‐flow reactor with a fixed bed placed in a nonuniform microwave field are predicted by considering specific on – off regulation of the microwave power and local distribution of the electric‐field intensity. The spatial and temperature dependence of the electric‐field intensity affects the heating appreciably, as shown by indirect measurements. Results of numerical calculations agree with experimental data of outlet temperature and explain temperature oscillations in the outlet flow. The model developed allows an accurate description of microwave heating in the continuous‐flow reactor associated with specific regulation of microwave power. The hydrolysis of sucrose catalyzed by the strongly acidic cation‐exchange resin Amberlite 200C in R‐H form was chosen as the reaction system. Calculated conversions of sucrose based on predicted temperature profiles agree with experimental data.

Resonant scattering of three-level Rydberg atoms in a microwave field

We examine the theory of potential scattering of Rydberg atoms in a microwave field. The model of a three-level atom is employed to calculate the radiative force emerging in the resonant coherent interaction with the microwave field for the case of a two-photon resonance and high intensities, using the method of quasienergies of the system consisting of the atom and the field. We determine the probabilities of Landau-Zener transitions in the spatial regions where under two-photon resonance conditions the quasienergies of the atoms approach one another by a small quantity. We also study the dynamics of the variation of the spatial profile of a beam of Rydberg atoms caused by resonant scattering. Finally, we give the results of the first experimental observation of the variation of the transverse beam profile when Rydberg atoms pass through a nonuniform microwave field formed in a rectangular waveguide and in resonance with the two-photon 36P–37P transition.

Determination of Spin-Wave Boundary Conditions by dc Effects in Spin-Wave Resonance

A galvanomagnetic spin-wave experiment is described which, in conjunction with resonance absorption measurements in both uniform and nonuniform microwave fields, yields more information about the distribution of magnetization in spin-wave modes in ferromagnetic films than hitherto available, particularly for spin waves of low mode number. The method is applied to determine the spin-wave boundary conditions in 1200-\AA{} Permalloy films, with the static field either parallel or perpendicular to the film plane. Homogeneous films show unpinned surfaces for the parallel orientation. In the perpendicular case, these films have a partly pinned film-glass interface, with anisotropies of a few tenths of 1 erg/${\mathrm{cm}}^{2}$, and an unpinned film-air surface. Parallel-orientation results for a film with a thin Ni over-coat indicate the excitation of a surface mode, which is also predicted by theory.

Second-Harmonic Generation in Plasmas by Nonuniform Microwave Fields

In this paper the authors give a Boltzmann-equation analysis of the nonlinear generation of the second harmonic in the current density in a slightly ionized plasma, arising because of the nonuniform nature of the applied microwave field. Under suitable approximations, the expression derived for the second-harmonic component in the current density reduces to the one derived on the basis of an elementary theory by Krenz and Kino. Some numerical results, illustrating the effect of collisions on the harmonic generation process, have also been presented.

Interaction of electron beams with strongly nonuniform microwave fields

Interaction of electron beams with strongly nonuniform microwave fields