Radek Sedláček

An HF coaxial bridge for measuring impedance ratios up to 1 MHz

A four-terminal pair coaxial ac bridge developed for calibrating both resistance and capacitance ratios and working in the frequency range from 100?kHz up to 1?MHz is described. A reference inductive voltage divider (IVD) makes it possible to calibrate ratios 1:1 and 10:1 with uncertainty of a few parts in 105. The IVD is calibrated by means of a series?parallel capacitance device (SPCD). Use of the same ac bridge with minimal changes for calibrating the SPCD, IVD and unknown impedances simplifies the whole calibration process. The bridge balance conditions are fulfilled with simple capacitance and resistance decades and by injecting voltage supplied from the auxiliary direct digital synthesizer. Bridge performance was checked on the basis of resistance ratio measurements and also capacitance ratio measurements.

Calibrating Standards of Mutual Inductance

As the final stage of development of a new system for impedance calibrations, a facility for calibrating standards of mutual inductance has been realized. Being based on the three-voltmeter method, this facility makes it possible to calibrate 1 mH to 100 mH standards at frequencies up to 5 kHz with uncertainties of the order of 0.01 %. An improved direct connection technique is used when routine calibrations of mutual inductors are made by means of a four terminal-pair LCR meter

A new capacitance device for calibration of N:1 HF inductive voltage dividers

A new series-parallel capacitance device (SPCD) for calibration of high-frequency inductive voltage dividers with nominal ratios N:1, N = 1…10, is described. A fully coaxial design and well-defined calibration planes with four terminal-pair (4-TP) interface make it possible to evaluate SPCD ratios with uncertainties of only a few parts per million at a frequency of 1 MHz.

Calibration of Inductance Standards

A double-balance bridge and a measurement set-up based on the three-voltmeter method are used to calibrate inductance standards by linking them with reference capacitors or resistors. For inductances from 1 mH to 100 mH and frequencies up to 5 kHz, the calibration uncertainties are less than 1 part in 104

Effects of thermal ageing on the static and cyclic mechanical properties of carbon fibres/PDMS composites for use in medicine.

composites for use in medicine T. Suchý, R. Sedláček*, Z. Sucharda, K. Balı́k and T. Bouda Department of Mechanics, Biomechanics and Mechatronics, Faculty of Mechanical Engineering, Czech Technical University in Prague, Technická 4, 166 07 Prague 6, Czech Republic; Department of Composite and Carbon Materials, Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, v.v.i, V Holešovickách 41, Prague 8, Czech Republic

Dry versus hydrated collagen scaffolds: are dry states representative of hydrated states?

AbstractCollagen composite scaffolds have been used for a number of studies in tissue engineering. The hydration of such highly porous and hydrophilic structures may influence mechanical behaviour and porosity due to swelling. The differences in physical properties following hydration would represent a significant limiting factor for the seeding, growth and differentiation of cells in vitro and the overall applicability of such hydrophilic materials in vivo. Scaffolds based on collagen matrix, poly(DL-lactide) nanofibers, calcium phosphate particles and sodium hyaluronate with 8 different material compositions were characterised in the dry and hydrated states using X-ray microcomputed tomography, compression tests, hydraulic permeability measurement, degradation tests and infrared spectrometry. Hydration, simulating the conditions of cell seeding and cultivation up to 48 h and 576 h, was found to exert a minor effect on the morphological parameters and permeability. Conversely, hydration had a major statistically significant effect on the mechanical behaviour of all the tested scaffolds. The elastic modulus and compressive strength of all the scaffolds decreased by ~95%. The quantitative results provided confirm the importance of analysing scaffolds in the hydrated rather than the dry state since the former more precisely simulates the real environment for which such materials are designed.

Improved calculable 4TP coaxial capacitance standards

This paper describes improved calculable 4TP coaxial capacitance standards with nominal values 10 pF and 100 pF. Modifications of construction and internal wiring made possible to lower and better define mutual inductions between some standards parts than in previous version. Frequency dependencies of the standards were calculated up to 1 MHz and first measurements shown a good consistency between theoretical calculations and measurements. Achievable relative accuracy of measurements with such a type of capacitors seems to be on the level 10-5 in frequency range at least up to 1 MHz.

Optimizing and evaluating the biocompatibility of fiber composites with calcium phosphate additives.

SummaryComposite materials based on a polyamide fabric (aramid) and a polydymethylsiloxane (PDMS) matrix were designed for application in bone surgery. In order to increase the bioactivity, 2, 5, 10, 15, 20, and 25 vol.% of nano/micro hydroxyapatite (HA) and tricalcium phosphate (TCP) were added. We studied the effect of the additives on the biocompatibility of the composite. It appears that nano additives have a more favorable effect on mechanical properties than microparticles. 15 vol.% of nano hydroxyapatite additive is an optimum amount for final application of the composites as substitutes for bone tissue: in this case both the mechanical properties and the biological properties are optimized without distinct changes in the inner structure of the composite.

Comparison of frequency dependences of resistance standards made from surface mount resistors

Two versions of four terminal-pair resistance standards have been made from surface mount resistors. Frequency characteristics of these standards, as well as frequency characteristics of a set of quadrifilar resistors with calculable frequency performance, have been measured by means of an Agilent 4285A precision LCR meter. In case of the quadrifilar resistors, the measured frequency characteristics agreed with the calculated ones within the stated uncertainties.

Testing calculable resistors of quadrifilar design

Quadrifilar resistors with nominal values of 100 Omega, 1000 Omega and 10000 Omega, as well as a 100 Omega coaxial resistor, have been realized and their frequency dependences have been calculated from their constructional parameters. With the aim of experimentally verifying correctness of mathematical models used in these calculations, frequency dependences of ratios of parallel equivalent resistances of the realized resistors have been measured by means of a four terminal-pair transformer bridge.