84 research outputs found
Use of zinc phosphate cement as a luting agent for Denzirâą copings: an in vitro study
BACKGROUND: The clinical success rate with zinc phosphate cemented Procera crowns is high. The objective with this study was to determine whether CADCAM processed and zinc phosphate cemented Denzir copings would perform as well as zinc phosphate cemented Procera copings when tested in vitro in tension. METHODS: Twelve Procera copings and twenty-four Denzir copings were made. After the copings had been made, twelve of the Denzir copings were sandblasted on their internal surfaces. All copings were then cemented with zinc phosphate cement to carbon steel dies and transferred to water or artificial saliva. Two weeks after cementation, half of the samples were tested. The remaining samples were tested after one year in the storage medium. All tests were done in tension and evaluated with an ANOVA. RESULTS: Sandblasted and un-sandblasted Denzir copings performed as well as Procera copings. Storage in water or artificial saliva up to one year did not decrease the force needed to dislodge any of the coping groups. Three copings fractured during testing and one coping developed a crack during testing. The three complete fractures occurred in Procera copings, while the partly cracked coping was a Denzir coping. CONCLUSION: No significant differences existed between the different material groups, and the retentive force increased rather than decreased with time. Fewer fractures occurred in Denzir copings, explained by the higher fracture toughness of the Denzir material. Based on good clinical results with zinc phosphate cemented Procera crowns, we foresee that zinc phosphate cement luted Denzir copings are likely to perform well clinically
Black Holes in the Early Universe
The existence of massive black holes was postulated in the sixties, when the
first quasars were discovered. In the late nineties their reality was proven
beyond doubt, in the Milky way and a handful nearby galaxies. Since then,
enormous theoretical and observational efforts have been made to understand the
astrophysics of massive black holes. We have discovered that some of the most
massive black holes known, weighing billions of solar masses, powered luminous
quasars within the first billion years of the Universe. The first massive black
holes must therefore have formed around the time the first stars and galaxies
formed. Dynamical evidence also indicates that black holes with masses of
millions to billions of solar masses ordinarily dwell in the centers of today's
galaxies. Massive black holes populate galaxy centers today, and shone as
quasars in the past; the quiescent black holes that we detect now in nearby
bulges are the dormant remnants of this fiery past. In this review we report on
basic, but critical, questions regarding the cosmological significance of
massive black holes. What physical mechanisms lead to the formation of the
first massive black holes? How massive were the initial massive black hole
seeds? When and where did they form? How is the growth of black holes linked to
that of their host galaxy? Answers to most of these questions are work in
progress, in the spirit of these Reports on Progress in Physics.Comment: Reports on Progress in Physics, in pres
Galaxy bulges and their massive black holes: a review
With references to both key and oft-forgotten pioneering works, this article
starts by presenting a review into how we came to believe in the existence of
massive black holes at the centres of galaxies. It then presents the historical
development of the near-linear (black hole)-(host spheroid) mass relation,
before explaining why this has recently been dramatically revised. Past
disagreement over the slope of the (black hole)-(velocity dispersion) relation
is also explained, and the discovery of sub-structure within the (black
hole)-(velocity dispersion) diagram is discussed. As the search for the
fundamental connection between massive black holes and their host galaxies
continues, the competing array of additional black hole mass scaling relations
for samples of predominantly inactive galaxies are presented.Comment: Invited (15 Feb. 2014) review article (submitted 16 Nov. 2014). 590
references, 9 figures, 25 pages in emulateApJ format. To appear in "Galactic
Bulges", E. Laurikainen, R.F. Peletier, and D.A. Gadotti (eds.), Springer
Publishin
Kinematics and simulations of the stellar stream in the halo of the Umbrella Galaxy
We study the dynamics of faint stellar substructures around the Umbrella Galaxy, NGC 4651, which hosts a dramatic system of streams and shells formed through the tidal disruption of a nucleated dwarf elliptical galaxy. We elucidate the basic characteristics of the system (colours, luminosities, stellar masses) using multiband Subaru/Suprime-Cam images. The implied stellar mass ratio of the ongoing merger event is âŒ1:50. We identify candidate kinematic tracers (globular clusters, planetary nebulae, Hâii regions) and follow up a subset with Keck/DEIMOS (DEep Imaging Multi-object Spectrograph) spectroscopy to obtain velocities. We find that 15 of the tracers are likely associated with halo substructures, including the probable stream progenitor nucleus. These objects delineate a kinematically cold feature in positionâvelocity phase space. We model the stream using single test particle orbits, plus a rescaled pre-existing N-body simulation. We infer a very eccentric orbit with a period of âŒ0.35âGyr and turning points at âŒ2â4 and âŒ40âkpc, implying a recent passage of the satellite through the disc, which may have provoked the visible disturbances in the host galaxy. This work confirms that the kinematics of low surface brightness substructures can be recovered and modelled using discrete tracers â a breakthrough that opens up a fresh avenue for unravelling the detailed physics of minor merging
Substructure and Tidal Streams in the Andromeda Galaxy and its Satellites
Tidal streams from existing and destroyed satellite galaxies populate the
outer regions of the Andromeda galaxy (M31). This inhomogeneous debris can be
studied without many of the obstacles that plague Milky Way research. We review
the history of tidal stream research in M31, and in its main satellite
galaxies. We highlight the numerous tidal streams observed around M31, some of
which reside at projected distances of up to 120 kpc from the center of this
galaxy. Most notable is the Giant Stellar Stream, a signature of the most
recent significant accretion event in the M31 system. This event involved an
early-type progenitor of ~10^9 solar masses that came within a few kpc of M31's
center roughly a gigayear ago; almost all of the inner halo debris (within 50
kpc) in M31 can be tied either directly or indirectly to this event. We draw
attention to the fact that most of M31's outer halo globular clusters lie
preferentially on tidal streams and discuss the potential this offers to use
these systems as probes of the accretion history. Tidal features observed
around M33, M32, NGC 205 and NGC 147 are also reviewed. We conclude by
discussing future prospects for this field.Comment: 29 pages, 9 figures. Book chapter in Tidal Streams in the Local Group
and Beyond, eds. Heidi Newberg and Jeff Carli
The nature and origin of substructure in the outskirts of M31-II. Detailed star formation histories
While wide-field surveys of M31 have revealed much substructure at large
radii, understanding the nature and origin of this material is not
straightforward from morphology alone. Using deep HST/ACS data, we have derived
further constraints in the form of quantitative star formation histories (SFHs)
for 14 inner halo fields which sample diverse substructures. In agreement with
our previous analysis of colour-magnitude diagram morphologies, we find the
resultant behaviours can be broadly separated into two categories. The SFHs of
'disc-like' fields indicate that most of their mass has formed since z~1, with
one quarter of the mass formed in the last 5 Gyr. We find 'stream-like' fields
to be on average 1.5 Gyr older, with <10 percent of their stellar mass formed
within the last 5 Gyr. These fields are also characterised by an
age--metallicity relation showing rapid chemical enrichment to solar
metallicity by z=1, suggestive of an early-type progenitor. We confirm a
significant burst of star formation 2 Gyr ago, discovered in our previous work,
in all the fields studied here. The presence of these young stars in our most
remote fields suggests that they have not formed in situ but have been
kicked-out from through disc heating in the recent past.Comment: 14 pages, 10 figures (+12 figures in appendix). MNRAS, in pres
Path integral solutions for Klein-Gordon particle with position-dependent mass in deformed Hulthén potential
The Green's function for a Klein-Gordon particle with position-dependent mass under the action of vector plus scalar q-deformed Hulthén potentials is evaluated by exact path integration. From the singularities of the latter Green's function, the bound states are extracted. For , the analytic expression of the energy spectrum and the normalized wave functions for the l states are obtained within the framework of an approximation to the centrifugal term. Particular cases of these potentials are also discussed briefly and it is found that the obtained results are in good agreement with those obtained in the literature
Bending, Buckling and Vibration of a Functionally Graded Porous Beam Using Finite Elements
This study presents the effect of porosity on mechanical behaviors of a power distribution functionally graded beam. The Euler-Bernoulli beam is assumed to describe the kinematic relations and constitutive equations. Because of technical problems, particle size shapes and micro-voids are created during the fabrication which should be taken into consideration. Two porosity models are proposed. The first one describes properties in the explicit form as linear functions of the porosity parameter. The second is a modified model which presents porosity and Youngâs modulus in an implicit form where the density is assumed as a function of the porosity parameter and Youngâs modulus as a ratio of mass with porosity to the mass without porosity. The modified proposed model is more applicable than the first model. The finite element model is developed to solve the problem by using the MATLAB software. Numerical results are presented to show the effects of porosity on mechanical behaviors of functionally graded beams
Effect of SiC addition on the mechanical properties and wear behavior of Al-SiC nanocomposites produced by accumulative roll bonding
In this study, we manufactured Al and Al-SiC nanocomposites (SiC vol.% = 1, 2 and 4) using accumulative roll bonding (ARB) at room temperature. Mechanical and wear characteristic of the manufactured samples was studied. The uniform dispersion of SiC particles was achieved after five ARB cycles. The EDX mapping images reveal that the SiC reinforcement particles were homogeneously distributed into the Al matrix. After five ARB cycles, relatively small amount of nano-grains were observed in Al-SiC nanocomposite, indicating the effectiveness of the ARB process of achieving near nanostructure. It was observed that Vickers microhardness improve with increasing accumulative roll bonding cycles. Furthermore after 5 cycles, Vickers microhardness tests results showed that the highest hardness value of 66 HV is attributed to the sample containing 4 vol.% of SiC, much higher than the hardness value of the ARB-processed aluminum of 51 HV. Experimental results indicated that the wear resistance of this nanocomposite improved by increasing the number of ARB cycles due to SiC particles act as a solid lubricant
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