Study of the Static and Microwave Magnetic Properties of Nanostructured BaFe12−xTixO19

The effect of Ti substitution on the microwave and magnetostatic properties of nanostructured hexagonal BaFe12−xTixO19 ferrite composites is studied. The microwave permeability is measured in the frequency range of 0.1–22 GHz by a coaxial technique. An analysis of the magnetostatic data is made by the law of approach to saturation. The ferrimagnetic resonance frequencies calculated from the magnetostatic data are consistent with those obtained from the microwave measurements. The natural ferrimagnetic resonance frequencies are located in the frequency range of 15 to 22 GHz, depending on the substitution level x. An increase in the amount of substitution elements results in a low-frequency shift of the ferrimagnetic resonance frequency for samples with x < 1. With x rising from 1 to 2.5, the resonance frequency increases. The results of the study demonstrate that the tailored optimization of the nano-structure of a functional material is a robust tool to fine-tune its microwave magnetic properties. The ferrites under study are promising materials to be applied as functional coatings intended to control electromagnetic interference in microwave devices

Control over magnetic spectrum of multilayer magnetic film metamaterial

A RLC electric circuit with magnetic core is studied experimentally and theoretically as a promising design of a metamaterial cell. Laminates made of multilayered ferromagnetic films are used as the magnetic core. The wire coiled around the core allows the frequency dependence of permeability to be adjusted according to needs of a particular task by creating a region of intensive magnetic loss below the ferromagnetic resonance frequency of the bare core. The theoretic analysis is based on the quasi-statics of magnetic fields and electric currents. The intensity of the loss peak is proportional to the value of μ′2/μ″, where μ′ and μ″ are the frequency-dependent components of permeability of the core material. The magnetic spectra of cells with cores made of laminates of NiFe films and FeCo films have been measured. It is shown that the application of the winding allows the magnetic loss peak to be shifted from 1 GHz to 0.3 GHz for NiFe and from 5 GHz to 0.7 GHz for FeCo. The effective imaginary permeability at the resonant frequency increases by the factors of 5 and 6, correspondingly. The theory agrees well with the measured data

Deposition of a SiO2 Shell of Variable Thickness and Chemical Composition to Carbonyl Iron: Synthesis and Microwave Measurements

Protective SiO2 coating deposited to iron microparticles is highly demanded both for the chemical and magnetic performance of the latter. Hydrolysis of tetraethoxysilane is the crucial method for SiO2 deposition from a solution. The capabilities of this technique have not been thoroughly studied yet. Here, two factors were tested to affect the chemical composition and the thickness of the SiO2 shell. It was found that an increase in the hydrolysis reaction time thickened the SiO2 shell from 100 to 200 nm. Moreover, a decrease in the acidity of the reaction mixture not only thickened the shell but also varied the chemical composition from SiO3.0 to SiO8.6. The thickness and composition of the dielectric layer were studied by scanning electron microscopy and energy-dispersive X-ray analysis. Microwave permeability and permittivity of the SiO2-coated iron particles mixed with a paraffin wax matrix were measured by the coaxial line technique. An increase in thickness of the silica layer decreased the real quasi-static permittivity. The changes observed were shown to agree with the Maxwell Garnett effective medium theory. The new method developed to fine-tune the chemical properties of the protective SiO2 shell may be helpful for new magnetic biosensor designs as it allows for biocompatibility adjustment

Incidence and risk factors for persistent symptoms in adults previously hospitalized for COVID-19

Background: The long-term sequalae of COVID-19 remain poorly characterized. We assessed persistent symptoms in previously hospitalized patients with COVID-19 and assessed potential risk factors. Methods: Data were collected from patients discharged from 4 hospitals in Moscow, Russia between 8 April and 10 July 2020. Participants were interviewed via telephone using an ISARIC Long-term Follow-up Study questionnaire. Results: 2,649 of 4755 (56%) discharged patients were successfully evaluated, at median 218 (IQR 200, 236) days post-discharge. COVID-19 diagnosis was clinical in 1291 and molecular in 1358. Most cases were mild, but 902 (34%) required supplemental oxygen and 68 (2.6%) needed ventilatory support. Median age was 56 years (IQR 46, 66) and 1,353 (51.1%) were women. Persistent symptoms were reported by 1247 (47.1%) participants, with fatigue (21.2%), shortness of breath (14.5%) and forgetfulness (9.1%) the most common symptoms and chronic fatigue (25%) and respiratory (17.2%) the most common symptom categories. Female sex was associated with any persistent symptom category OR 1.83 (95% CI 1.55 to 2.17) with association being strongest for dermatological (3.26, 2.36 to 4.57) symptoms. Asthma and chronic pulmonary disease were not associated with persistent symptoms overall, but asthma was associated with neurological (1.95, 1.25 to 2.98) and mood and behavioural changes (2.02, 1.24 to 3.18), and chronic pulmonary disease was associated with chronic fatigue (1.68, 1.21 to 2.32). Conclusions: Almost half of adults admitted to hospital due to COVID-19 reported persistent symptoms 6 to 8 months after discharge. Fatigue and respiratory symptoms were most common, and female sex was associated with persistent symptoms