GHz Microwave Cavity Research Articles

Evaluating the coefficient of thermal expansion of additive manufactured AlSi10Mg using microwave techniques

In this paper we have used laser powder bed fusion (PBF) to manufacture and characterize metal microwave components. Here we focus on a 2.5 GHz microwave cavity resonator, manufactured by PBF from the alloy AlSi10Mg. Of particular interest is its thermal expansion coefficient, especially since many microwave applications for PBF produced components will be in satellite systems where extreme ranges of temperature are experienced. We exploit the inherent resonant frequency dependence on cavity geometry, using a number of TM cavity modes, to determine the thermal expansion coefficient over the temperature range 6–450 K. Our results compare well with literature values and show that the material under test exhibits lower thermal expansion when compared with a bulk aluminium alloy alternative (6063).

Design and implementation of a non-invasive real-time microwave sensor for assessing water hardness in heat exchangers

A non-invasive-monitoring of concentration and dielectric properties of calcium hardness in heat exchanger cooling water was conducted with a 2.5 GHz microwave cavity resonator designed and fabricated locally for the experiment. The principle of electric dipole moment theories were used to analyse the sample solution that occurs as a function of calcium ion content. Artificial difference of water hardness was prepared by mixing CaCl2 in deionised water. The sample was centrally positioned in the electric field of the TM010 mode of a resonant cylindrical cavity. COMSOL simulation package was used to compare and validate the experimental cavity resonator frequency. Transmission signal (S21) measurements via vector network analyser at different concentrations were observed a linear relationship in amplitude with different frequency changes. In addition, calcium absorption provides a first-order change in material polarisation (i.e. real permittivity), and second-order transitions associated dielectric losses (i.e. imaginary permittivity). These research findings introduce a novel technique of real-time monitoring of water hardness concentration by using non-invasive microwave sensor.

Exploring constant substrate temperature and constant high pressure SCD growth using variable pocket holder depths

SCD substrates were successfully synthesized using an optimized pocket holder design. The pocket holder design creates an appropriate thermal environment to shield the diamond substrate from the intense microwave discharge. This substrate configuration allows for a polycrystalline diamond (PCD) rimless, uniform single crystal diamond (SCD) growth process. Square shaped pocket holders with a constant width but with varying depths were used. The SCD growth procedures were carried out via microwave plasma assisted chemical vapor deposition (MPACVD) in a 2.45GHz microwave cavity plasma reactor C at a constant pressure of 240Torr and high power density of ~500W/cm3. By continuously adjusting incident microwave power (Pinc), the substrate temperature (Ts) was held at 1020°C±5°C throughout the entire process cycle. Under these growth conditions, the crystal morphologies exhibited a smooth and flat surface. The average growth rate of the SCDs varied between 30 and 24μm/h as the pocket depth increased from 2.0 to 2.9mm. The PCD rim was eliminated for all pocket depths and the shape of the final, as grown, top SCD surface varied from a square to a circular shape as the pocket holder depth increased. When using the 2.0mm and 2.3mm pockets, the final top SCD surface area increased to almost twice the original HPHT diamond seed area and the lateral growth rate was slightly larger than the vertical growth rate. Birefringence imaging of the grown CVD diamond indicated low stress and SIMS analysis indicated nitrogen concentration levels in the freestanding, CVD diamond plates were in the range of 50ppb to 140ppb. Prime novelty statementSingle crystal diamond is grown by high power density, microwave plasma assisted chemical vapor deposition in a pocket holder held at a constant temperature of 1020°C, a constant pressure of 240Torr and with 5% CH4/H2. Experimental variables were pocket depth and deposition time. Thick, PCD rimless growth was achieved, and for shallow holders, top growth surface area gains of 1. 4–1.9 were demonstrated.

A non-invasive microwave method for assessing solid-state ammonia storage

A 2.45GHz microwave cavity resonator is used to measure the change in dielectric properties of alkali halide salts (such as CaI2) when exposed to ammonia gas. This technique is based on the change in electric dipole moment of the material that occurs as a function of ammonia content, and so can be used to determine ammonia concentration in solids in a non-invasive way. When a powdered sample is placed in the electric field of the TM010 mode of a resonant cylindrical cavity, we find that ammonia absorption gives a first order change in material polarisation (i.e. real permittivity), related to the ammonia sequestered within the solid. The associated dielectric losses (i.e. imaginary permittivity) exhibit second order transitions, which we believe are due to order–disorder transitions between the different coordination complexes of the halide salt.

一种用于微小型铷原子频标的6.83 GHz谐振腔

研制了一种用于微小型铷原子频标的6.83 GHz微波谐振腔。对谐振腔内的电磁场、谐振频率与探针的关系进行了分析，并应用Ansoft HFSS软件进行填充介质的谐振腔进行了仿真分析。在此基础上对介质谐振腔进行设计与加工制造，测量出填充不同厚度的氧化铝陶瓷时谐振腔的谐振频率。结果表明，当矩形谐振腔取12 mm × 15 mm × 18 mm，填充5.66 mm厚的氧化铝陶瓷介质，介质谐振腔频率可达6.83 GHz，模式为TE101，谐振腔体积大幅减小至3.24 cm3, 是传统的未填充谐振腔体积的三分之一，同时电磁场分布均匀，适用于铷气室型微小型原子频标。 This paper presents a 6.83 GHz microwave cavity for miniaturized rubidium atomic frequency standard. Electromagnetic fields in the microwave cavity are analyzed, and relationships of the resonance frequency and the size of the field excitation probe are studied. The Ansoft HFSS software is used to simulate the electromagnetic distributions in microwave cavity. Based on these analyses, the microwave cavity is manufactured. By using the vector network analyzer, the S11 parameters for the micro- wave cavity filled with different thickness of aluminum oxide ceramics are acquired. Tested results demonstrate that the electromagnetic field mode is TE101 when the resonance frequency reaches 6.83 GHz, and cavity size is 12 mm × 15 mm × 18 mm with the thickness of filled aluminum oxide ceramics (96%) of 5.66 mm. The volume of the developed 6.83 GHz microwave cavity is 3.24 cm3, which is about 1/3 of the volume of the conventional unfilled cavity. It fits very well for loading the MEMS- based rubidium filter cell and absorption cell for miniaturized rubidium atomic frequency standards. This paper presents a 6.83 GHz microwave cavity for miniaturized rubidium atomic frequency standard. Electromagnetic fields in the microwave cavity are analyzed, and relationships of the resonance frequency and the size of the field excitation probe are studied. The Ansoft HFSS software is used to simulate the electromagnetic distributions in microwave cavity. Based on these analyses, the microwave cavity is manufactured. By using the vector network analyzer, the S11 parameters for the micro- wave cavity filled with different thickness of aluminum oxide ceramics are acquired. Tested results demonstrate that the electromagnetic field mode is TE101 when the resonance frequency reaches 6.83 GHz, and cavity size is 12 mm × 15 mm × 18 mm with the thickness of filled aluminum oxide ceramics (96%) of 5.66 mm. The volume of the developed 6.83 GHz microwave cavity is 3.24 cm3, which is about 1/3 of the volume of the conventional unfilled cavity. It fits very well for loading the MEMS- based rubidium filter cell and absorption cell for miniaturized rubidium atomic frequency standards.

Efficient microwave heating of microfluidic systems

This paper presents a 2.45GHz microwave cavity resonator with the novel dual function of both sensitive dielectric characterisation and directed, volumetric heating of fluids in a microfluidic chip. This method is shown to have a higher efficiency (>90% for methanol) than previous microfluidic microwave heating methods and can regulate heating using real time cavity characterisation. The system is simple, robust and does not require on-chip integration of microwave waveguide structures. Furthermore, this method can be combined with miniaturised resonators (such as split rings) for localisation of heating within a microfluidic chip.

Microwave-assisted synthesis and characterization of Nd 1.5Mg 17Ni 0.5–Fe 3O 4 hydrogen storage composite

This work reports the preparation of Nd 1.5Mg 17Ni 0.5–Fe 3O 4 hydrogen storage composite in a single mode 2.45 GHz microwave cavity. The physicochemical properties (thermodynamic and kinetic characteristics, hydrogen absorption/desorption properties, thermal behavior, phase composition and morphology) were characterized by pressure-composition isotherms, differential scanning calorimetry, X-ray diffraction, scanning electron microscope with an energy dispersive X-ray spectrometer, transmission electron microscopy, and laser granulometry. The proposed microwave synthesis, in contrast with conventional sintering method, offers rapid heating, makes homogenous composition and hence improves the hydrogen storage properties of the composite.

Synthesis of amorphous Si–Ge alloys using microwave energy

Amorphous silicon–germanium alloys with 70% Si and 30% Ge have been formed by rapid heating in a pure E-field in a single mode 2.45 GHz microwave cavity. The synthesis of amorphous Si–Ge alloy by all conventional processing is difficult. This polarized microwave process provides another striking example of its unique abilities: producing bulk amorphous Si–Ge alloys. The microwave fabricated amorphous Si0.7Ge0.3 alloy composition exhibited relatively high Seebeck coefficient with high potential of its application in high efficient thermoelectric materials.

Infrared and microwaves at 5.8 GHz in a catalytic reactor

An improved micro-reactor cell for IR spectroscopic studies of heterogeneous catalysis was built around a 5.8 GHz microwave cavity. The reactor can operate at 20 bars and with conventional heating up to 720 K, with reactant gas flows velocities (GHSV) from 25,000 to 50,000 h(-1). The temperature of the sample under microwave irradiation was measured by time resolved IR emission spectroscopy. The first experiment performed was the IR monitoring of the desorption of carbonates induced by irradiating an alumina sample by microwaves at 5.8 GHz.

Rapid alloying of silicon with germanium in microwave field using single mode cavity

Silicon and germanium alloys have been formed for the first time by heating powdered mixtures of 75%Si and 25%Ge in just 2 min in the H field in a single mode 2˙45 GHz microwave cavity. As is well known, alloy formation in conventionally heated silicon–germanium mixtures is difficult, even when processed for much longer time durations at the same temperature. Further, any oxide contaminants, if present in the starting material or formed during heating, somehow, appear on the surface of the specimen after microwave treatment.

Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications

In this paper, we report on microwave CVD deposition of high quality polycrystalline diamond and on related post-processing steps to produce smooth, flat and uniformly thick films or diamond substrates. The deposition reactor is a 2.45 GHz microwave cavity applicator with the plasma confined inside a 12 cm diameter fused silica bell jar. The deposition substrates utilized are up to 75 mm diameter silicon wafers. The substrate holder is actively cooled with a water-cooled substrate holder to achieve a substrate surface temperature of 600–1150 C. The pressure utilized is 60–180 Torr and the microwave incident power is 2–4.5 kW. Important parameters for the deposition of thick films with uniform quality and thickness include substrate temperature uniformity as well as plasma discharge size and shape. As deposited thickness uniformities of ± 5% across 75 mm diameters are achieved with simultaneous growth rates of 1.9 μm/h. The addition of argon to the deposition gases improves film deposition uniformity without decreasing growth rate or film quality, over the range of parameters investigated. Post-processing includes laser cutting of the diamond to a desired shape, etching, lapping and polishing steps.

Microwave effects on the electrochemical deposition of copper

The effects of microwave radiation on the electro-deposition of copper metal on platinum, gold, and carbon electrode surfaces are studied by cyclic voltammetry and scanning electron microscopy. Electrochemical experiments under microwave conditions are carried out in a conventional three-electrode electrochemical cell placed into a 2.45 GHz microwave cavity. The microwave effect is associated with steady state electrode temperatures of up to 400 K and mass transport enhancements, which are both electrode size and electrode material dependent. The most dramatic mass transport effects are observed at 25 μm diameter platinum microelectrodes with limiting current increases of more than two orders of magnitude. The morphology of the copper deposit at 50 μm diameter platinum electrodes is shown to change from dendritic to cauliflower-type at high microwave intensities. In situ thermal de-complexation of Cu2+ cations in the presence of a strong complexing agent bovine serum albumin (BSA) is achieved under the microwave conditions.

New driving parameters for diamond deposition reactors: pulsed mode versus continuous mode

Experimental investigation and modeling of pulsed H2/CH4 plasmas used for diamond deposition are presented. Two plasma configurations are studied : a 2.45 GHz microwave cavity configuration and a 915 MHz surface-wave configuration. Time-resolved measurements of the gas temperature determined from the Doppler broadening of the Balmer ­Ha line, of the H-atom relative density and of the discharge volume (Vpl) are reported. The experimental time-variations of the gas temperature are characterized by a sharp increase at the beginning of the pulse (t 1 ms). The simulations enable us to estimate time-variations of the electron energy distribution function, gas temperature and chemical species densities. The in-pulse steady state temperature obtained from the model is in agreement with the measured one, although a discrepancy is obtained on the shape of the early time-variation. Calculations were carried out in order to study the effects of the in-pulse power, the duty cycle and the off-plasma time on the H-atom and CH3-radical densities. It is seen that, at a constant power density averaged over a period, low duty cycles favor high H-atom and CH3 - radical densities, while too long off-plasma times reduce the H-atom density during the pulse. In addition, the production of H atoms was seen to be governed by thermal dissociation in the 2.45 GHz microwave cavity system, and by electronic impact dissociation in the 915 MHz surface wave system, the latter operating under high gas velocities.

Microwave study of screened two-dimensional electron crystals on helium films

We report investigations at microwave frequencies on surface-state electrons (SSE) which were supported by helium films adsorbed on different polymer substrates at temperatures above 1 K. The real and imaginary parts of the dielectric response $\ensuremath{\epsilon}(k,\ensuremath{\omega})$ of the SSE were measured at fixed $k$ and \ensuremath{\omega} inside a 10 GHz microwave cavity. The solidification of the two-dimensional electron crystal was detected by a kink occurring simultaneously in both components of $\ensuremath{\epsilon}(k,\ensuremath{\omega})$ as the electron density was slowly increased at constant temperature. The influence of the dielectric substrate on such a freezing transition has been measured at different temperatures and at various helium-film thicknesses. The results are found to be in good agreement with existing theories. This work represents the first systematic study of a screened two-dimensional electron crystal on helium.

Microwave resonators from YBa2Cu3O7−δ thin films made by plasma-enhanced metalorganic chemical vapor deposition

Epitaxial YBa2Cu3O7−δ thin films on LaAlO3 deposited by plasma-enhanced metalorganic chemical vapor deposition were tested in a 100 GHz microwave cavity and as patterned 5.6 GHz resonators. Two films, both having high critical temperature (≳90 K) and high critical current density (≳106 A/cm2), exhibited low-power Q’s of 5400 and 8000 as 5.6 GHz microstrip resonators when operated at 77 K. The equivalent surface resistances at 10 GHz are 1.7 and 1 mΩ, respectively. Despite the fact that these films are thinner than typical laser ablation films, the results are within a factor of 2 of the best laser ablation prepared YBa2Cu3O7−δ films.

Polymethyl Methacrylate Binder Removal from an Alumina Compact: Microwave Versus Conventional Heating

ABSTRACTCompact samples of alumina and polymethyl methacrylate have been heated in a 2.45 GHz microwave cavity and by conventional heating in an electric furnace. Various heating schedules have been used to effect the removal of the polymeric binder by thermal decomposition. Dielectric properties, porosity and other physical properties have been investigated in order to better understand the binder removal process in a microwave field. Results of the study emphasize the amount of carbon residuals remaining in the bulk.

Microwave Sintering of Nanocrystalline TiO2

ABSTRACTNanocrystalline TiO2 compacts having initial approximate mean grain sizes of 14 nm and approximate green densities of 70% of theoretical were sintered by short-time exposure in a 2.45 GHz microwave cavity to maximum temperatures of 800, 1000 or 1200 ºC. Sample densities were measured before and after exposure to microwaves using Archimede's method. Transmission electron microscopy and x-ray diffraction were utilized to monitor grain growth and phase changes. Rutherford backscattering was used to monitor any changes in oxygen stoichiometry. The results of this study indicate that enhanced densification behavior is obtained for microwavesintered samples relative to samples sintered using conventional pressureless-sintering techniques.

Simultaneous Measurements of Basis Weight and Moisture Content of Sheet Materials by Microwave Cavity

In an ordinary measurement, different sensors separately monitor the basis weight (B) and the moisture weight (M) of papers; thus, measurements of the BM weights are not always correct owing to the different measuring parts and times. The purpose of this note is to develop a method for the simultaneous measurements of the BM weights of paper by using a 3 GHz microwave cavity.

Plasma density measurement in an imperfect microwave cavity

The density of a plasma generated in a TM010/2.45 GHz microwave cavity has been measured from the shift of the TM110 mode excited at a higher frequency. Since the design was aimed at obtaining optimum power coupling, the cavity has several openings and is imperfect from the viewpoint of the density measurement. A technique based on the dipole resonance behavior of cavity modes with nonzero axial mode number has been utilized to calibrate the observed TM110 mode frequency shift, which is found to differ considerably from the ideal cavity formula. A new theoretical formula which accounts for the particular form of cavity imperfection used in the experiment, has been derived and verified. It is also shown that the cavity shift method is conveniently applicable to pulsed microwave discharges.