Microstructural Enhancement of Dental Composite and Ceramic Materials by Plasma Etching

Radio frequency plasma etching of a selected number of dental materials , including glass filled polymeric composites, alumina reinforced ceramic, glass ionomer, and zinc oxide eugenol enhanced microstructural detail for scanning electron microscopy analyses. All four plasmas, argon , oxygen, carbon tetrafluoride (CF4), and CF4 + 4% 0 2 , proved effective, but the latter was the most effective. The etching of all materials , except the ceramic, was accomplished by the selective removal of a polymeric component. For the glass filled composites , this involved removal of the Bis-GMA matrix resin, leaving the filler particles in relief. For a microfilled composite, excellent delineation of the microfine particles contained within the prepolymerized polymer blocks was obtained , while for a hybrid composite, delineation of a bimodal particle distribution was revealed. For glass ionomer material , plasma etching enhanced the delineation between the silica gel lined unreacted glass particles and the polyacrylate matrix, as well as clearly defining regions in the matrix that were totally polysalt in nature. For a resin filled zinc oxide eugenol material, plasma etching removed the resin, leaving behind a porous material made-up of zinc oxide particles held together in a matrix partly composed of crystallites

Microstructural Effects Regarding Fracture of Clinically Retrieved Dental Sonic and Hand Scalers

The fracture surfaces of sonic and hand scaler instruments fractured under clinical and laboratory conditions were compared to their etched sections, and analyzed for evidence revealing the process that led to their failure. Scanning electron microscopy and energy-dispersive spectroscopy indicated the quality of the martensitic stainless steel comprising the sonic scalers, all from one manufacturer, was inferior to the steel comprising the hand scalers from another manufacturer. The sonic scalers contained stringer inclusions aligned longitudinally and consisting of calcium, aluminum, silicon, and other elements up to 50 µm in length. The sonic scalers displayed brittle fracture while the hand scalers displayed mainly ductile fracture. Microcracks occurred between stringer inclusions. Microhardness for sonic scalers, although slightly higher and significantly different from hand scalers, proved ineffective for detecting a structure-property relationship. Microscopy, however, proved very useful for this purpose and also well-suited for analyzing the stress state occurring on the instruments at the time of failure. All sonic scalers were stressed by being bent inwards, while a hand scaler was pulled and torqued by twisting. Retained coarse grinding grooves also affected fracture. Recommendations are made for hand instrument standards to include checks on steel quality for inclusions of the stringer type

Surface Analysis of Tarnished Dental Alloys

Six crown and bridge alloys ranging in nobility between 25-63 wt % (18-45 at %) were analyzed by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and secondary ion mass spectroscopy (SIMS), as well as by L*a*b* colorimetry before and after in vitro tarnishing in artificial saliva with and without additions of 0.00016, 0.016, and 1.6 % Na2S with a rotating wheel apparatus. All alloys except the lowest of 18 at % changed colors to about the same degree after 72 h of tarnishing. All alloys decreased in L*, while increased in both a* and b*, thus appearing darker and with increased redness and yellowness. This was due to localized darkening and to other products. For all alloys except one, saliva without sulfide promoted color changes more severe than for saliva with 0.016% Na2S. For the most part, analysis by EDS was unable to detect differences between the tarnished films and the as-polished surfaces. SIMS analysis, however, showed changes in the substrate ion (Cu, Ag, Pd, and In) peak intensities. In most cases the intensities decreased and with the decrease greater with the sulfide-free saliva than with sulfide-containing. This indicated that sulfide promoted insoluble deposition of products. Changes in the Ag, Pd, and In peak intensities followed much the same pattern as with Cu. The as-polished surfaces, even though carefully prepared, showed much contamination in the form of organics, namely C, CH, N, NH, 0, CHN, CN, as well as from Na, K, Ca, Si, S, Cl, and others. Most tarnished surfaces showed large increases in Na, K, and Ca, and with the sulfide-free saliva being more severe in this regard. The mass spectrum also showed peaks with atomic mass units in the ranqe 55-58 related to only some of the tarnished surfaces

Further Development of the Sextupole and Decapole Spool Corrector Magnets for the LHC

In the Large Hadron Collider (LHC) the main dipoles will be equipped with sextupole (MCS) and decapole (MCD) spool correctors to meet the very high demands of field quality required for the satisfactory operation of the machine. Each decapole corrector will in addition have an octupole insert (MCO) and the assembly of the two is designated MCDO. These correctors are needed in relatively large quantities, i.e. 2464 MCS Sextupoles and 1232 MCDO Decapole-Octupole assemblies. Half the number of the required spool correctors will be made in India through a collaboration between CERN and CAT (Centre for Advanced Technology, Indore, India), the other half will be built by European industry. The paper describes final choices concerning design, materials, production techniques, and testing so as to assure economic magnet manufacture but while maintaining a homogenous magnetic quality that results in a robust product

Experience with the Fabrication and Testing of the Sextupole Superconducting Corrector Magnets for the LHC

The LHC main dipoles will be equipped with sextupole corrector magnets with a field strength of 1700 x2 (T,m) and a magnetic length of 110 mm to correct sextupole field errors. Within the LHC magnet programme CERN has developed in collaboration with CAT a cosine-q type of design where much emphasis has been put on the cost reduction. The magnet features a two-layer racetrack coil, without end spacers, wound from a rectangular NbTi-wire. The two layers are wound simultaneously turning in opposite directions. The yoke is made of a scissor-type of lamination, which allows bringing the iron close to the coil for field enhancement. In this paper we review the manufacturing experiences with the first 12 prototypes built at CERN and CAT. The results of the training at 4.2 K and 1.9 K are presented along with the magnetic field quality measured at room temperature and at 1.9 K

The Effects of Shilling on Final Bid Prices in Online Auctions

An increasing number of reports of online auction fraud are of growing concern to auction operators and participants. In this research, we discuss reserve price shilling, where a bidder shills in order to avoid paying auction house fees, rather than to drive up the price of the final bid. We examine the effect that premium bids, since they are linked with reserve price shill bids, have upon the final selling price. We use 10,260 eBay auctions during April 2001, and identify 1,389 auctions involving 493 sellers and 1,314 involved in concurrent auctions that involving the exact same item. We find that premium bidding occurs 23 % of the time, in 263 of the 1,389 auctions. Using a theoretical perspective involving valuation signals, we show that other bidders may view high bids as signals that an item is worth more. Thus, they may be willing to pay more for the item than items that do not receive premium bids. The implications are disturbing in that sellers may be more motivated to enter a shill bid in order to drive up the final price in an online auction. We also examine and report on alternative hypotheses involving winner’s curse and the possibility of reserve price shill bids. Our results are developed in the context of a weighted leas

Magnetic stray fields in nanoscale magnetic tunnel junctions

The magnetic stray field is an unavoidable consequence of ferromagnetic devices and sensors leading to a natural asymmetry in magnetic properties. Such asymmetry is particularly undesirable for magnetic random access memory applications where the free layer can exhibit bias. Using atomistic dipole-dipole calculations we numerically simulate the stray magnetic field emanating from the magnetic layers of a magnetic memory device with different geometries. We find that edge effects dominate the overall stray magnetic field in patterned devices and that a conventional synthetic antiferromagnet structure is only partially able to compensate the field at the free layer position. A granular reference layer is seen to provide near-field flux closure while additional patterning defects add significant complexity to the stray field in nanoscale devices. Finally we find that the stray field from a nanoscale antiferromagnet is surprisingly non-zero arising from the imperfect cancellation of magnetic sublattices due to edge defects. Our findings provide an outline of the role of different layer structures and defects in the effective stray magnetic field in nanoscale magnetic random access memory devices and atomistic calculations provide a useful tools to study the stray field effects arising from a wide range of defects

Cooperative Sentry Vehicles And Differential GPS Leapfrog

As part of a project for the Defense Advanced Research Projects Agency, Sandia National Laboratories Intelligent Systems and Robotics Center is developing and testing the feasibility of using a cooperative team of robotic sentry vehicles to guard a perimeter, perform a surround task, and travel extended distances. This paper describes the authors most recent activities. In particular, this paper highlights the development of a Differential Global Positioning System (DGPS) leapfrog capability that allows two or more vehicles to alternate sending DGPS corrections. Using this leapfrog technique, this paper shows that a group of autonomous vehicles can travel 22.68 kilometers with a root mean square positioning error of only 5 meters