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<div>Understanding grain growth in dielectric ceramics is essential to controlling the electrical and mechanical properties necessary to produce ceramic capacitors and sensors. The effect of alloying barium titanate with strontium titanate on the equilibrium crystal shape was investigated in order to determine possible impacts on grain growth. The equilibrium crystal shape was studied through three experimental methods to identify possible changes in grain boundary energy or anisotropy with changing composition.</div><div>The first method was by imaging intergranular pores to observe faceting behavior and relative interfacial energies. Intergranular pores were reconstructed to determine the relative surface energies of the identified facets. The second method was to perform atomic force microscopy on surface facets to collect topography data. The topography data was combined with orientation data obtained by EBSD analysis from the same region, and used to calculate the normal vector of the surface facets. These datasets were plotted in a stereographic projection to study the faceting anisotropy. The third method involved collecting EBSD orientation data and images of surface faceting behavior. The surface faceting behavior of each grain was categorized by type of facet and plotted on a stereographic projection at the corresponding orientation. This allowed for the analysis of faceting transitions and the differentiation of faceted and continuous regions of the equilibrium crystal shape. The analysis of faceting behavior across compositions has implications on grain growth of the barium titanate/strontium titanate system.</div>

京都大学 / 0048 / 新制・論文博士 / 博士(工学) / 乙第13015号 / 論工博第4140号 / 新制||工||1650(附属図書館) / 32943 / (主査)教授 斧 髙一, 教授 木村 健二, 教授 立花 明知 / 学位規則第4条第2項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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This thesis reports on the chemical vapour deposition (CVD) growth of carbon nanostructures (mainly carbon nanotubes (CNTs)) on perovskite oxide surfaces with the aid of various catalysts. Two types of perovskite oxide, single crystal SrTiO3 (001) and polycrystalline BaSrTiO₃, have been used as catalyst supports (in metal-catalyst-involved CVD routes) or as catalysts (via metal-catalyst-free CVD routes) for the growth of carbon nanostructures. In metal-catalyst-involved cases, SrTiO₃ (001) single crystal has been proven, for the first time, to serve as a substrate for the growth of CNTs. Fe and Ni catalysts can be tailored in a controllable manner on SrTiO3 (001) surfaces prior to the CNT synthesis, forming truncated pyramid shaped nanocrystals with uniform size distributions. The growth of vertically aligned CNT carpets was realised with the aid of Fe on SrTiO₃ (001) surfaces, and it was further found that the CNTs grow via a base growth model. Furthermore, it is possible to grow helical carbon nanostructures on BaSrTiO3 substrates by introducing a Sn catalyst into the system. The synthesised helical carbon nanostructures follow a tip growth mode, where the structural and chemical aspects of catalyst particles gave rise to a wide range of carbon morphologies. CNTs were also grown on single crystal SrTiO₃ (001) and polycrystalline BaSrTiO3 substrates via metal-catalyst-free routes. The surface-roughness-tailored growth of CNTs was surprisingly achieved on a series of engineered SrTiO₃ (001) surfaces, where a correlation between the surface roughness/morphology of the substrates and the relevant catalytic activity was revealed. The growth of CNTs arises because the catalyst fabrication methods lead to the formation of SrTiO₃ asperities with nanoscale curvatures, over which the CNTs are generated throughout a lift-off process. Facet-selective growth of CNTs was observed on polycrystalline BaSrTiO₃ surfaces, where BaSrTiO₃ (110) facets lead to the growth of CNTs on them, whereas the (001) facets result in no growth at all. This observation was further analysed in the content of the adsorption and diffusion of carbon species on distinct BaSrTiO₃ facets, before reaching the conclusion that the formation of CNTs occurs through a metal-free, stack-up process driven by the assembly of the carbon fragments.

Microwave materials have been widely used in a variety of applications ranging from communication devices to military satellite services, and the study of materials properties at microwave frequencies and the development of functional microwave materials have always been among the most active areas in solid-state physics, materials science, electrical and electronic engineering. In recent years, the increasing requirements for the development of high speed, high frequency circuits and systems require complete understanding of the properties of materials function at microwave frequencies. Ferroelectric materials usually have high dielectric constant, and their dielectric properties are temperature and electric field dependent. The change in permittivity as a function of electric field is the key to a wide range of applications. Ferroelectric materials can be used to fabricate capacitors for electronic industry because of their high dielectric constant, and this is important in the trend toward miniaturization and high functionality of electronic products. The simple tunable passive component based on ferroelectric films is a varactor which can be made as a planar structure and used for electrically tunable microwave integrated circuits. It is an important task to sinter highly tunable and low loss ferroelectrics, fabricate and test the properties of microwave ferroelectric components. This thesis shows experimental results on growth, crystalline and microwave properties of Na0.5K0.5NbO3 (NKN), AgTa0.5Nb0.5O3 (ATN), Ba0.5Sr0.5TiO3 (BST) as well as AgTaO3 (ATO), and AgNbO3 (ANO) thin films. The films were grown by Pulsed Laser Deposition (PLD) and rf-magnetron sputtering techniques from stoichiometric high density ceramic NKN, ATN, ATO, ANO and BST targets onto LaAlO3 (LAO), Al2O3 (r-cut sapphire), Nd:YAlO3 single crystals and amorphous glass substrates. Advanced X-ray diffraction examinations showed NKN, ATN, BST films on LAO substrates grow epitaxially, whereas films on r-cut sapphire were found to be preferentially (00l) oriented. Coplanar waveguide 2 µm finger gap interdigital capacitor (CPWIDC) structures were fabricated by photolithography process and metal lift-off technique. On-wafer tests up to 40 GHz were performed to characterize microwave properties of the ferromagnetic film CPWIDC devices. The measurement setup is composed of network analyzer, probe station, and microwave G-S-G probes. External electric field was applied to planar capacitors to measure tunability. Original de-embedding technique has been developed to calculate capacitance, loss tan δ, and tunability of varactors from the measured S-parameters. NKN film interdigital capacitors on Nd:YAlO3 showed superior performance compared to ATN in the microwave range from 1 to 40 GHz. Within this range, the voltage tunability (40V, 200 kV/cm) was about 29%, loss tangent ~ 0.13, K-factor = tunability/tan δ from 152% @ 10GHz to 46% @ 40GHz. The ATN/sapphire CPWIDCs showed the lowest dispersion ~ 4.3% in whole frequency range from 1 to 40 GHz, voltage tunability 4.7% @ 20GHz and 200 kV/cm, lowest loss tangent ~ 0.068 @ 20GHz, K-factor = tunability/tan δ ranged from 124% @ 10GHz to 35% @ 40GHz. BST film CPWIDCs on sapphire showed frequency about 17%, the highest voltage tunability ~ 22.2%, loss tangent ~ 0.137 @ 20GHz, and K-factor = 281% @ 10GHz to 95% @ 40GHz. / QC 20100906

The present study describes the growth and characterization of pulsed laser ablated Bao.sSro.sTiOs (BST) thin films. Emphasis has been laid on the study of a plausible correlation between structure and property in order to optimize the processing parameters suitably for required application. An attempt has been made to understand the basic properties such as, origin of dielectric response, charge transfer under low and high-applied electric fields across the BST capacitor and finally the dielectric breakdown process. Chapter 1 gives a brief introduction on the application of ferroelectric thin films in microelectronic industry and its growth techniques. It also addresses the present issues involved in the introduction of BST as a capacitor material for high-density dynamic random access memories. Chapter 2 outlines the motivation for the present study and briefly outlines the research work involved. Chapter 3 describes the experimental procedure involved in the growth and characterization of BST thin films using pulsed laser ablation technique. Details include the setup design for PLD growth, material synthesis for the ceramic targets, deposition conditions used for thin film growth and basic characterizations methods used for study of the grown films. Chapter 4 describes the effect of systematic variation of deposition parameters on the physical and electrical properties of the grown BST films. The variation in processing conditions has been found to directly affect the film crystallinity, structure and morphology. The change observed in these physical properties may also be correlated to the observed electrical properties. This chapter summarizes the optimal deposition conditions required for growing BST thin films using a pulsed laser ablation technique. Microstructure of BST films has been categorized into two types: (a) Type I structure, with multi-grains through the film thickness, for amorphous as-grown films after high temperature annealing (exsitu crystallized), and (b) columnar structure (Type II) films, which were as-grown well-crystallized films, deposited at high temperatures. The ac electrical properties have been reviewed in detail in Chapter 5. Type I films showed a relatively lower value of dielectric constant (e ~ 426) than Type II films with dielectric constant around 567. The dissipation factors were around 0.02 and 0.01 for Type I and Type II films respectively. The dispersion in the frequency domain characteristics has been quantitatively explained using Jonscher's theory. Complex impedance spectroscopy employed showed significant grain boundary response in the case of multi-grained Type I films while negligible contribution from grain boundaries has been obtained in the case of columnar grained Type II BST films. The average relaxation time r obtained from the complex impedance plane plots show almost three orders higher values for Type I films. The obtained results suggest that in multi-grained samples, grain boundary play a major role in electrical properties. This has been explained in accordance to a model proposed on the basis of depleted grains in the case of Type I films where the grain sizes are smaller than the grain boundary depletion width. Chapter 6 describes the dc leakage properties of the grown BST thin films and the influence of microstructure on the leakage properties. It was evident from the analysis of the graph of leakage current against measurement temperature, that, the observed leakage behavior in BST films, can not be attributed to a single charge transport mechanism. For Type I films, the Arrhenius plot of the leakage current density with 1000/T exhibits different regions with activation energy values in the range of 0.5 and 2.73 for low fields (2.5kV/cm). The activation energy changes over to 1.28 eV at high fields (170 kV/cm). The obtained values agree well with that obtained from the ac measurements, thus implying a similarity in the origin of the transport process. The activation energy value in the range of 0.5 eV is attributed to the electrode/film Schottky barrier, while the value in the range of 2.73 eV is due to deep trap levels originating from Ti+3 centers. The value in the range of 1.28 eV has been attributed to oxygen vacancy motion. Similar results have been obtained from the Arrhenius plot of the leakage current for Type II films. In this case, only two different activation energy values can be identified in the measured temperature and applied electric field range. At low fields the activation energy value was around 0.38 eV while at high fields the value was around 1.06 eV. These values have been identified to be originating from the electrode/film Schottky barrier and oxygen vacancy motion respectively. Thus a complete picture of the charge transport process in the case of BST thin film may be summarized as comprising of both electronic motion as well as contribution from oxygen vacancy motion. The effect of electrical stress on the capacitance-voltage (C-V) and the leakage current has been analyzed in Chapter 7. From the change in the zero bias capacitance after repeated electron injection through the films the values of the electronic capture cross-section and the total trap density for Type I and II films have been estimated. The results showed higher values for Type I film in comparison to Type II films. The difference has been attributed to the presence of grain boundaries and a different interface in the case of Type I films when compared to Type II films where the absence of grain boundaries is reflected in the columnar microstructure. A study of the time-dependent-dielectric-breakdown (TDDB) characteristics under high fields for Type I and Type II films showed higher endurance for Type I film. On the other hand space-charge-transient characteristics have been observed in the case of Type II films at elevated temperature of measurement. Mobility and activation energy values extracted from the transient characteristics are found to be in the range of 1 x 10~12 cm2 /V-sec and 0.73 eV respectively, suggesting a very slow charge transport process, which has been attributed to the motion of oxygen vacancies. An overall effect of electrical stress suggested that oxygen vacancy motion can be related to the observed resistance degradation and TDDB, which has been further enhanced by the combination of high temperature and high electric fields. Chapter 8 deals with the effect of intentional doping in the BST films. The doping includes Al at the Ti-site, Nb in the Ti-site and La at the Ba/Sr-site. The effect of doping was observed both on the structure and electrical properties of the BST films. Acceptor doping of 0.1 atomic 7c Al was found to decrease the dielectric constant as well as the leakage current. For higher concentration of acceptor-dopant, the leakage current was found to increase while showing space-charge-transient in the TDDB characteristics, again suggesting the effect of increased concentration of oxygen vacancies. Donor doping using 2 atomic % La and Xb significantly improved the leakage as well as the TDDB characteristics by reducing the concentration of oxygen vacancies. A further procedure using graded donor doping in the BST films exhibits even better leakage and TDDB properties. An unconventional, graded doping of donor cations has been carried out to observe the impact on leakage behavior, in particular. The leakage current measured for a graded La-doped BST film show almost six orders of lower leakage current in comparison to undoped BST films, while endurance towards breakdown has been observed to increase many-fold. Chapter 9 highlights the main findings of the work reported in this thesis and lists suggestions for future work, to explore new vistas ahead.

Σύνθετα που ενσωματώνουν σιδηροηλεκτρικά και πιεζοηλεκτρικά νανοσωματίδια, ομοιογενώς διεσπαρμένα μέσα σε μήτρα άμορφου πολυμερούς, αντιπροσωπεύουν μια νέα κατηγορία υλικών. Τα νανοδιηλεκτρικά σύνθετα ανήκουν σε ένα νέο τύπο υλικών που παρασκευάζονται για βελτιωμένες επιδόσεις, σαν διηλεκτρικά και ηλεκτρικοί μονωτές. Ορισμένα κεραμικά υλικά μπορούν να επιλεγούν και να αναμιχθούν με πολυμερή για να επιτευχθεί συνέργια μεταξύ της υψηλής διηλεκτρικής αντοχής των πολυμερών και της υψηλή διηλεκτρικής σταθεράς των κεραμικών. Τα εν λόγω συστήματα μπορούν να χρησιμοποιηθούν σε πολλές εφαρμογές, όπως σε ολοκληρωμένους πυκνωτές αποσύζευξης, ακουστικούς αισθητήρες εκπομπής, επιταχυνσιόμετρα γωνιακής επιτάχυνσης και ελεγκτές ρεύματος διαρροής, καθώς και σε στρατιωτικούς εξοπλισμούς και εφαρμογές στις μεταφορές. Έχει διαπιστωθεί πως τα νανοσύνθετα παρουσιάζουν βελτιωμένες ιδιότητες, αλλά όχι πλήρως κατανοητές, συγκριτικά με τα μικροσύνθετα. Επιπλέον, μελέτες δείχνουν ενδιαφέρουσες συμπεριφορές, όταν πρόκειται για πολύ χαμηλές περιεκτικότητες σε νανοσωματίδια. Στην παρούσα μελέτη, νανοσύνθετα εποξειδικής ρητίνης και κεραμικών νανοσωματιδίων BaSrTiO3 (μεικτό οξείδιο τιτανικού στροντίου βαρίου, BST) (<100 nm), παρασκευάστηκαν με διαδικασία ανάμειξης σε ένα ευρύ φάσμα συγκεντρώσεων, με σκοπό να μελετηθεί η επίδραση της πολύ χαμηλής (ή πολύ υψηλής) περιεκτικότητας στα χαρακτηριστικά του συστήματος. Οι διηλεκτρικές ιδιότητες και τα φαινόμενα χαλάρωσης μελετήθηκαν με τη βοήθεια της διηλεκτρικής φασματοσκοπίας (BDS) στο εύρος θερμοκρασιών από 30 oC έως 160 oC και συχνοτήτων 10-1 Hz έως 107 Hz. Ο μορφολογικός χαρακτηρισμός έγινε μέσω της ηλεκτρονικής μικροσκοπίας σάρωσης (SEM) και διαπιστώθηκε πως η νανοδιασπορά των εγκλεισμάτων είναι επιτυχής. Ο δομικός χαρακτηρισμός, των δοκιμίων αλλά και των σωματιδίων BaSrTiO3, έγινε μέσω περίθλασης ακτίνων-Χ (XRD) και διαπιστώθηκε πως τα φάσματα έρχονται σε συμφωνία με τη σχετική βιβλιογραφία. Από την ανάλυση των αποτελεσμάτων της διηλεκτρικής φασματοσκοπίας καταγράφονται τρεις διηλεκτρικές χαλαρώσεις, που αποδίδονται με φθίνουσα σειρά των χρόνων χαλάρωσης, στη διεπιφανειακή πόλωση, την μετάπτωση από την υαλώδη στην ελαστομερική φάση της μήτρας (α-χαλάρωση) και σε επαναδιευθετήσεις πολικών πλευρικών ομάδων της κύριας πολυμερικής αλυσίδας (β-χαλάρωση). Η εξέταση της επίδρασης της περιεκτικότητας σε νανοεγκλέισματα, στη διηλεκτρική απόκριση των σύνθετων αποκαλύπτει μη-αναμενόμενες συμπεριφορές, σε χαμηλές (αλλά και υψηλές) περιεκτικότητες. Πιο συγκεκριμένα στις χαμηλές περιεκτικότητες εμφανίζονται φαινόμενα ακινητοποίησης των μακροαλυσίδων και επακόλουθα των διπόλων, με αποτέλεσμα τη μείωση της διαπερατότητας και την αύξηση της θερμοκρασίας υαλώδους μετάπτωσης του συστήματος. Αντίστοιχα σε υψηλές περιεκτικότητες υπάρχει εκ νέου αύξηση της θερμοκρασίας υαλώδους μετάπτωσης, λόγω περιορισμένης κινητικότητας των διπόλων. Συμπερασματικά, θεωρείται πως υπάρχουν τρεις ‘’ζώνες περιεκτικοτήτων’’ που προσδίδουν διαφορετικά χαρακτηριστικά στη διηλεκτρική συμπεριφορά του συστήματος, μέσω της ρύθμισης των αλληλεπιδράσεων ρητίνης-εγκλεισμάτων. / Ceramic–polymer composites incorporating ferroelectric and piezoelectric crystal nanoparticles, homogeneously dispersed within an amorphous polymer matrix represent a novel class of materials. Nanodielectric composites belong to a new type of engineering materials suitable for improved performance as dielectrics and electrical insulators. Certain ceramic materials can be selected to be blended with polymers providing synergy between the high breakdown strength of polymers and the high permittivity of ceramic materials. These type of material systems can be used in plenty applications such as integrated decoupling capacitors, acoustic emission sensors, angular acceleration accelerometers, smart skins and leakage current controllers, as well as in military equipment and transport applications. It has been found that nanocomposites exhibit enhanced properties, yet not fully understandable, comparably to microcomposites. Furthermore, literature demonstrates some interesting dielectric behaviors when filler’s concentration comes to very low nanoparticle loadings. In the present study, nanocomposites of epoxy resin and ceramic BaSrTiO3 (Barium-Strontium Titanate, BST) nanoparticles (<100 nm), were prepared with a mixing procedure in a wide range of nanofiller concentrations, aiming to investigate the impact of very low (or very high) loadings on the system’s properties. The dielectric properties and the related relaxation phenomena were studied by means of Broadband Dielectric Spectroscopy (BDS) in the temperature range from 30 oC to 160 oC and frequency range from 10-1 Hz to 107 Hz. Scanning electron microscopy (SEM) was employed in order to examine the morphology of the produced specimens. The dispersion of nanoinclusions can be considered as satisfactory. Structural characterization of the systems as well as of the BaSrTiO3 nanopowder was examined via x-ray diffraction (XRD). Obtained results are in accordance with literature. Three dielectric relaxation processes were detected form the analysis of the dielectric spectra. They are attributed, with descending order of relaxation time, to interfacial polarization, glass to rubber transition of the polymer matrix (α-relaxation), and re-arrangement of polar side groups of the main macromolecular chain (β-relaxation). The influence of the nanoparticles content upon the dielectric response of the composites, reveal unexpected behaviours at low and high filler loading. In particular, effects of immobilization/entanglement of macromolecular chains and subsequently of dipoles, resulting to a decrease of the permittivity values and enhancement of the glass to rubber transition temperature of the systems, were ascertained. At the opposite edge, at high filler loading, an increase of glass to rubber transition temperature was also found, due to the limited mobility of the chains and dipoles. Concluding, the existence of three “zones of filler content” is assumed, providing different characteristics in the dielectric response of the systems, because of the tunable polymer-inclusion interactions.