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Abstract: Two models have recently been proposed to describe dielectric spectroscopy measurements of ferroelectric nematics (NF) in thin planar capacitors. The polarisation-external capacitance Goldstone reorientation mode (PCG model) considers the NF layer between the electrodes as an effective low resistivity material; the resistivity is inversely proportional to the square of polarisation magnitude. The ‘high-ε’ model considers the NF material as having a huge permittivity due to the ease of polarisation rotation. We study implications of both models and show, why both models describe the majority of the observed dielectric spectroscopy results equally well. We point out differences among the models’ predictions and explain why some observations can be explained only by the high-� model. The high-� model predicts that the frequency range within which capacitors filled with NF material effectively store energy can be increased by using thicker cells, while within the PCG model this frequency range reduces with increasing capacitor thickness. Within both models, a crucial parameter that determines the behaviour of the capacitors filled with an NF material is parasitic resistance, primarily due to the electrode resistance. We present measurements of electrode resistance and find that in ITO cells it is of the order of few hundred ohms.
Keywords: ferroelectric nematics, dielectric measurements, high-permittivity model, electrode resistance
Published in DKUM: 14.08.2025; Views: 0; Downloads: 2
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Abstract: V magistrski nalogi so predstavljeni različni modeli pospešenih testov in klimatskih komor, v katerih se ti izvajajo. Za izvedbo deseturnega pospešenega testiranja preprostega vezja z LED diodo je bil uporabljen standard IEC 60068-2-38. Med testiranjem so se spremljali vpliv temperature in relativne vlažnosti na upornost predupora in LED diodo. Za namene testiranja ročnega mešalnika je bilo izvedeno HALT testiranje, pri katerem se je preverjala pravilnost delovanja glede na podano specifikacijo proizvajalca. Pri tem testu smo se osredotočili na čas delovanja ročnega mešalnika pri polni hitrosti do pojava odpovednih mehanizmov.
Keywords: klimatska komora, pospešeno testiranje, temperatura, relativna vlažnost, IEC standard, HASS
Published in DKUM: 27.03.2025; Views: 0; Downloads: 24
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Abstract: The magnitude of the relative permittivity of the ferroelectric nematic phase (NF ) has been the subject of lively scientific discussion since the phase was recently discovered. Dielectric spectroscopy measurements (DSMs) give a huge value of relative permittivity, which depends on the cell thickness, but this is argued to result from a misinterpretation of the DSM results. We have conducted DSM using a set of cells differing in thickness of the NF layer, type of electrodes, and presence/absence of nanoscale-thick surface polymer layers. To model the DSM results, cells are presented by an equivalent electric circuit that includes a capacitor due to the NF layer with frequency dependent complex relative permittivity, capacitors due to surface layers, and a resistor describing the limited conductivity of electrodes. DSM results for different cells with the same liquid crystal in the NF phase are semiquantitatively reproduced by the same set of physical parameters if a huge relative permittivity of the NF , which is even orders of magnitude larger than the measured apparent values, is assumed. We show that the capacitance of surface layers should also be considered in cells with no polymer alignment layer on the electrodes.
Keywords: liquid crystals, statistical physics
Published in DKUM: 27.11.2024; Views: 0; Downloads: 10
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Abstract: Namen tega projekta je bilo ustvariti vezje, ki bo omogočalo napajanje poljubnega bremena in istočasno merjenje stanja napolnjenosti dveh zaporedno vezanih Li – ionskih baterij, ki breme napajata ter posredovanje podatkov na mobilni telefon preko Bluetooth komunikacije. Izdelano je bilo tiskano vezje, ki vključuje mikrokrmilnik za katerega je bil napisan program, ki omogoča branje napetosti na njegovih vhodih in vse potrebne izračune. Vezje in program sta od začetka delovala kot pričakovano. Vsi potrebni podatki so bili uspešno posredovani na mobilni telefon. Poleg tega je projekt vključeval tudi eksperimentalno določanje kapacitete baterij in pa večkratno analizo praznjenja, katere ugotovitve so služile za izboljšavo programa.
Keywords: Li-ionska baterija, stanje napolnjenosti, električni tok, kapaciteta, praznjenje
Published in DKUM: 07.10.2024; Views: 0; Downloads: 18
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Abstract: We proposed a method for measuring frequency differences of the order of a few Hz with an experimental error lower than 0.0001% by using two 4 MHz quartz oscillators, the frequencies of which are very close (a few 10 Hz difference) due to the dual mode operation (differential mode with two temperature-compensated signal frequencies or a mode with one signal and one reference frequency). We compared the existing methods for measuring frequency differences with the new method which is based on counting the number of transitions through zero within one beat period of the signal. The measuring procedure requires equal experimental conditions (temperature, pressure, humidity, parasitic impedances etc.) for both quartz oscillators. To ensure equal resonant conditions for oscillation two quartz crystals are needed, which form a temperature pair. The frequencies and resonant conditions of both oscillators must be almost equal, which is achieved by an external inductance or capacitance. In such a way, we minimized all the external effects and ensured highly stable oscillations and high sensitivity of the differential sensors. The counter detects one beat period by an external gate signal former. By using the method of counting transitions through zero within one beat period, we reduced the measuring error by three orders of magnitude, compared to the existing methods.
Keywords: small frequency difference measurement, temperature-compensated quartz sensors, differential sensors, quartz oscillators, transitions-through-zero method
Published in DKUM: 26.03.2024; Views: 357; Downloads: 246
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