Millimeter-wave Absorption Research Articles

Continuum absorption of millimeter waves in nitrogen

We report the results of experimental study for the N2 absorption in the millimeter wavelength range at pressure and temperature ranging, respectively, from 750 to 1500 Torr and from 265 to 310 K. The obtained data are shown to be in a good agreement with the previously published experimental results, as well as with the predictions issued from available empirical models of nonresonant absorption and from the simulation of the N2–N2 rototranslational collision-induced absorption performed recently using the classical trajectories method relying on complete ab initio potential and induced dipole surfaces. Analysis of the first spectral moments leads to a conclusion that the role of true bound (N2)2 dimer absorption is negligible at least in the range of atmospheric temperatures. New experimental data reported in this paper are supported by our theoretical modelling being however somewhat in excess of the data presented in the HITRAN/CIA database for the same spectral range. We consider our data as an update of the previously available so-called “dry” atmospheric continuum which is conventionally employed nowadays in the mm-waves propagation models.

Additive Manufacturing of Ceramic Products Based on Millimeter-Wave Heating

Additive manufacturing of ceramic articles making use of concentrated energy flows attracts the research interest worldwide. While the application of laser beams faces serious problems associated with high temperature of sintering and low thermal conductivity of ceramics, layer-by-layer sintering by focused millimeter-wave radiation appears to be a promising method of additive manufacturing. This paper describes the studies of fast millimeter-wave sintering of yttria-stabilized zirconia and hydroxyapatite ceramics. Coefficients of the millimeter-wave absorption have been determined in broad frequency and temperature ranges. Rapid sintering of compacted ceramics samples was accomplished using volumetric microwave heating in a work chamber of a 24 GHz / 5 kW gyrotron system. In addition, using a 263 GHz / 1 kW cwgyrotron millimeter-wave source and a purposely designed electrodynamic focusing structure, radiation intensities of up to 20 kW/cm2could be achieved, which was sufficient for fast localized heating of ceramic layers to the solidification temperature. The results of a study of the microstructure and mechanical properties of the sintered ceramics are presented.

Transparently curved metamaterial with broadband millimeter wave absorption

We present a conformal metamaterial with simultaneous optical transparency and broadband millimeter-wave absorption for a curved surface. By tailoring the reflection response of meta-atoms at oblique angles, it is possible to achieve excellent absorption performance from 26.5 to 40.0 GHz within a wide angular range from 0° to 60° for transverse-electric and transverse-magnetic waves. In the meantime, by employing transparent substrates, including polyvinyl chloride and polyethylene terephthalate, good optical transmittance (80.1%) and flexibility are obtained simultaneously. The reflectivity of a curved metallic surface coated with the proposed curved metamaterial is simulated and measured experimentally. Both results demonstrate excellent absorption performance of the metamaterial, which is highly favored for practical applications.

Resonance Enhanced Multiphoton Ionization Detected Millimeter-Wave Absorption: The 115 GHz Line of CO.

We investigate the potential for sensitively detecting millimeter-wave absorption through multiphoton ionization using the lowest rotational transition in CO (X1Σ, J = 1 ← J = 0) at 115 GHz as a diagnostic tool. This transition represents an almost perfect realization of a quantum mechanical two-level system. In the experiment, the output of a powerful continuous millimeter-wave source with sub-kilohertz resolution propagates counter to a pulsed molecular beam, causing Doppler shifted resonances. Absorption is detected by monitoring the involved rotational levels through resonance enhanced multiphoton ionization (REMPI). Observed polarization, saturation and Doppler effects demonstrate that the interaction with the millimeter-wave field is coherent and can be simulated well using the results for the two-level system. The combination of these results indicates that the millimeter-wave field is approximated well by a Gaussian beam with beam waist around 15 mm and constant effective electric field amplitude on the molecular beam axis. Time-correlated frequency modulation in the form of a short rectangular pulse is used to follow the molecular trajectory from the source to the laser interaction region. The experiments clearly demonstrate a great potential of extending high-resolution molecular beam spectroscopy with mass specific REMPI detection into the sub-terahertz or terahertz regimes.

Achieving impressive millimeter-wave absorption properties in Nb5+ doped barium ferrite by simply controlling the sintering atmosphere

To enhance the millimeter-wave absorption properties around atmospheric window of 35 GHz, Nb5+ doped barium ferrite precursors are sintered at 1400 °C for 3 h in air, Ar, N2 respectively. The magnetic loss region covers 30 ∼ 40 GHz in the sample sintered in air and extends to 26.5 ∼ 40 GHz in the sample sintered in N2. The ε′ and ε″ vary from ∼5 to ∼0.4 by sintering in air to ∼10 and ∼0.3 by sintering in Ar, and to ∼13 and ∼2.0 in N2. Ultimately, contributed by enhanced permittivity and extended magnetic-loss region, the sample sintered in N2 exhibits impressive millimeter-wave absorption properties around 35 GHz with broad bandwidth more than 12 GHz and maximum reflectivity of −37 dB under thin thickness of 0.55 mm.

The pure rotational spectrum of the ZnBr radical (X2Σ+): Trends in the zinc halide series.

The pure rotational spectrum of ZnBr (X2Σ+) has been recorded in the frequency range 259-310 GHz using millimeter-wave direct absorption techniques. This study is the first quantitative spectroscopic investigation of this free radical. ZnBr was synthesized in a DC discharge by the reaction of zinc vapor in argon with one of three reagents: BrCH3, Br2CH2, or Br2. Eight rotational transitions were measured for six isotopologues (64Zn79Br, 64Zn81Br, 66Zn79Br, 66Zn81Br, 68Zn79Br, and 68Zn81Br), all of which exhibited spin-rotation interactions. Furthermore, transitions originating in the v = 1 through 3 excited vibrational states were obtained for certain isotopologues. Five rotational transitions were also recorded for 67Zn79Br, in which hyperfine splittings were observed arising from the 67Zn nucleus (I = 5/2). The spectra were analyzed using a Hund's case (bβJ) Hamiltonian, and rotational, spin-rotation, and 67Zn magnetic hyperfine constants were determined. Equilibrium parameters were also derived for the 64Zn79Br, 64Zn81Br, 66Zn79Br, and 66Zn81Br isotopologues, including the vibrational constant, ωe = 286 cm-1. The equilibrium bond length was derived to be re = 2.268 48(90) Å. Analysis of the 67Zn hyperfine parameters suggest a decrease in ionic character in ZnBr from the other known zinc halides, ZnF and ZnCl.

Gas phase detection and rotational spectroscopy of ethynethiol, HCCSH

ABSTRACTWe report the gas-phase detection and spectroscopic characterisation of ethynethiol (), a metastable isomer of thioketene () using a combination of Fourier-transform microwave and submillimetre-wave spectroscopies. Several a-type transitions of the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency, b-type lines using double resonance provided accurate predictions in the submillimetre region. With these, searches using a millimetre-wave absorption spectrometer equipped with a radio frequency discharge source were conducted in the range 280–660 GHz, ultimately yielding nearly 100 transitions up to and . From the combined data set, all three rotational constants and centrifugal distortion terms up to the sextic order were determined to high accuracy, providing a reliable set of frequency predictions to the lower end of the THz band. Isotopic substitution has enabled both a determination of the molecular structure of HCCSH and, by inference, its formation pathway in our nozzle discharge source via the bimolecular radical-radical recombination reaction , which is calculated to be highly exothermic (−477 kJ/mol) using the HEAT345(Q) thermochemical scheme.

Millimeter-wave spectroscopy and modeling of 1,2-butanediol

Context. The continuously enhanced sensitivity of radioastronomical observations allows the detection of increasingly complex organic molecules. These systems often exist in a large number of isomers leading to very congested spectra. Aims. We explore the conformational space of 1,2-butanediol and provide sets of spectroscopic parameters to facilitate searches for this molecule at millimeter wavelengths. Methods. We recorded the rotational spectrum of 1,2-butanediol in the 59.6–103.6 GHz frequency region (5.03–2.89 mm) using a free-jet millimeter-wave absorption spectrometer, and we analyzed the properties of 24 isomers with quantum chemical calculations. Selected measured transition lines were then searched on publicly available ALMA Band 3 data on IRAS 16293-2422 B. Results. We assigned the spectra of six conformers, namely aG′Ag, gG′Aa, g′G′Ag, aG′G′g, aG′Gg, and g′GAa, to yield the rotational constants and centrifugal distortion constants up to the fourth or sixth order. The most intense signal belong to the aG′Ag species, that is the global minimum. Search for the corresponding 30x,30 − 29x,29 transition lines toward IRAS 16293-2422 B was unsuccessful. Conclusions. Our present data will be helpful for identifying 1,2-butanediol at millimeter wavelengths with radio telescope arrays. Among all possible conformers, first searches should be focused on the aG′Ag conformers in the 400–800 GHz frequency spectral range.

Millimeter-wave absorption properties of BaTiO3/Co3O4composite powders controlled by high-frequency resonances of permittivity and permeability

Millimeter-wave resonance of permittivity–permeability-contributed double absorption peaks in BaTiO3/Co3O4, exhibiting RL of ∼−40 dB and a bandwidth of 5 GHz around 35 GHz.

Multiphysics Analysis of Millimeter-Wave Absorber with High-Power Handling Capability

For the fifth generation wireless communication system, millimeter-wave absorber has been frequently used in the measurement of high-power devices, such as transmitting antennas of base stations or satellite communication antennas, etc. The overheat inside the absorber can always be a matter of concern because it may lead to catastrophic fire in the anechoic chamber. A novel pyramidal structure with vent holes and heat sinks was proposed to improve the heat dissipation. The power-handling capability of the wave absorber could be improved by increasing the heat dissipation path without changing the characteristics of the base material. Multiphysics simulation was carried out to analyze the thermal-electromagnetic interaction. This absorber can be used to measure the high-power antenna and the radar system inside anechoic chambers, with power density up to 7 kW/m2.

Examining transition metal hydrosulfides: The pure rotational spectrum of ZnSH (X̃2A').

The pure rotational spectrum of the ZnSH (X̃2A') radical has been measured using millimeter-wave direct absorption and Fourier transform microwave (FTMW) methods across the frequency range 18-468 GHz. This work is the first gas-phase detection of ZnSH by any spectroscopic technique. Spectra of the 66ZnSH, 68ZnSH, and 64ZnSD isotopologues were also recorded. In the mm-wave study, ZnSH was synthesized in a DC discharge by the reaction of zinc vapor, generated by a Broida-type oven, with H2S; for FTMW measurements, the radical was made in a supersonic jet expansion by the same reactants but utilizing a discharge-assisted laser ablation source. Between 7 and 9 rotational transitions were recorded for each isotopologue. Asymmetry components with Ka = 0 through 6 were typically measured in the mm-wave region, each split into spin-rotation doublets. In the FTMW spectra, hyperfine interactions were also resolved, arising from the hydrogen or deuterium nuclear spins of I = 1/2 or I = 1, respectively. The data were analyzed using an asymmetric top Hamiltonian, and rotational, spin-rotation, and magnetic hyperfine parameters were determined for ZnSH, as well as the quadrupole coupling constant for ZnSD. The observed spectra clearly indicate that ZnSH has a bent geometry. The rm(1) structure was determined to be rZn-S = 2.213(5) Å, rS-H = 1.351(3) Å, and θZn-S-H = 90.6(1)°, suggesting that the bonding occurs primarily through sulfur p orbitals, analogous to H2S. The hyperfine constants indicate that the unpaired electron in ZnSH primarily resides on the zinc nucleus.

Experimental results on the frequency dependence of attenuation, scattering, and absorption of millimeter waves in a dry snow cover

The reflectivity and transmissivity and coherent transmittance of dry homogeneous snow are measured as functions of the thickness of a snow layer at frequencies of 22.2, 37.5, 60, and 94 GHz. The experimental characteristics are approximated using the FIRE, HUT, and Kubelka–Munk models. The experimental frequency dependences of the extinction coefficient and model quantities: absorption, scattering, and flux attenuation coefficients are analyzed. It is shown that the dependence of the model quantities is governed by a power function of frequency in which the exponent for each quantity depends on the structure of snow and the frequency interval in which the remote sensing of the snow cover is performed. It is found that the frequency dependence of the scattering coefficient for coarse-grained snow in the millimeter-wave band is close to linear.

Effective Microwave Dielectric Properties of Ensembles of Spherical Metal Particles

Knowledge of dielectric and magnetic properties is crucially important in the numerical modeling of high-temperature microwave processing of materials. To enhance the accuracy of modeling for metal powders and metal-containing composites, adequate description is required for the interaction of the electromagnetic field with the electrically conductive particles. A theoretical model for the calculation of effective complex dielectric permittivity of an ensemble of spherical metal particles surrounded by thin insulating layers is presented. The model is based on the effective medium approximation. It accounts for the nonuniform distribution of the electromagnetic field that exists inside individual particles when the electromagnetic wavelength or penetration depth is smaller than the particle size. The effective dielectric losses predicted by the model can be highly significant compared with the magnetic losses that arise due to the eddy currents induced in metal particles by microwaves. Reasonable agreement with the experiment has been demonstrated for the millimeter-wave absorption in metal–ceramic composites with significant volume fraction of metal particles. The model can be utilized for assessing the feasibility of microwave heating of powder metals and metal–ceramic composites.

The microwave spectroscopy study of 1,2-dimethoxyethane

The pure rotational spectrum of 1,2-dimethoxyethane was measured in the 6–18 and 59.6–74.4GHz frequency ranges using a cavity-based pulsed jet Fourier transform microwave spectrometer and a Stark modulated free-jet millimeter-wave absorption spectrometer, respectively. Two conformers, TGT and TGG′, have been identified, together with their mono-substituted 13C isotopologues. Each rotational transition is split into several components due to the two methyl groups internal rotations. The corresponding V3 barriers have been experimentally determined. MP2/6-311++G(d,p) calculations have provided the determination of the conformational stabilities, structures and spectroscopic parameters.

In situ polymerized polyaniline nanofiber-based functional cotton and nylon fabrics as millimeter-wave absorbers

Polyaniline nanofibers and their composite with graphite have been synthesized by a simple chemical polymerization method. Polyaniline nanofiber graphite composites with a thickness of 1 mm exhibit excellent electromagnetic interference (EMI) shielding of above 80 dB in the frequency range of 8.2–18 GHz. EMI shielding fabrics of 0.1 mm thickness based on polyaniline nanofibers and their composite have been developed by an in situ polymerization route. These fabrics combine the properties of polyaniline nanofibers as well as their composite and fabrics (cotton and nylon). The developed functional fabrics with 0.1 mm thickness exhibit EMI shielding effectiveness in the range of 11–15 dB in the 8.2–18 GHz frequency range. Optical and scanning electron microscopy studies indicate the uniform coating of the polyaniline nanofibers over the individual fibers and interweave regions. Thin and flexible shielding materials suitable for a broad range of millimeter-wave shielding applications have been developed using this simple and potentially profitable method. Schematic representation depicts the synthesis, microstructure and EMI shielding properties of functional cotton and nylon fabrics based on polyaniline nanofibers.

A millimeter-wave gain equalizer using E-plane microstrip-line probe and waveguide resonator

This paper presents a millimeter-wave wideband gain equalizer-based metallic waveguide, which is implemented by E-plane microstrip-line probe, metallic waveguide resonator, waveguide coupling groove, and millimeter-wave absorber. The proposed gain equalizer has advantages of high Q-factor and high power-handling capacity. In order to validate the new proposed topology, an ultra-wideband millimeter-wave gain equalizer, operating from 26.8 to 40 GHz, is designed and fabricated using rectangular waveguide. The R320 standard waveguide is used as main transmission line for convenient measurement. The fabricated gain equalizer exhibits excellent performances: the input return loss and the output return loss are all less than −14 dB in whole operating frequency band, and the maximum equalization value is about 11 dB.

Machine safety issues with respect to the extension of ECRH systems at ASDEX Upgrade

The beam intensity of electron cyclotron resonance heating at ASDEX Upgrade has the potential to seriously damage in-vessel components, whenever not fully absorbed by the plasma. Operation is, therefore, interlocked with both plasma current and density above a given threshold. Microwave protection detectors installed in several ports on the low field side switch the heating system off, in case the stray radiation exceeds a given threshold. During regular inspections, however, damages were reported in the vicinity of the launchers and in particular around the tiles of the heat shield. On one hand, it was found that insulating material, which may not face the plasma, degraded due to millimetre wave absorption. The waves entered the free space behind the heat shield through gaps. On the other hand, local damage even of metallic components was observed on surfaces, which were directly exposed to the microwave beam. Polarisation errors, which led to a local shine through of significant beam power, were responsible. We note that this happened mainly on the high field side in a certain distance to the microwave protection detectors, which were not triggered by the events. In order to increase the level of protection, we identify three necessary measures: Firstly, polarisation control is to be automated such, that mode content and shine through can be monitored. Secondly, by installing additional detectors, the spatial coverage of stray radiation monitoring is enlarged. Thirdly, the heat shield tiles will be redesigned in order to increase the shielding against millimetre waves.

Structural determination and gas-phase synthesis of monomeric, unsolvated IZnCH3 (X̃(1)A(1)): a model organozinc halide.

The first experimental structure of a monomeric organozinc halide, IZnCH3, has been measured using millimeter-wave direct absorption spectroscopy in the frequency range 256-293 GHz. IZnCH3 is a model compound for organozinc halides, widely used in cross-coupling reactions. The species was produced in the gas phase by reaction of zinc vapor with iodomethane in the presence of a dc discharge. IZnCH3 was identified on the basis of its pure rotational spectrum as well as those of the isotopically substituted species I(66)ZnCH3, I(64)Zn(13)CH3, and I(64)ZnCD3. IZnCH3 is unmistakably a symmetric top molecule (X̃(1)A1) belonging to the C3v point group, in agreement with DFT calculations, with the following experimentally determined structural parameters: rIZn = 2.4076(2) Å, rZnC = 1.9201(2) Å, rCH = 1.105(9) Å, and ∠H-C-H = 108.7(5)°. The basic methyl group geometry is not significantly altered in this molecule. Experimental observations suggest that IZnCH3 is synthesized in the gas phase by direct insertion of activated atomic zinc into the carbon-iodine bond of iodomethane.

Conformational landscape of indan-5-ol: A free-jet millimetre wave study

The rotational spectra of the normal and O-D species of two conformers of indan-5-ol, having about the same energy, have been assigned and measured by free jet millimetre wave absorption spectroscopy. The two conformers differ on the orientation of the hydroxyl group, towards or outwards with respect to the a-axis of the molecule, and have been identified on the basis of the different substitution coordinates of the hydroxyl hydrogen. No splittings of the rotational lines due to the ring puckering of the cyclopentane ring have been observed.

The millimeter-wave spectrum of CCN [formula omitted]: A combined Fourier transform and direct absorption study

New pure rotational measurements have been conducted for the CCN radical (X∼2Πr) in the Ω=1/2 spin–orbit ladder using a combination of Fourier transform millimeter-wave (FTmmW) and millimeter direct absorption techniques. For the FTmmW work, the species was created in a supersonic jet in a pulsed DC discharge from a dilute mixture of CH4 and (CN)2 in argon, while an AC discharge of pure (CN)2 in argon was used for the direct absorption study. Six hyperfine components arising from the nitrogen nuclear spin in the lambda-doublets of the J=5/2→3/2 transition of CCN near 59GHz were recorded, partly as a test of the new FTmmW system. Four transitions were also measured in the 224–296GHz range using direct absorption, as well; at the higher frequencies, lambda-doubling was resolved but the hyperfine structure was effectively collapsed. All measurements required significant signal-averaging. The spectra were analyzed in a combined fit with the previous J=3/2→1/2, FTMW data, using a case (a) Hamiltonian. Rotational and lambda-doubling constants for CCN were significantly improved over previous analyses. Based on the hyperfine and quadrupole constants, the most dominant resonance structure for this radical is the CCN arrangement. Unlike CN and C3N, however, there is still substantial electron density on the nitrogen nucleus such that the CCN structure is also important, as found for CCP and CCAs.