Flux-Line-Lattice Melting and Upper Critical Field of Bi1.65Pb0.35Sr2Ca2Cu3O10+delta Ceramic Samples

We have conducted magnetoresistance measurements rho(T,H) in applied magnetic fields up to 18 T in Bi1.65Pb0.35Sr2Ca2Cu3O10+delta ceramic samples which were subjected to different uniaxial compacting pressures. The anisotropic upper critical fields H (c2)(T) were extracted from the rho(T,H) data, yielding and the out-of-plane superconducting coherence length xi (c) (0)similar to 3 . We have also estimated and xi (ab) (0) similar to 90 . In addition to this, a flux-line-lattice (FLL) melting temperature T (m) has been identified as a second peak in the derivative of the magnetoresistance d rho/dT data close to the superconducting transition temperature. An H (m) vs. T phase diagram was constructed and the FLL boundary lines were found to obey a temperature dependence H (m) ae(T (c) /T-1) (alpha) , where alpha similar to 2 for the sample subjected to the higher compacting pressure. A reasonable value of the Lindemann parameter c (L) similar to 0.29 has been found for all samples studied.

Clinical evaluation of ceramic inlays and onlays fabricated with two systems: five-year follow-up

This study evaluated the five-year clinical performance of ceramic inlays and onlays made with two systems: sintered Duceram (Dentsply-Degussa) and pressable IPS Empress (Ivoclar Vivadent). Eighty-six restorations were placed by a single operator in 35 patients with a median age of 33 years. The restorations were cemented with dual-cured resin cement (Variolink II, Ivoclar Vivadent) and Syntac Classic adhesive under rubber dam. The evaluations were conducted by two independent investigators at baseline, and at one, two, three, and five years using the modified United States Public Health Service (USPHS) criteria. At the five-year recall, 26 patients were evaluated (74.28%), totalling 62 (72.09%) restorations. Four IPS restorations were fractured, two restorations presented secondary caries (one from IPS and one from Duceram), and two restorations showed unacceptable defects at the restoration margin and needed replacement (one restoration from each ceramic system). A general success rate of 87% was recorded. The Fisher exact test revealed no significant difference between Duceram and IPS Empress ceramic systems for all aspects evaluated at different recall appointments (p>0.05). The McNemar chi-square test showed significant differences in relation to marginal discoloration, marginal integrity, and surface texture between the baseline and five-year recall for both systems (p<0.001), with an increased percentage of Bravo scores. However, few Charlie or Delta scores were attributed to these restorations. In conclusion, these two types of ceramic materials demonstrated acceptable clinical performance after five years

Chain extension of poly (ethylene terephthalate) by reactive extrusion with secondary stabilizer

Poly(ethylene tereftalate) (PET) is a polymer highly susceptible to the hydrolytic reactions that occur during applications and mainly in thermomechanical processing. These reactions lead to the decrease of molecular weight of the polymer, limiting the recycling number of the material. The reactive extrusion of the PET in presence of chain extenders is an alternative to recover mechanical and rheological properties that were depreciated by the polymer degradation. In this study, PET wastes from nonwoven fabrics production were extruded in presence of the secondary stabilizer Irgafos 126 (IRG) on variable concentrations. The results showed that Irgafos 126 increased molecular weight, decreased crystallinity and changed processing behavior of the PET, similarly to the effects produced by the well-known chain extender pyromellitic dianhydride (PMDA), showing that the secondary stabilizer Irgafos 126 can also act as a chain extender for the PET.

Internal fit of two all-ceramic systems and metal-ceramic crowns

OBJECTIVES: The aim of this study was to investigate the internal fit (IF) of glass-infiltrated alumina (ICA - In-Ceram Alumina), yttria-stabilized tetragonal zirconia polycrystals (Y-TZP - IPS e.max ZirCAD), and metal-ceramic (MC - Ni-Cr alloy) crowns. MATERIAL AND METHODS: Sixty standardized resin-tooth replicas of a maxillary first molar were produced for crown placement and divided into 3 groups (n=20 each) according to the core material used (metal, ICA or Y-TZP). The IF of the crowns was measured using the replica technique, which employs a light body polyvinyl siloxane impression material to simulate the cement layer thickness. The data were analyzed according to the surfaces obtained for the occlusal space (OS), axial space (AS) and total mean (TM) using two-way ANOVA with Tukey s multiple comparison test (p<0.05). RESULTS: No differences among the different areas were detected in the MC group. For the Y-TZP and ICA groups, AS was statistically lower than both OS and TM. No differences in AS were observed among the groups. However, OS and TM showed significantly higher values for ICA and Y-TZP groups than MC group. Comparisons of ICA and Y-TZP revealed that OS was significantly lower for Y-TZP group, whereas no differences were observed for TM. CONCLUSIONS: The total mean achieved by all groups was within the range of clinical acceptability. However, the metal-ceramic group demonstrated significantly lower values than the all-ceramic groups, especially in OS.

Development and characterization of non-oxide ceramic composites for mechanical and tribological applications

The main reasons for the attention focused on ceramics as possible structural materials are their wear resistance and the ability to operate with limited oxidation and ablation at temperatures above 2000°C. Hence, this work is devoted to the study of two classes of materials which can satisfy these requirements: silicon carbide -based ceramics (SiC) for wear applications and borides and carbides of transition metals for ultra-high temperatures applications (UHTCs). SiC-based materials: Silicon carbide is a hard ceramic, which finds applications in many industrial sectors, from heat production, to automotive engineering and metals processing. In view of new fields of uses, SiC-based ceramics were produced with addition of 10-30 vol% of MoSi2, in order to obtain electro conductive ceramics. MoSi2, indeed, is an intermetallic compound which possesses high temperature oxidation resistance, high electrical conductivity (21·10-6 Ω·cm), relatively low density (6.31 g/cm3), high melting point (2030°C) and high stiffness (440 GPa). The SiC-based ceramics were hot pressed at 1900°C with addition of Al2O3-Y2O3 or Y2O3-AlN as sintering additives. The microstructure of the composites and of the reference materials, SiC and MoSi2, were studied by means of conventional analytical techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (SEM-EDS). The composites showed a homogeneous microstructure, with good dispersion of the secondary phases and low residual porosity. The following thermo-mechanical properties of the SiC-based materials were measured: Vickers hardness (HV), Young’s modulus (E), fracture toughness (KIc) and room to high temperature flexural strength (σ). The mechanical properties of the composites were compared to those of two monolithic SiC and MoSi2 materials and resulted in a higher stiffness, fracture toughness and slightly higher flexural resistance. Tribological tests were also performed in two configurations disco-on-pin and slideron cylinder, aiming at studying the wear behaviour of SiC-MoSi2 composites with Al2O3 as counterfacing materials. The tests pointed out that the addition of MoSi2 was detrimental owing to a lower hardness in comparison with the pure SiC matrix. On the contrary, electrical measurements revealed that the addition of 30 vol% of MoSi2, rendered the composite electroconductive, lowering the electrical resistance of three orders of magnitude. Ultra High Temperature Ceramics: Carbides, borides and nitrides of transition metals (Ti, Zr, Hf, Ta, Nb, Mo) possess very high melting points and interesting engineering properties, such as high hardness (20-25 GPa), high stiffness (400-500 GPa), flexural strengths which remain unaltered from room temperature to 1500°C and excellent corrosion resistance in aggressive environment. All these properties place the UHTCs as potential candidates for the development of manoeuvrable hypersonic flight vehicles with sharp leading edges. To this scope Zr- and Hf- carbide and boride materials were produced with addition of 5-20 vol% of MoSi2. This secondary phase enabled the achievement of full dense composites at temperature lower than 2000°C and without the application of pressure. Besides the conventional microstructure analyses XRD and SEM-EDS, transmission electron microscopy (TEM) was employed to explore the microstructure on a small length scale to disclose the effective densification mechanisms. A thorough literature analysis revealed that neither detailed TEM work nor reports on densification mechanisms are available for this class of materials, which however are essential to optimize the sintering aids utilized and the processing parameters applied. Microstructural analyses, along with thermodynamics and crystallographic considerations, led to disclose of the effective role of MoSi2 during sintering of Zrand Hf- carbides and borides. Among the investigated mechanical properties (HV, E, KIc, σ from room temperature to 1500°C), the high temperature flexural strength was improved due to the protective and sealing effect of a silica-based glassy phase, especially for the borides. Nanoindentation tests were also performed on HfC-MoSi2 composites in order to extract hardness and elastic modulus of the single phases. Finally, arc jet tests on HfC- and HfB2-based composites confirmed the excellent oxidation behaviour of these materials under temperature exceeding 2000°C; no cracking or spallation occurred and the modified layer was only 80-90 μm thick.

Utilizzo di dispositivi magnetici nel trattamento della patologia artrosica del ginocchio

The use of scaffolds for Tissue Engineering (TE) is increasing due to their efficacy in helping the body rebuild damaged or diseased tissue. Hydroxyapatite (HA) is the most suitable bioactive ceramic to be used in orthopaedic reconstruction since it replicates the mineral component of the hard tissues, and it has therefore excellent biocompatibility properties. The temporal and spatial control of the tissue regeneration process is the limit to be overcome in order to treat large bone and osteochondral defects. In this thesis we describe the realization of a magnetic scaffolds able to attract and take up growth factors or other bio-agents in vivo via a driving magnetic force. This concept involves the use of magnetic nanoparticles (MNP) functionalized with selected growth factors or stem cells. These functionalized MNP act as shuttles transporting the bio-agents towards and inside the scaffold under the effect of the magnetic field, enhancing the control of tissue regeneration processes. This scaffold can be imagined as a fixed “station” that provides a unique possibility to adjust the scaffold activity to the specific needs of the healing tissue. Synthetic bone graft substitutes, made of collagen or biomineralized collagen (i.e. biomimetic Hydroxyapatite/collagen composites) were used as starting materials for the fabrication of magnetic scaffolds. These materials are routinely used clinically to replace damaged or diseased cartilaginous or bone tissue. Our magnetization technique is based on a dip-coating process consisting in the infilling of biologically inspired porous scaffolds with aqueous biocompatible ferrofluids’ suspensions. In this technique, the specific interconnected porosity of the scaffolds allows the ferrofluids to be drawn inside the structure by capillarity. A subsequent freeze-drying process allows the solvent elimination while keeping very nearly the original shape and porosity of the scaffolds. The remaining magnetic nanoparticles, which are trapped in the structure, lead to the magnetization of the HA/Collagen scaffold. We demonstrate here the possibility to magnetize commercially available scaffolds up to magnetization values that are used in drug delivery processes. The preliminary biocompatibility test showed that the investigated scaffolds provide a suitable micro-environment for cells. The biocompatibility of scaffold facilitates the growth and proliferation of osteogenic cells.

I materiali ceramici trasparenti: preparazione, caratterizzazione e correlazione microstruttura-proprietà

This thesis focuses on the ceramic process for the production of optical grade transparent materials to be used as laser hosts. In order to be transparent a ceramic material must exhibit a very low concentration of defects. Defects are mainly represented by secondary or grain boundary phases and by residual pores. The strict control of the stoichiometry is mandatory to avoid the formation of secondary phases, whereas residual pores need to be below 150 ppm. In order to fulfill these requirements specific experimental conditions must be combined together. In addition powders need to be nanometric or at least sub-micrometric and extremely pure. On the other hand, nanometric powders aggregate easily and this leads to a poor, not homogeneous packing during shaping by pressing and to the formation of residual pores during sintering. Very fine powders are also difficult to handle and tend to absorb water on the surface. Finally, the powder manipulation (weighting operations, solvent removal, spray drying, shaping, etc), easily introduces impurities. All these features must be fully controlled in order to avoid the formation of defects that work as scattering sources thus decreasing the transparency of the material. The important role played by the processing on the transparency of ceramic materials is often underestimated. In the literature a high level of transparency has been reported by many authors but the description of the experimental process, in particular of the powder treatment and shaping, is seldom extensively described and important information that are necessary to reproduce the described results are often missing. The main goal of the present study therefore is to give additional information on the way the experimental features affect the microstructural evolution of YAG-based ceramics and thus the final properties, in particular transparency. Commercial powders are used to prepare YAG materials doped with Nd or Yb by reactive sintering under high vacuum. These dopants have been selected as the more appropriate for high energy and high peak power lasers. As far as it concerns the powder treatment, the thesis focuses on the influence of the solvent removal technique (rotavapor versus spray drying of suspensions in ethanol), the ball milling duration and speed, suspension concentration, solvent ratio, type and amount of dispersant. The influence of the powder type and process on the powder packing as well as the pressure conditions during shaping by pressing are also described. Finally calcination, sintering under high vacuum and in clean atmosphere, and post sintering cycles are studied and related to the final microstructure analyzed by SEM-EDS and HR-TEM, and to the optical and laser properties.

Charged colloidal suspensions in confined geometries

Im Zentrum dieser Arbeit steht das Verhalten von geladenen kolloidalen Suspensionen in eingeschränkten Geometrien. Es wurden verschiedene keilförmige Zellen verwendet, die eine kontinuierliche Variation der Abstände zwischen den Platten ermöglichen. In Zellen mit fluid geordneten Suspensionen bei niedrigen Salzkonzentrationen akkumulieren die kolloidalen Partikel in der Keilspitze und bilden kristallin geordnete Strukturen. Systematische Experimente zu diesem Akkumulationseffekt führen zu dem Schluss, dass es um eine elektrostatischer Fallensituation handeln muss, was durch ein einfaches theoretisches, von Löwen et al vorgeschlagenes Modell bestätigt wird. In Abhängig von der Zellhöhe lässt sich in den auftretenden kristallinen Strukturen eine charakteristische Abfolge erkennen. Diese Struktursequenz wurde schon zuvor in eingeschränkten Keilgeometrien beobachtet, jedoch ermöglichen die in unseren Experimenten realisierbaren kleinen Keilwinkel die Beobachtung neuer Strukturen. Einige dieser neuen Strukturen zeigen eine exotische Anordnung die keine atomare Entsprechung besitzen. Basierend auf experimentellen Beobachtungen schlagen wir Modelle für unterschiedliche Übergangsmechanismen zwischen den verschiedenen Strukturen vor, unter der physikalisch motivierten Vorraussetzung, dass sich die Partikel wie einem hohen Druck unterworfene harte Kugeln verhalten. Des Weiteren wurde eine Zelle mit variabler Höhe konstruiert, die zur Untersuchung des vollständigen Phasenverhaltens geladener, zwischen parallelen Platten eingeschlossener Kugeln dient. Die vorläufigen Ergebnisse werden mit theoretischen Prognosen verglichen.

Ab initio quantum mechanical investigation of structural and chemical-physical properties of selected minerals for minero-petrological, structural ceramic and biomaterial applications

The purpose of this thesis is the atomic-scale simulation of the crystal-chemical and physical (phonon, energetic) properties of some strategically important minerals for structural ceramics, biomedical and petrological applications. These properties affect the thermodynamic stability and rule the mineral-environment interface phenomena, with important economical, (bio)technological, petrological and environmental implications. The minerals of interest belong to the family of phyllosilicates (talc, pyrophyllite and muscovite) and apatite (OHAp), chosen for their importance in industrial and biomedical applications (structural ceramics) and petrophysics. In this thesis work we have applicated quantum mechanics methods, formulas and knowledge to the resolution of mineralogical problems ("Quantum Mineralogy”). The chosen theoretical approach is the Density Functional Theory (DFT), along with periodic boundary conditions to limit the portion of the mineral in analysis to the crystallographic cell and the hybrid functional B3LYP. The crystalline orbitals were simulated by linear combination of Gaussian functions (GTO). The dispersive forces, which are important for the structural determination of phyllosilicates and not properly con-sidered in pure DFT method, have been included by means of a semi-empirical correction. The phonon and the mechanical properties were also calculated. The equation of state, both in athermal conditions and in a wide temperature range, has been obtained by means of variations in the volume of the cell and quasi-harmonic approximation. Some thermo-chemical properties of the minerals (isochoric and isobaric thermal capacity) were calculated, because of their considerable applicative importance. For the first time three-dimensional charts related to these properties at different pressures and temperatures were provided. The hydroxylapatite has been studied from the standpoint of structural and phonon properties for its biotechnological role. In fact, biological apatite represents the inorganic phase of vertebrate hard tissues. Numerous carbonated (hydroxyl)apatite structures were modelled by QM to cover the broadest spectrum of possible biological structural variations to fulfil bioceramics applications.

Oxide thermoelectrics via a glass-ceramic route

Thermoelektrizität beschreibt die reversible Beeinflussung und Wechselwirkung von Elektrizität und Temperatur T in Systemen abseits des thermischen Gleichgewichtes. In diesen führt ein Temperaturgradient entlang eines thermoelektrischen Materials zu einem kontinuierlichen Ungleichgewicht in der Energieverteilung der Ladungsträger. Dies hat einen Diffusionsstrom der energiereichen Ladungsträger zum kalten Ende und der energiearmen Ladungsträger zum heißen Ende zur Folge. Da in offenen Stromkreisen kein Strom fließt, wird ein Ungleichgewicht der Ströme über das Ausbilden eines elektrischen Feldes kompensiert. Die dadurch entstehende Spannung wird als Seebeck Spannung bezeichnet. Über einen geeigneten Verbraucher, folgend aus dem Ohm'schen Gesetz, kann nun ein Strom fließen und elektrische Energie gewonnen werden. Den umgekehrten Fall beschreibt der sogenannte Peltier Effekt, bei dem ein Stromfluss durch zwei unterschiedliche miteinander verbundene Materialien ein Erwärmen oder Abkühlen der Kontaktstelle zur Folge hat. Die Effizienz eines thermoelektrischen Materials kann über die dimensionslose Größe ZT=S^2*sigma/kappa*T charakterisiert werden. Diese setzt sich zusammen aus den materialspezifischen Größen der elektrischen Leitfähigkeit sigma, der thermischen Leitfähigkeit kappa und dem Seebeck Koeffizienten S als Maß der erzeugten Spannung bei gegebener Temperaturdifferenz. Diese Arbeit verfolgt den Ansatz glaskeramische Materialien mit thermoelektrischen Kristallphasen zu synthetisieren, sie strukturell zu charakterisieren und ihre thermoelektrischen Eigenschaften zu messen, um eine Struktur-Eigenschaft Korrelation zu erarbeiten. Hierbei werden im Detail eine elektronenleitende (Hauptphase SrTi_xNb_{1-x}O_3) sowie eine löcherleitende Glaskeramik (Hauptphase Bi_2Sr_2Co_2O_y) untersucht. Unter dem Begriff Glaskeramiken sind teilkristalline Materialien zu verstehen, die aus Glasschmelzen durch gesteuerte Kristallisation hergestellt werden können. Über den Grad der Kristallisation und die Art der ausgeschiedenen Spezies an Kristallen lassen sich die physikalischen Eigenschaften dieser Systeme gezielt beeinflussen. Glaskeramiken bieten, verursacht durch ihre Restglasphase, eine niedrige thermische Leitfähigkeit und die Fermi Energie lässt sich durch Dotierungen in Richtung des Leitungs- oder Valenzbands verschieben. Ebenso besitzen glaskeramische Materialien durch ihre Porenfreiheit verbesserte mechanische Eigenschaften gegenüber Keramiken und sind weniger anfällig für den Einfluss des Sauerstoffpartialdruckes p_{O_2} auf die Parameter. Ein glaskeramisches und ein gemischt keramisch/glaskeramisches thermoelektrisches Modul aus den entwickelten Materialien werden konzipiert, präpariert, kontaktiert und bezüglich ihrer Leistung vermessen.

Performance of different polishing techniques for direct CAD/CAM ceramic restorations

This in vitro study evaluated the performance of three ceramic and two commonly used polishing methods on two CAD/CAM ceramics. Surface roughness and quality were compared. A glazed group (GLGR) of each ceramic material served as reference. One-hundred and twenty specimens of VITABLOCS Mark II (VITA) and 120 specimens of IPS Empress CAD (IPS) were roughened in a standardized manner. Twenty VITA and 20 IPS specimens were glazed (VITA Akzent Glaze/Empress Universal Glaze). Five polishing methods were investigated (n=20/group): 1) EVE Diacera W11DC-Set (EVE), 2) JOTA 9812-Set (JOTA), 3) OptraFine-System (OFI), 4) Sof-Lex 2382 discs (SOF) and 5) Brownie/Greenie/Occlubrush (BGO). Polishing quality was measured with a surface roughness meter (Ra and Rz values). The significance level was set at alpha=0.05. Kruskal Wallis tests and pairwise Wilcoxon rank sum tests with Bonferroni-Holm adjustment were used. Qualitative surface evaluation of representative specimens was done with SEM. On VITA ceramics, SOF produced lower Ra (p<0.00001) but higher Rz values than GLGR (p=0.003); EVE, JOTA, OFI and BGO yielded significantly higher Ra and Rz values than GLGR. On IPS ceramics, SOF and JOTA exhibited lower Ra values than GLGR (p<0.0001). Equivalent Ra but significantly higher Rz values occurred between GLGR and EVE, OFI or BGO. VITA and IPS exhibited the smoothest surfaces when polished with SOF. Nevertheless, ceramic polishing systems are still of interest to clinicians using CAD/CAM, as these methods are universally applicable and showed an increased durability compared to the investigated silicon polishers.

Push-out bond strength of CAD/CAM-ceramic luted to dentin with self-adhesive resin cements

OBJECTIVES: This study evaluated the initial and the artificially aged push-out bond strength between ceramic and dentin produced by one of five resin cements. METHODS: Two-hundred direct ceramic restorations (IPS Empress CAD) were luted to standardized Class I cavities in extracted human molars using one of four self-adhesive cements (SpeedCEM, RelyX Unicem Aplicap, SmartCem2 and iCEM) or a reference etch-and-rinse resin cement (Syntac/Variolink II) (n=40/cement). Push-out bond strength (PBS) was measured (1) after 24h water storage (non-aged group; n=20/cement) or (2) after artificial ageing with 5000 thermal cycles followed by 6 months humid storage (aged group; n=20/cement). Nonparametrical ANOVA and pairwise Wilcoxon rank-sum tests with Bonferroni-Holm adjustment were applied for statistical analysis. The significance level was set at alpha=0.05. In addition, failure mode and fracture pattern were analyzed by stereomicroscope and scanning electron microscopy. RESULTS: Whereas no statistically significant effect of storage condition was found (p=0.441), there was a significant effect of resin cement (p<0.0001): RelyX Unicem showed significantly higher PBS than the other cements. Syntac/Variolink II showed significantly higher PBS than SmartCEM2 (p<0.001). No significant differences were found between SpeedCEM, SmartCem2, and iCEM. The predominant failure mode was adhesive failure of cements at the dentin interface except for RelyX Unicem which in most cases showed cohesive failure in ceramic. SIGNIFICANCE: The resin cements showed marked differences in push-out bond strength when used for luting ceramic restorations to dentin. Variolink II with the etch-and-rinse adhesive Syntac did not perform better than three of the four self-adhesive resin cements tested.