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We study how crystal size influences magnetic ordering in arrays of molecular nanomagnets coupled by dipolar interactions. Compressed fluid techniques have been applied to synthesize crystals of Mn6 molecules (spin S=12) with sizes ranging from 28μm down to 220 nm. The onset of ferromagnetic order and the spin thermalization rates have been studied by means of ac susceptibility measurements. We find that the ordered phase remains ferromagnetic, as in the bulk, but the critical temperature Tc decreases with crystal size. Simple magnetostatic energy calculations, supported by Monte Carlo simulations, account for the observed drop in Tc in terms of the minimum attainable energy for finite-sized magnetic domains limited by the crystal boundaries. Frequency-dependent susceptibility measurements give access to the spin dynamics. Although magnetic relaxation remains dominated by individual spin flips, the onset of magnetic order leads to very long spin thermalization time scales. The results show that size influences the magnetism of dipolar systems with as many as 1011 spins and are relevant for the interpretation of quantum simulations performed on finite lattices., We acknowledge support from Grants No. RTI2018- 096075-B-C21 and No. RTI2018-096075-A-C22, funded by MCIN/AEI/10.13039/501100011033 and ERDF A way of making Europe, Grant No. RYC2019-028429-I, funded by MCIN/AEI/10.13039/501100011033 and ESF Investing in your future, the Gobierno de Aragón Grant No. E09-20R-QMAD, and the CSIC Quantum Technology Platform PT-001. This letter used resources of the Advanced Light Source, a Department of Energy, Office of Science User Facility under Contract No. DEAC02-05CH11231. The synthesis and size measurements of Mn6 micro- and nanocrystals were done at the U6 unit of NANBIOSIS ICTS, located at the Institute of Materials Science of Barcelona (CSIC)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

L'òxid de cadmi (CdO) i l'òxid de zinc (ZnO) han estat postulats com dos materials amb gran potencialitat en un ampli ventall d'aplicacions tecnològiques. Pel que fa a les seues propietats físiques, ambdós presenten un caràcter semiconductor de tipus “n”, sense dopatge intencionat. A més, el seu bandgap directe a 2.2 i 3.3 eV, respectivament, asseguren un bon grau de transparència òptica en la regió infraroja i visible de l'espectre electromagnètic. La projecció d'aquests compostos en camps com la plasmònica o les tecnologies basades en transicions intersubbanda fa convenient el control per a l'obtenció de capes molt primes. Malgrat això, gairebé tots els treballs publicats reporten capes per sobre del centenar de nanòmetres. Tenint en compte la possibilitat de controlar la densitat de portadors i l'energia de la banda prohibida, el creixement de l'aliatge ternari dels òxids de cadmi i zinc cobra una especial rellevància. No obstant això, la diferència d'estructures cristal·lines entre el CdO (cúbica) i el ZnO (wurtzita), ha resultat ser un dels principals inconvenients en el procés de creixement del ternari de bona qualitat cristal·lina i amb propietats físiques que confirmen el seu potencial, constituint, per tant, un atractiu repte hui en dia. L'estudi del ternari s'ha centrat en els darrers anys en l'obtenció de la regió rica en zinc, motivat per la possibilitat de sintonitzar el bandgap del ZnO en un ampli rang de l'espectre amb la incorporació de Cd. Tanmateix, la regió rica en cadmi de l'aliatge també emergeix per la seua potencial aplicació com a òxid conductor transparent, si bé el nombre d'estudis és molt més reduït i el desenvolupament d'aquesta opció ha estat menor. Així la majoria de treballs envers el ternari en la fase cúbica reporten capes policristal·lines i amb un rang d'incorporació del zinc generalment reduït. En aquesta tesi s'ha estudiat el creixement dels òxids de cadmi i zinc mitjançant la tècnica MOCVD (sigles de l'anglés: Metal Organic Chemical Vapor Deposition) a pressió atmosfèrica sobre substrats de safir-r. D'una banda, s'ha aprofundit en el procés de creixement dels composts binaris, CdO i ZnO, sobre aquest tipus de substrats, amb l'objectiu d'obtenir capes de gruix nanomètric tot mantenint una bona qualitat morfològica i estructural. I d'altra banda, s'ha analitzat el creixement de l'aliatge ternari, fent una clara distinció entre la regió d'estructura wurtzita (rica en zinc) i la fase cúbica (rica en cadmi). Donant així una entitat pròpia a cadascuna d'aquestes, per tal d'establir en cada cas les condicions favorables al creixement de les dues fases amb vista a l'obtenció d'un material resultant de bona qualitat. D'acord amb aquests objectius, s'ha realitzat un estudi de les primeres etapes del creixement en el cas dels binaris, on s'ha demostrat un mecanisme de creixement del tipus Volmer-Weber, constant-se l'existència d'un gruix llindar que marca la fita inferior a l'obtenció de capes planes i compactes. Gràcies a la incidència sobre els paràmetres del creixement i la promoció d'un augment de la densitat de punts de nucleació, aquest gruix s'ha vist reduït considerablement respecte dels valors típics de la bibliografia. Obtenint capes planes i compactes amb una única orientació out-plane, [002] per al cas de CdO fins als 20 nm i [11-20] per al cas del ZnO de 45 nm. Pel que fa a l'aliatge ternari en la regió rica en Zn i estructura wurtzita, Zn1-xCdxO, la temperatura de creixement ha resultat crítica en aquest procés, on apareix un efecte competitiu entre el creixement de la xarxa hoste estable i la incorporació de l'element alié. L'estudi de la regió rica en Cd ha permés demostrar la incorporació de fins a un 10.4% de Zn mesurat per EDX en una xarxa cúbica Cd1-xZnxO amb una única orientació out-plane [002] i orientació in-plane ben definida. Aquest tipus de creixement ha estat clau per a l'obtenció de les propietats optoelectròniques associades. Cadmium oxide (CdO) and zinc oxide (ZnO) have been proposed as two potential materials in a wide range of technological applications. Regarding its physical properties, both exhibit an “n” type semiconductor character, without intentional doping. Besides, its direct bandgap, at 2.2 and 3.3 eV respectively, assure a good level of optical transparency in the infrared and visible regions of the electromagnetic spectrum. The potential of these compounds in fields such as plasmonics or technologies based on intersubband transitions can demand the growth of very thin films. However, almost all the publications report layers over a hundred nanometers on a wide variety of substrates. The growth of the ternary alloy of the cadmium and zinc oxides is of special importance, taking into account the possibility to control the density of carriers and the energy of the bandgap. Nevertheless, the difference of crystalline structures among CdO (cubic) and ZnO (wurtzite) seems to be one of the main problems in the growth process of the ternary alloy with good crystalline quality and physical properties that confirm its potential. This fact, therefore, constitutes an attractive challenge nowadays. Over the last years, the study of the ternary alloy has been focused on the obtaining of the Zn-rich region, motivated for the possibility of tunning the ZnO bandgap in a wide range of the spectrum by Cd incorporation. Nevertheless, the Cd-rich region of the alloy also emerges for its potential application as transparent conductive oxide, even though this option has been less studied. However, most of the works towards the ternary alloy in the cubic phase reported polycrystalline films and with generally low zinc incorporation range. In the present work, the growth of cadmium and zinc oxides has been studied using the MOCVD (Metal Organic Chemical Vapor Deposition) technique at atmospheric pressure on r-sapphire substrates. On the one hand, the process of growth of the binary compounds, CdO and ZnO, on this type of substrates has been studied in depth, with the aim of obtaining layers with nanometric thickness while maintaining a good morphological and structural quality. On the other hand, the growth of the ternary alloy has been analyzed, making a clear distinction between the wurtzite (Zn-rich) and the cubic (Cd-rich) regions, in order to establish the favorable growth conditions of each phase in a resulting material with good quality. According to these objectives, a study of the early stages of growth for the binary compounds has been carried out, where a Volmer-Weber growth mechanism type has been demonstrated. Also, the existence of a threshold thickness that marks the lower bound to obtaining flat and compact layers has been established. This thickness has been reduced considerably compared to the typical values of the bibliography, by acting on the growth parameters and on the promotion of an increase in the density of nucleation points. Flat and compact films with a single out-plane orientation have been obtained. In the case of CdO [002] films have been grown with thicknesses as low as 20 nm and [11-20] films of 45 nm for ZnO. Regarding the ternary alloy in the Zn-rich region of wurtzite structure, Zn1-xCdxO, the growth temperature has been revealed as critical in this process, where there is a competitive effect between the growth of the stable host lattice and the incorporation of the foreigner element. The study of the rich region in Cd has shown the incorporation of up to 10.4% Zn measured by EDX in a cubic Cd1-xZnxO lattice with a single [002] out-plane orientation and well-defined in-plane orientation. This type of growth has been key for the associated optoelectronic properties.

Els materials escintil·ladors són àmpliament utilitzats com a detectors en els sistemes de detecció de diverses aplicacions, com la imatge mèdica, la física d’alta energia, l’astrofísica i en proves no destructives (seguretat aeroportuària, control industrial, etc.). Hi ha diversos requisits importants per avaluar favorablement el rendiment d’un escintil·lador. Donat que no existeix el escintil·lador ideal, s’estan dedicant molts esforços a buscar nous escintil·ladors inorgànics amb millors propietats que els dels escintil·ladors existents. En aquesta tesi, es creixen monocristalls amb alta qualitat cristal·lina de KGd(PO3)4 tipus III dopats amb Ce3+ i Pr3+, i també nanocristalls de Pr:KGd(PO3)4 tipus III per posteriorment caracteritzar-los estructuralment i òpticament com a possibles nous materials escintil·ladors. Aquests monocristalls es creixen a partir de solucions d’alta temperatura mitjançant la tècnica Top Seeded Solution Growth-Slow Cooling (TSSG-SC) i els nanocristalls es sintetitzen mitjançant el mètode de Pechini. La caracterització estructural inclou la morfologia cristal·lina, estabilitat i expansió tèrmiques, distribució de mida de les partícules, entre altres. S’estudia amb detall l’espectroscòpia de les transicions 4f–5d, sobre les quals es basa el mecanisme d’escintil·lació. Inclou mesures d'absorció òptica, mesures de luminescència i del temps de decaiguda d’aquesta excitant amb llum de sincrotró en el rang espectral des de l'ultraviolat de buit fins l'ultraviolat (VUV-UV), mesures de radioluminescència excitant amb raigs X, entre altres. A més, es proporcionen les dades espectroscòpiques bàsiques dels monocristalls de Pr:KGd(PO3)4 tipus III per aplicacions làser en la gamma de longitud d'ona visible basades en les transicions electròniques 4f-4f. També, es determina la regió de cristal·lització primària del KYP4O12 tipus B i del KY(PO3)4 tipus IV en el sistema ternari K2O–Y2O3–P2O5, ja que són candidats interessants per actuar com a matriu per ions lantànids actius en aplicacions de centelleig. Los materiales centelleadores son ampliamente utilizados como detectores en los sistemas de detección de diversas aplicaciones, como la imagen médica, la física de alta energía, la astrofísica y en pruebas no destructivas (seguridad aeroportuaria, control industrial, etc.). Hay varios requisitos importantes para evaluar favorablemente el rendimiento de un centelleador. Dado que no existe el centelleador ideal, se están dedicando muchos esfuerzos para buscar nuevos centelleadores inorgánicos con mejores propiedades que los de los centelleadores existentes. En esta tesis, se crecen monocristales con alta calidad cristalina de KGd(PO3)4 tipo III dopados con Ce3+ y Pr3+, y también nanocristales de Pr:KGd(PO3)4 tipo III para posteriormente caracterizarlos estructuralmente y ópticamente como posibles nuevos materiales centelleadores. Estos monocristales se crecen a partir de soluciones de alta temperatura mediante la técnica Top Seeded Solution Growth-Slow Cooling (TSSG-SC) y los nanocristales se sintetizan mediante el método de Pechini. La caracterización estructural incluye la morfología cristalina, estabilidad y expansión térmicas, distribución de tamaño de las partículas, entre otros. Se estudia con detalle la espectroscopia de las transiciones 4f-5d, sobre las que se basa el mecanismo de centelleo. Incluye medidas de absorción óptica, medidas de luminiscencia y del tiempo de decaída de ésta excitando con luz de sincrotrón en el rango espectral desde el ultravioleta de vacío hasta el ultravioleta (VUV-UV), medidas de radioluminiscencia excitando con rayos X, entre otros. Además, se proporcionan los datos espectroscópicos básicos de los monocristales de Pr:KGd(PO3)4 tipo III para aplicaciones láser en la gama de longitud de onda visible basadas en las transiciones electrónicas 4f-4f. También, se determina la región de cristalización primaria de KYP4O12 tipo B y de KY(PO3)4 tipo IV en el sistema ternario K2O-Y2O3-P2O5, ya que son candidatos interesantes para actuar como matriz para iones lantánidos activos en aplicaciones de centelleo. Scintillators materials are widely used as detectors in the detection systems of a variety of applications, such as medical imaging, high energy physics, astrophysics and non-destructive testing (airport security, industrial control, etc.). There are several important requirements to evaluate favourably the performance of a scintillator. Since the ideal scintillator does not exist, many efforts are dedicated to find new inorganic scintillators with better properties than those of the existing scintillators. Here, Ce3+- and Pr3+-doped type III KGd(PO3)4 bulk single crystals with high crystalline quality and type III Pr:KGd(PO3)4 nanocrystals are grown and structurally and optically characterized as possible new scintillator materials. These bulk single crystals are grown from high temperature solutions by the Top Seeded Solution Growth-Slow Cooling (TSSG-SC) technique, while the nanocrystals are synthesized by the Pechini method. The structural characterization includes the crystal morphology, thermal stability, thermal expansion, particle size distribution, among others. The spectroscopy of the 4f–5d transitions, on which the scintillation mechanism is based, is studied in detail. The spectroscopic characterization includes optical absorption measurements, luminescence and decay time measurements under synchrotron vacuum ultraviolet-ultraviolet (VUV-UV) excitation, radioluminescence measurements after synchrotron X-ray irradiation, among others. In addition, the basic spectroscopic data of the type III Pr:KGd(PO3)4 crystal for lasing applications in the visible wavelength range based on the 4f–4f electronic transitions is provided. Moreover, the primary crystallization region of type B KYP4O12 and type IV KY(PO3)4 in the K2O–Y2O3–P2O5 ternary system is determined, since these are interesting candidates as hosts for active lanthanide ions in scintillation applications.

[eng] The work presented in this dissertation is focused on the fabrication, integration and test of chemoresistive gas sensor devices and systems based on semiconducting nanowires (NWs). The first objective of this dissertation was to grow monocrystalline In2O3 and Ga2O3 NWs via the vapor-liquid-solid mechanism using a chemical vapor deposition furnace and solid precursors. Subsequently devices based on individual NWs were fabricated, contacted on top of microhotplates using Focused Electron Beam Induced Deposition (FEBID), and their gas sensing properties were characterized. The gas sensors based on individual In2O3 NWs present considerable response towards ethanol at 300 ºC, with a response time of about 4 minutes. On the other hand, the gas sensors based on individual Ga2O3 NWs showed high selectivity towards relative humidity at room temperature, with resistance variations up to 90% in times as short as 2 minutes, and with minimal response to other gases (NO2, CO, ethanol and H2). This behavior is completely different from that reported on this material and is a direct consequence of the NW growth method, which gives rise to a carbon shell around the NWs. Furthermore, the sensing behavior ressembles that of carbon-containing materials.. A second objective was to deepen into methodologies to integrate the sensing material in the substrates where the gas sensing devices are fabricated, with the aim of simplifying the integration procedures and increasing the throughput. With this in mind, dielectrophoretic alignment of NWs was the first methodology proposed to fabricate chemoresistors based on arrays of individual WO3 NWs. The maximum gas response of the fabricated arrays of individual NWs was towards 5 ppm of NO2 for the pristine and towards 100 ppm of EtOH for the Pt-functionalized WO3 NWs, respectively. This higher response of the Pt-functionalized WO3 NWs-based gas sensors is related to the surface decoration of these NWs, which increases the amount of oxygen adsorbed species at their surface, allowing EtOH molecules to be more easily adsorbed than on pristine NWs. The second approach proposed for contacting individual SnO2 NWs on top of suspended microhotplates was Electron Beam Lithography (EBL) in combination with lift-off. The method allows fabricating several devices sequentially but without breaking the vacuum of the EBL system and required optimization of the holder for spinning and of the electron dose used to modify the photoresist properties. The gas characterization of these devices showed higher resistance variations than that obtained for the reference fabrication technique, FEBID. This superior behavior can be the result of the better electrical characteristics of the Ti/Pt contacts in front of the FEBID Pt-deposition. This demonstrates the potentiality of this techniques for contacting individual NWs on top of micromembranes. The third objective was, to a certain point, a natural extension of the device integration activity when considering one of the major drawbacks of chemoresistors: their lack of selectivity. For this, SnO2, WO3 and Ge NWs, have been grown on well-defined and pre-specified regions of one single chip, allowing their simultaneous operation. Here, NO2, CO and relative humidity (RH), diluted in dry synthetic air, have been tested. The calibration of each individual sensor has been carried out exposing the whole chip to the individual gases but with only this particular sensor heated and biased, while the others were unheated and unbiased. This has allowed determining the optimal operation conditions for each sensor. Next, at these optimal temperatures, all the sensors have been tested, simultaneously, towards each gas specie alone. Finally, tests of the three sensors, operating simultaneously, towards mixtures of the three gases were performed. The data from all the mentioned measurements have been treated according to the Principal Component Analysis (PCA) methodology and the results demonstrate that the fabricated sensor system can discriminate and quantify the concentration of the three different studied analytes. The three sensors, made of three different materials and operating simultaneously, constitute an electronic nose, which we here call nano electronic nose., [cat] Aquesta tesi doctoral està enfocada al desenvolupament de dispositius i sistemes sensors de gas basats en nanofils semiconductors monocristal·lins. El primer objectiu aconseguit és el creixement de nanofils d’In2O3 i de Ga2O3 utilitzant un forn pel dipòsit químic en fase vapor. Els nanofils fabricats s’han transferit a xips amb micromembranes suspeses amb calefactor i nanofils individuals s’han contactat amb els seus elèctrodes, emprant el dipòsit assistit per feixos d’electrons. Els nanofils d’In2O3 mostren una resposta considerablement selectiva enfront d’etanol a partir dels 200 ºC i els de Ga2O3 són molt selectius a la humitat relativa a temperatura ambient, fruit del mètode de creixement. Un segon objectiu d’aquesta tesi ha estat explorar dos mètodes d’integració de nanofils individuals: la dielectroforesi, per alinear nanofils de WO3, que han mostrat respostes enfront d’etanol i NO2, a 250 ºC, i la litografia per feixos d’electrons, per contactar nanofils de SnO2 sobre microplataformes calefactores suspeses, que han mostrat respostes satisfactòries enfront d’amoníac a 200 ºC. En ambdós casos s’ha demostrat la viabilitat d’aquestes tècniques. L’últim objectiu ha estat la fabricació i caracterització d’un nas electrònic basat en xarxes de nanofils de SnO2, WO3 i Ge integrades en cadascuna en una micromembrana d’un mateix xip. Aquests nanofils s’han fet créixer directament en regions localitzades i predefinides del xip, les micromembranes, integrats directament en ell, evitant la necessitat de transferir-los després de fer-los créixer. Primer s’ha realitzat el calibratge de cada sensor individual enfront de tres gasos individualment, després enfront de tres gasos simultàniament i, finalment, els tres sensors s’han fet funcionar alhora enfront dels tres gasos. Les dades de totes les mesures s’han tractat segons la metodologia d’anàlisi de components principals i els resultats demostren que aquest sistema és capaç de discriminar entre diverses mescles de NO2 i CO diluïts en aire sintètic sota diferents nivells d’humitat relativa. Els tres sensors, formats per els tres materials diferents i funcionant simultàniament, constitueixen un nano nas electrònic.

This work bases on the solid‐state study of a chiral ZnII‐porphyrin derivative (5,10,15,20‐tetra[(4‐R,R,R,R)‐methyl‐2‐phenoxy‐propanoate, 1) building block and its achiral analogous (2). Here, foreseen the rich molecular recognition of the designed metallo‐porphyrins (1 and 2) and tendency to crystallize, we recrystallized both using two sets of solvents (CH2Cl2/CH3OH and CH2Cl2/hexane). As a result, four different crystalline arrangements (1a‐b, 2a‐b, from 0D to 2D) were successfully achieved. We performed solid state studies for all the species, analysing the role played by chirality, solvent mixtures and surfaces (mica and HOPG), on the supramolecular arrangements. As for the combination of solvents and substrates we obtained a variety of micro‐sized species, from vesicles to flower‐shaped arrays, including geometrical microcrystals. Overall, our results emphasize the environmental susceptibility of metallo‐porphyrins and how this feature must be taken into account in their design.

Analysis of the chiral composition of the crystal mixture obtained from samples of boiling solutions of NaClO3 (see picture) indicates that symmetry breaking towards homochiral compositions may begin in the metastable stage preceding crystallization, that is, at the level of subcritical clusters.

This PhD thesis is focused in structural aspects of alkali nitrates. In the first chapter, we recollected the most important advancement in this field in alkali nitrates compounds published since 1970. Along this lines, crystal structure, polymorphism and phase transition is discussed first; second, crystal growth, and third, the morphology. And, finally a section concerning solid state miscibility and binary phase diagrams between these compounds is presented. The following chapters present a deeper study of growth morphology and how the impurities affect to the morphology and growth rate. These two topics are closely related because the growth morphology is a consequence of the growth rate ratio of different faces. Evidently, we chose only one compound of all the alkali nitrate family. It turned to be sodium nitrate, nitratine or NaNO3. First, in chapter 2, we calculate the morphology of nitratine by applying the two most common approaches to determine the theoretic morphology of crystals: the Bravais-Friedel-Donnay-Harker (BFDH) methodology and the periodic bond chains (PBCs) procedure proposed by Hartman and Perdok (HP). Then we compare the obtained morphology with the experimental growth shape. This compound is interesting from the morphological and structural point of view because it is isostructural with calcite, a calcium carbonate (CaCO3) polymorph. This fact will permit us to discuss how the charges affect the final crystal morphology; an introduction to this question is in chapter 2. In chapter 3 we sought to determination of the normal growth rates (R104) of {104} faces of sodium nitrate single crystals under isothermal conditions in the temperature interval 288 K  297.5 K and hence find the most probable growth mechanisms. Chapter 4 is devoted to the morphology change in NaNO3 crystals by the effect of impurities. It is followed by an atomic force microscopy (AFM) investigation of heterogeneous nucleation of nitratine on calcite in chapter 5. Finally, the NaNO3 – KNO3 phase diagram has been studied from both experimental and theoretical aspects in chapter 6.

In this paper, we focus on the measurement of the normal growth rates (R104) of {10.4} faces of nitratine (NaNO3) single crystals under isothermal conditions in the temperature interval 288-297.5 K in order to unravel the rate determining steps during crystal growth, depending on the hydrodynamic conditions. Owing to the elevated solubility of nitratine in water, the supersaturated system is very sensitive to temperature variation. In this sense, we have constructed a device to control the temperature (its accuracy is below 0.1 K) and the concentration throughout the experiments. We managed to measure the crystal growth of three symmetry equivalent {10.4} faces of NaNO3 under different hydrodynamic conditions. We found that growth of these faces depends on different rate determining steps (being either surface or volume diffusion limited) according to their position with respect to the solution flow. Finally, some thermodynamic crystallization parameters have been calculated to assess the rate determining steps.

Treball de recerca realitzat per un alumne d'ensenyament secundari i guardonat amb un Premi CIRIT per fomentar l'esperit científic del Jovent l'any 2009. Aquest treball de recerca es basa en l'experimentació i, posteriorment, l'obtenció i anàlisi de resultats de l'experiment creador d'anells de Liesegang. Aquest experiment, consistent en la precipitació d'un compost en una base gelificada formant anells distanciats logarítmicament els uns dels altres, ha estat durant més d'un segle objecte d'investigació de moltíssims científics, els quals no han sabut mai treure'n una explicació lògica i raonable d'aquest rar comportament. L'autor ha pretès recrear els curiosos anells intentant formar-los amb diferents inhibidors i compostos als trobats en la bibliografia. Després de realitzar més d'una trentena d'experiments, s'ha realitzat una anàlisi exhaustiva dels resultats. Aquest apartat ha estat un dels més enriquidors, ja que s'han dut a terme en ell comparacions sorprenents i troballes molt curioses, com per exemple la similitud entre els anells de Liesegang i les estructures de Turing, la qual intenta explicar les formes presents en els ocels dels éssers vius; i l'aparició d'anells de Liesegang segons l’òptica visual, efecte inexistent en l’àmplia bibliografia consultada. A més a més, també s'han efectuat una sèrie d'estudis: un en què es confirmen les distàncies logarítmiques entre els anells i on es realitza una comparació entre les dades empíriques i el patró matemàtic; i un altre en què s'estudia el comportament dels anells al variar els factors que regulen la velocitat de reacció. Research project carried out by a pupil of secondary education and rewarded with a Prize CIRIT to foster the scientific spirit of the Youth in year 2009. This research project is based on the experiments and, later, the gathering and analysis of the results of the creator of the Liesegang rings. This experiment, which consisted on the precipitation of one compost on a gel base creating rings which are separated with logarithmic distances, reach not yet a logical, reasoned explanation for this extraordinary behaviour. In this piece of work, the author's aim has been to recreate the rings, trying to form them with different methods. After having carried out more than 30 experiments, the author has compiled an exhaustive analysis of the results. This has proved to be a truly enriching experience as it has presented surprising comparisons and interesting behaviours. For example, the similarity between the Liesegang rings and the Turing structures, which attempts to explain the forms in skin of animals and the apparition of Liesegang rings depending on the optical sight. In addition, a series of studies have been carried out: one of which confirms the logarithmic distances between the rings and which focuses on a comparison of empirical data and the mathematic pattern; and another which studies the behaviour of the rings when the factors which regulate the velocity of the reaction are varied.

We obtained Ba3Yb(BO3)3 single crystals by the flux method with solutions of the BaB2O4–Na2O–Yb2O3 system. The evolution of the cell parameters with temperature shows a slope change at temperatures near 873 K, which may indicate a phase transition that is not observed by changes appearing in the x-ray powder patterns or by differential thermal analysis (DTA). The evolution of the diffraction patterns with the temperature shows incongruent melting at temperatures higher than 1473 K. DTA indicates that there is incongruent melting and this process is irreversible. Ba3Yb(BO3)3 has a wide transparency window from 247 to 3900 nm. We recorded optical absorption and emission spectra at room and low temperature, and we determined the splitting of Yb3+ ions. We used the reciprocity method to calculate the maximum emission cross section of 0.28 × 10−20 cm2 at 966 nm. The calculated lifetime of Yb3+ in Ba3Yb(BO3)3 is τrad = 2.62 ms, while the measured lifetime is τ = 3.80 ms.

S'anomenen cristalls bifuncionals aquells que posseeixen simultàniament propietats d'òptica no lineal i propietats làser. Aquest tipus de materials són molt interessants ja que, combinant aquestes dues propietats, poden auto-generar radiació làser en la regió electromagnètica del blau (≈ 450 nm) o del verd (≈ 550 nm). Els materials làser d'estat sòlid que generen radiació làser en el rang electromagnètic visible (≈ 400-800 nm) tenen actualment un paper molt important en el desenvolupament tecnològic de dispositius làser i òptics en general ja que són interessants per un gran nombre d'aplicacions com són: l'emmagatzematge òptic d'informació d'alta densitat (HDVD, per exemple), reprografia, pantalles de color, medicina, biotecnologia, comunicacions submarines, comunicacions atmosfèriques transparents, etc.Aquest tipus de fonts làser compactes emetent en el visible poden ésser desenvolupades utilitzant un cristall bifuncional. Aquests làsers auto-dobladors de freqüència tenen associades menys pèrdues (absorció, reflexió i dispersió) que làsers dobladors de freqüència intracavitat, que significa que el disseny del ressonador és més simple i compacte. Actualment, la recerca en aquest camp està centrada en el desenvolupament de nous materials no-centrosimètrics que presentin propietats d'òptica no-lineal i que puguin allotjar ions làser actius, majoritàriament ions lantànid. Els materials d'òptica no-lineal tenen la capacitat de generar el segon harmònic de la radiació, és a dir, de doblar la seva freqüència. Si un material d'òptica no-lineal conté un ió lantànid que generi radiació làser en la regió infraroja d'aproximadament =1000 nm, podrà auto-doblar-ne la freqüència, és a dir, reduir-ne a la meitat la longitud d'ona, =500 nm, que correspon a la regió electromagnètica del blau o el verd.El fosfat doble de potassi i gadolini de tipus III, KGd(PO3)4 (KGP), que és objecte d'aquesta tesi doctoral, és un cristall d'estructura no-centrosimètrica i per tant amb el requisit estructural necessari per a posseir propietats d'òptica no-lineal. Es va plantejar l'objectiu de sintetitzar i caracteritzar aquest cristall i a més de substituir parcial o totalment l'ió gadolini per un ió lantànid actiu en la zona de 1000 nm, com són l'iterbi i el neodimi, per tal d'aconseguir desenvolupar un nou cristall auto-doblador de freqüència.Així doncs, l'objectiu de la present tesi doctoral va ésser l'estudi del creixement cristal.lí del KGP i del KGP dopat amb iterbi o neodimi i la seva posterior caracterització. Com que el fosfats doble de potassi i lantànid fonen incongruentment, és a dir, que es descomponen abans de fondre, és necessari utilitzar un mètode de creixement cristal.lí a partir de solució a alta temperatura per créixer cristalls. Aquest mètodes permeten créixer a una temperatura per sota la temperatura de fusió del material, i per tant evitar-ne la seva descomposició. El mètode que nosaltres em utilitzat per créixer monocristalls de KGP pur i dopat és l'anomenat top seeded solution growth-slow cooling (TSSG-SC), que permet créixer els cristalls lliures en la solució disminuint lentament la seva temperatura a partir de la temperatura se cristal.lització. Això permet obtenir cristalls volúmics amb formes cristal.logràfiques desenvolupades lliurament i determinades a partir de l'estructura cristal.lina. Després d'optimitzar el procés de creixement, hem aconseguitr sintetitzar cristalls lliures de macrodefectes i d'una mida suficient pel seu estudi i possible posterior aplicació. Posteriorment, els cristalls han estat caracteritzats estructuralment, físicament, òpticament i espectroscòpicament.Finalment, s'han realitzat experiències de laserat amb Yb:KGP, obtenint uns resultats força encoratjadors, ja que s'ha aconseguit generar radiació làser per primer cop en aquest cristall dopat amb iterbi amb un nivell de dopatge força baix.Com a conclusió i després dels resultats de l'estudi, podem dir que els cristalls de Yb:KGP i Nd:KGP poden ésser considerats com a candidats prometedors per a ser aplicats com a cristalls auto-dobladors de freqüència ja que s'ha provat la seva capacitat tant de generar segon harmònic com de generar radiació làser., Bifunctional crystals are those in which the nonlinear optical process and the laser effect occur simultaneously. This kind of materials is very promising for compact solid-state laser designs operating in the visible because solid-state lasers operate mainly in the infrared. So, the nonlinear and laser properties can combine to generate blue (≈ 450 nm) or green (≈550 nm) laser radiation by self-frequency doubling. Solid-state laser sources operating in the visible spectral region (≈400-800 nm) play an important role in laser technology they are potentially interesting for numerous applications such as high-density optical data storage, reprographics, colour displays, medicine, biotechnology, submarine communications, transparent atmosphere communications, etc.Such compact laser sources can be managed from frequency conversion by nonlinear optical processes such as frequency doubling and sum-frequency mixing by using a bifunctional crystal. Such self-frequency doubling (SFD) lasers involve fewer losses (absorption, reflection and scattering) than the intracavity frequency doubling lasers, which means that resonator designs are simpler and more compact.Currently, efforts are focussed on the development of new noncentrosymmetric crystals with nonlinear optical properties to be used as a host for active laser ions, and mostly lanthanide ions. Nonlinear optical materials are able to double the frequency of the laser emission generated by the active ion hosted in it. Nonlinear crystals with suitable sites for lanthanide ions which emit in the infrared region around 1 m may be able to self-double the frequency, i.e. to reduce the wavelength to the half, =500 nm, which is in the blue or green spectral region.The aim of this doctoral thesis is to synthesize and characterize the type III double phosphate of potassium and gadolinium, KGd(PO3)4 (KGP), either undoped or doped ordoped with ytterbium or neodymium, both of them emitting in the 1 m region.The KGP crystal is noncentrosymmetric, so with the indispensable requirement to have nonlinear optical properties. We propose to use this crystal as a nonlinear host for alternatively ytterbium (Yb3+) or neodymium (Nd3+) to develop a new promising selffrequency doubling laser crystal to generate blue-green laser radiation.As double phosphates of potassium and lanthanide melt incongruently, i.e. they decompose before melting, a high-temperature solution growth method has to be used to growth the crystals. This kind of growth methods allows growing the crystal under its melting point and so avoiding its decomposition. We use the top seeded solution growth-slow cooling (TSSG-SC) to grow the undoped and Yb- or Nd-doped KGP single crystals. This kind of growth method allows to freely growing the crystals in the solution by slowly cooling its temperature from the crystallization temperature. It allows obtaining bulk crystals with freely developed crystallographic forms determined by the crystals structure. By optimizing the growth process, we have successfully grown macrodefect-free crystals large enough for later characterizations and possible final application. The crystals have been structurally, physically, optically and spectroscopically characterized. Finally, laser operation with Yb:KGP has been proved for the first time. Although the low ion doping level in the crystal, the rather high slope efficiency obtained with this first sample is rather promising for the future.In conclusion, single crystals of Yb:KGP and Nd:KGP can be regarded as a promising candidates to be applied as a self-frequency doubling crystals since it has been proved both the second harmonic generation ability and the laser operation.

Flow induced by a random acceleration field (g-jitter) is considered in two related situations that are of interest for microgravity fluid experiments: the random motion of an isolated buoyant particle and coarsening of a solid-liquid mixture. We start by analyzing in detail actual accelerometer data gathered during a recent microgravity mission, and obtain the values of the parameters defining a previously introduced stochastic model of this acceleration field. We then study the motion of a solid particle suspended in an incompressible fluid that is subjected to such random accelerations. The displacement of the particle is shown to have a diffusive component if the correlation time of the stochastic acceleration is finite or zero, and mean squared velocities and effective diffusion coefficients are obtained explicitly. Finally, the effect of g-jitter on coarsening of a solid-liquid mixture is considered. Corrections due to the induced fluid motion are calculated, and estimates are given for coarsening of Sn-rich particles in a Sn-Pb eutectic fluid, experiment to be

We communicate a detailed study of the epitaxial growth of CeOz on MgO. The key feature of the growth is the dependence of the in-plane orientation of the Ce02 epitaxial layer on the MgO surface morphology. Atomic force microscopic (APM) measurements, x-ray analyses, as well as high-resolution transmission electron microscopy (HRTEM) investigations reveal that on rough substrates a cube-on-cube growth of Ce02 on MgO occurs while on smooth substrates the CeO, unit cell is rotated around the surface normal by 45” with respect to the MgO unit cell when the deposition rate is low (-0.3 A/s) during the first stages of growth. This growth mechanism can be used for a defined fabrication of 45” grain boundaries in the CeO, layer by controlling the surface roughness of the MgO substrate. This report demonstrates that these 45” grain boundaries may be used to fabricate YBazCus07-, Josephson junctions. 0 1995 American Institute of Physics.

Textured Ag foils 200 /spl mu/m thick have been obtained after cold rolling and recrystallization at 800/spl deg/C with a reduction ratio of up to 95%. Subsequently, a thick layer of YBa/sub 2/Cu/sub 3/O/sub 7/ (20 /spl mu/m) has been deposited by screen printing, A directional solidification process under a temperature gradient with displacement velocity between 1 mm/h and 10 mm/h has been performed to induce a biaxial texture to the superconducting layer. This last treatment has been performed at low P(O/sub 2/) atmosphere and at maximum temperatures below 945/spl deg/C. The texture of the layers has been studied by X-ray diffraction, rocking curves and pole figures, whilst the critical currents have been measured inductively by SQUID magnetometry. The relationship between the in-plane crystalline orientation and the observed critical currents has been discussed.

Polycrystalline silicon (poly-Si) films were obtained at moderate temperatures (280-500ºC) from a mixture of silane and hydrogen in a hot wire CVD reactor. SEM and TEM results revealed a columnar growth of poly-Si grains with a preferential orientation of the crystals perpendicular to the substrate along the [110] direction. Plain view examinations along the [110] axis revealed a needled shape of the crystals 0.3-1 µm) with the largest axis randomly distributed on the plane. The high quality of the polycrystalline samples obtained makes the hot-wire technique very promising.