Anizotropne lastnosti lesa v mikrovalovnem območju

doktorska disertacija

Saša Ziherl (Author), Mojca Čepič (Mentor), Jure Bajc (Co-mentor)

Abstract

Tekoči kristali so eni izmed novejših tehnološko uporabnih materialov, ki so prisotni vsepovsod okrog nas. V zadnjem desetletju so postali vsakodnevni spremljevalec večine ljudi po svetu. In prav zaradi tega bi jih lahko uporabili kot motivacijsko sredstvo za povečanje želje za učenje fizike med študenti. Toda poučevanje o lastnostih tekočih kristalov, ki so osnova za delovanje večine tehnoloških naprav s tekočimi kristali, ni enostavno. Pri poučevanju zahtevnejših vsebin velikokrat uporabljamo analogije, ki pripomorejo predvsem k boljši predstavljivosti. Za namen boljše predstavljivosti tipičnih optičnih lastnosti tekočih kristalov smo razvili zbirko eksperimentov z mikrovalovi in s primernimi kosi lesa. Namen disertacije je predstaviti anizotropijo lesa, ki je očitna že zaradi dobro vidnih lesnih vlaken. Osrednji del predstavljajo eksperimenti z vzorci lesa in s šolskim mikrovalovnim kompletom, ki ga sestavljajo mikrovalovni oddajnik in sprejemnik, ter digitalni multimeter. S temi preprostimi pripomočki pokažemo, da je les anizotropen za mikrovalove in da ima dvoosno simetrijo. Le-ta je opazna tudi s prostim očesom, saj so vlakna in letnice dobro vidne. Tako vidimo tri med seboj pravokotne smeri, v katerih ima les različne dielektrične in mehanske lastnosti. Ena smer je orientirana vzporedno z vlakni, drugi dve pa sta pravokotni na vlakna. Ena od teh dveh smeri ima smer radialno glede na letnice in druga tangencialno na letnice. Kljub temu, da se lastnosti lesa razlikujejo v teh dveh smereh, ki sta pravokotni na vlakna, so razlike tako majhne, da jih lahko zanemarimo. Pri teoretični obravnavi širjenja mikrovalov skozi les smo se zato posvetili zgolj enoosni simetriji. Lomna količnika za izredno in redno valovanje izmerimo pri eksperimentu z leseno prizmo. V naslednjem eksperimentu z leseno ploščo s spremenljivo debelino opazujemo in izmerimo linearni dikroizem in dvolomnost kosa lesa. Ker les lahko režemo v poljubnih smereh, s ploščami z različnimi orientacijami vlaken enostavno prikažemo kotno odvisnost izrednega absorpcijskega koeficienta in lomnega količnika. V zadnjem eksperimentu pokažemo, da les sam po sebi ni optično aktivna snov. V nadaljevanju je predstavljena analogija modelov iz lesa s tekočimi kristali. Primerno odžagan kos lesa predstavlja model za nematično fazo tekočih kristalov, saj so tako tekoči kristali kot tudi les dvolomni. Vidna lesna vlakna so analogna podolgovatim molekulam, ki sestavljajo tekoči kristal. Odličen model za izotropno fazo tekočih kristalov predstavlja iverna plošča, saj je analogna izotropni fazi tako po sestavi, kot tudi po lastnostih. še en model, ki predstavlja analogijo holesteričnim tekočim kristalom, je sestavljen iz tankih furnirnih plošč, kjer je vsaka plošča zamaknjena za določen kot glede na predhodno. V disertaciji torej pokažemo, da leseni modeli in mikrovalovi predstavljajo odlično analogijo tekočim kristalom in vidni svetlobi, poleg tega pa z eksperimenti z lesom in mikrovalovi pridobimo dodaten vpogled v makroskopske lastnosti kompleksnih tekočekristalnih struktur in v njihove optične lastnosti.

Keywords

anizotropija;les;mikrovalovi;tekoči kristali;absorpcija;dvolomnost;disertacije;

Data

Language:	Slovenian
Year of publishing:	2013
Source:	[Maribor
Typology:	2.08 - Doctoral Dissertation
Organization:	UM FNM - Faculty of Natural Sciences and Mathematics
Publisher:	S. Ziherl]
UDC:	630*81:539.22(043.3)
COBISS:	19894792
Views:	2241
Downloads:	339
Average score:	0 (0 votes)
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Other data

Secondary language:	English
Secondary title:	Anisotropy of wood in the microwave region
Secondary abstract:	Among many novel technological materials, liquid crystals are present all around us. Over the last decade they have become daily companions of many people. That is why liquid crystals are promising as a topic to provoke interest of students for physics. But on the other hand it is difficult to show and explain various properties of liquid crystals in an easy-to-understand way. Using analogies during teaching can be very helpful for the students to better visualize certain phenomena. That is why a series of experiments, using microwaves and suitably prepared wooden models were developed to show typical optical properties of liquid crystals. The thesis is based on the anisotropy of wood, which is obvious due to clearly visible fibres. The main focus of the thesis is on the experiments with wood samples and a simple school microwave kit, consisting of a microwave transmitter, a microwave receiver, and a multimeter. It is proven that wood is anisotropic for microwaves and it has biaxial symmetry. Three orthogonal directions with different mechanical and also dielectric properties are easily observed. One is directed along the wood fibres and the other two are perpendicular to the wood fibres; one is radial to the annual rings and the second is tangential to the annual rings. Although there is pronounced anisotropy in all three directions, the difference in the dielectric properties between the two directions perpendicular to the fibres is small and it can be neglected. Therefore a uniaxial symmetry is used for wood when measurements are fitted with theoretical models. With a simple experiment using a wooden prism both (ordinary and extraordinary) principal refractive indices are measured. In another experiment the measurements of linear dichroism and birefringence of a wooden plate with variable thickness are presented. As wood can easily be cut in any direction, samples with different fibre orientation were prepared for the purpose of showing extraordinary absorption coefficient and refractive index dependence on the fibre orientation. In the following experiment we show that a piece wood is not optically active. In the continuation the analogy between wood models and liquid crystals is studied in detail. A piece of wood presents a model for a nematic phase of liquid crystals, as they both are anisotropic in refractive index and visible wood fibres are analogous to ordered elongated molecules. A piece of splinter wood represents an isotropic phase. Another model is composed of thin wood veneer sheets; every consecutive sheet is rotated for a fixed angle with respect to the preceding sheet. This model represents an analogue for cholesteric liquid crystals. We therefore show that on one hand, wooden samples and microwaves represent excellent analogy of liquid crystals and visible light, and on the other hand, experiments with microwaves and wooden models provide additional insight into the macroscopic properties of complex liquid crystal structures and their optical properties.
Secondary keywords:	anisotropy;wood;microwaves;liquid crystals;absorption;double refraction;dissertations;
URN:	URN:SI:UM:
Type (COBISS):	Dissertation
Thesis comment:	Univ. v Mariboru, Fak. za naravoslovje in matematiko, Oddelek za fiziko
Pages:	XII, 102 str.
Keywords (UDC):	applied sciences;medicine;technology;uporabne znanosti;medicina;tehnika;agriculture and related sciences and techniques;forestry;farming;wildlife exploitation;kmetijstvo ter sorodne vede in tehnologije;forestry;gozdarstvo;forest products and their utilization;gozdni proizvodi in njihova uporaba;mathematics;natural sciences;naravoslovne vede;matematika;physics;fizika;physical nature of matter;fizikalna zgradba snovi;properties and structure of molecular systems;lastnosti in sestava molekularnih sistemov;
ID:	81763