Evaluation of the RapidAir 457 Air Void Analyzer, March 2012

An adequate air void system is imperative to produce concrete with freeze-thaw durability in a wet freeze environment such as found in Iowa. Specifications rely on a percentage of air obtained in the plastic state by the pressure meter. Actual, in place air contents, of some concrete pavements in Iowa, have been found with reduced air content due to a number of factors such as excessive vibration and inadequate mixing. Determining hardened air void parameters is a time consuming process involving potential for human error. The RapidAir 457 air void analyzer is an automated device used to determine hardened air void parameters. The device is used in Europe and has been shown to quickly produce accurate and repeatable hardened air results. This research investigates how well the RapidAir 457 results correlate to plastic air content and the image analysis air technique. The repeatability and operator variation were also investigated, as well as, the impact of aggregate porosity and selection of threshold value on hardened air results.

Diffusion in spatially and temporarily inhomogeneous media: Effects of turbulent mixing

We consider diffusion of a passive substance C in a phase-separating nonmiscible binary alloy under turbulent mixing. The substance is assumed to have different diffusion coefficients in the pure phases A and B, leading to a spatially and temporarily dependent diffusion ¿coefficient¿ in the diffusion equation plus convective term. In this paper we consider especially the effects of a turbulent flow field coupled to both the Cahn-Hilliard type evolution equation of the medium and the diffusion equation (both, therefore, supplemented by a convective term). It is shown that the formerly observed prolonged anomalous diffusion [H. Lehr, F. Sagués, and J.M. Sancho, Phys. Rev. E 54, 5028 (1996)] is no longer seen if a flow of sufficient intensity is supplied.

Feedback control of unstable cellular solidification fronts

We present a feedback control scheme to stabilize unstable cellular patterns during the directional solidification of a binary alloy. The scheme is based on local heating of cell tips which protrude ahead of the mean position of all tips in the array. The feasibility of this scheme is demonstrated using phase-field simulations and, experimentally, using a real-time image processing algorithm, to track cell tips, coupled with a movable laser spot array device to heat the tips locally. We demonstrate, both numerically and experimentally, that spacings well below the threshold for a period-doubling instability can be stabilized. As predicted by the numerical calculations, cellular arrays become stable with uniform spacing through the feedback control which is maintained with minimal heating.

Déficit cognitivo de la indefensión aprendida: Una aplicación de la detección de señales

Este estudio, trató de examinar la naturaleza del elemento cognitivo de la Indefensión Aprendida. En recientes trabajos (Alloy and Abramson, 1980; Ahramson, Alloy and Rosoff, 1981; Alloy and Seligman, 1979), se sugiere que el lugar del efecto de indefensión se encuentra en distorsiones del mecanismo expectacional involucrado en analizar contingencias de respuesta-consecuencia. En contraste, existen otras posturas alternativas que consideran que el efecto dv indefensión es debido a una falta de atención, por parte del sujeto, durante la fase de prueba. Estos conceptos están relacionados con la teoria de la detección de señales (SDT), y las predicciones serian que 0, en el primer caso, y d', o nada, en el segundo, variarian en función de la exposición a indefensión. La ejecución en una tarea de detección de señales fue evaluada, usando un análisis de SDT siguiendo a un pretratamiento de indefensión. Los resultados mostraron un efecto significativo en B, pero no en P(A). Los resultados son compatibles con hipótesis que sugieren como causa del déficit cognitivo una distorsión del mecanismo expectacional

Production of nanometric particles in radio frequency glow discharges in mixtures of silane and methane

The formation of silicon particles in rf glow discharges has attracted attention due to their effect as a contaminant during film deposition or etching. However, silicon and silicon alloy powders produced by plasma¿enhanced chemical vapor deposition (PECVD) are promising new materials for sintering ceramics, for making nanoscale filters, or for supporting catalytic surfaces. Common characteristics of these powders are their high purity and the easy control of their stoichiometry through the composition of the precursor gas mixture. Plasma parameters also influence their structure. Nanometric powders of silicon¿carbon alloys exhibiting microstructural properties such as large hydrogen content and high surface/volume ratio have been produced in a PECVD reactor using mixtures of silane and methane at low pressure (-1 Torr) and low frequency square¿wave modulated rf power (13.56 MHz). The a¿Si1¿xCx:H powders were obtained from different precursor gas mixtures, from R=0.05 to R=9, where R=[SiH4]/([SiH4]+[CH4]). The structure of the a¿Si1¿xCx:H powder was analyzed by several techniques. The particles appeared agglomerated, with a wide size distribution between 5 and 100 nm. The silane/methane gas mixture determined the vibrational features of these powders in the infrared. Silicon-hydrogen groups were present for every gas composition, whereas carbon¿hydrogen and silicon¿carbon bonds appeared in methane¿rich mixtures (R-0.6). The thermal desorption of hydrogen revealed two main evolutions at about 375 and 660¿°C that were ascribed to hydrogen bonded to silicon and carbon, respectively. The estimated hydrogen atom concentration in the sample was about 50%.

Microstructural evolution of primary crystallization in diffusion-controlled grain growth

A model has been developed for evaluating grain size distributions in primary crystallizations where the grain growth is diffusion controlled. The body of the model is grounded in a recently presented mean-field integration of the nucleation and growth kinetic equations, modified conveniently in order to take into account a radius-dependent growth rate, as occurs in diffusion-controlled growth. The classical diffusion theory is considered, and a modification of this is proposed to take into account interference of the diffusion profiles between neighbor grains. The potentiality of the mean-field model to give detailed information on the grain size distribution and transformed volume fraction for transformations driven by nucleation and either interface- or diffusion-controlled growth processes is demonstrated. The model is evaluated for the primary crystallization of an amorphous alloy, giving an excellent agreement with experimental data. Grain size distributions are computed, and their properties are discussed.

Ferromagnetic shape memory alloys: structural and thermal properties

The most extensively studied Heusler alloys are those based on the Ni-Mn-Ga system. However, to overcome the high cost of Gallium and the usually low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing In, Sn or Sb. In this work, two alloys (Mn50Ni35.5In14.5 and Ni50Mn35In15) have been obtained by melt spinning. We outline their structural and thermal behaviour. Mn50Ni35.5In14.5 alloy has the transformation above room temperature whereas Ni50Mn35In15 does not have this transformation in the temperature range here analyzed

Atomic Ordering and Martensitic Transitions in Cu-Zn-Al Shape Memory Alloys

We present results from both, calorimetric and dilatometric studies of the isothermal ordering process taking place in a Cu-Zn-Al shape memory alloy after quenches from Tq temperatures ranging from 350 K to 1200 K. The dissipated energy and the length variations of the system are obtained during the process. The change of these quantities in the whole process have been compared with the difference [MATH] between Ms, measured after the relaxation and Ms measured just after the quench. We obtain that these three quantities present, as a function of Tq, the same qualitative behaviour. These changes are then associated with changes of the L21 ordering after the quench in the system. The relaxational process does not follow a single exponential decay. Instead, a continuous slowing down is observed. A relaxation time [MATH] has been defined to characterize the relaxation rate. We show that [MATH] depends on both the annealing and the quenching (Tq [MATH] 800 K) temperatures through an Arrhenius law.

Systematic study of the temperature dependence of the saturation magnetization in Fe, Fe-Ni and Co-based amorphous alloys

Magnetization versus temperature in the temperature interval 2-200 K was measured for amorphous alloys of three different compositions: Fe 81.5B14.5Si4, Fe40Ni38 Mo4B18, and Co70Fe5Ni 2Mo3B5Si15. The measurements were performed by means of a SQUID (superconducting quantum interference device) magnetometer. The aim was to extract information about the different mechanisms contributing to thermal demagnetization. A powerful data analysis technique based on successive minimization procedures has demonstrated that Stoner excitations of the strong ferromagnetic type play a significant role in the Fe-Ni alloy studied. The Fe-rich and Co-rich alloys do not show a measurable contribution from single-particle excitations.

Electrochemical synthesis of mesoporous CoPt nanowires for methanol oxidation

A new electrochemical method to synthesize mesoporous nanowires of alloys has been developed. Electrochemical deposition in ionic liquid-in-water (IL/W) microemulsion has been successful to grow mesoporous CoPt nanowires in the interior of polycarbonate membranes. The viscosity of the medium was high, but it did not avoid the entrance of the microemulsion in the interior of the membrane"s channels. The structure of the IL/W microemulsions, with droplets of ionic liquid (4 nm average diameter) dispersed in CoPt aqueous solution, defined the structure of the nanowires, with pores of a few nanometers, because CoPt alloy deposited only from the aqueous component of the microemulsion. The electrodeposition in IL/W microemulsion allows obtaining mesoporous structures in which the small pores must correspond to the size of the droplets of the electrolytic aqueous component of the microemulsion. The IL main phase is like a template for the confined electrodeposition. The comparison of the electrocatalytic behaviours towards methanol oxidation of mesoporous and compact CoPt nanowires of the same composition, demonstrated the porosity of the material. For the same material mass, the CoPt mesoporous nanowires present a surface area 16 times greater than compact ones, and comparable to that observed for commercial carbon-supported platinum nanoparticles.

Effect of Shielding Gases in Laser Welding of Aluminium Alloys

High reflectivity to laser light, alloying element evaporation during high power laser welding makes aluminium alloys highly susceptibility to weld defects such as porosity, cracking and undercutting. The dynamic behaviour of the keyhole, due to fluctuating plasma above the keyhole and the vaporization ofthe alloying elements with in the keyhole, is the key problem to be solved for the improvement of the weld quality and stabilization of the keyhole dynamics isperhaps the single most important development that can broaden the application of laser welding of aluminium alloys. In laser welding, the shielding gas is commonly used to stabilize the welding process, to improve the welded joint features and to protect the welded seam from oxidation. The chemicalcomposition of the shielding gas is a key factor in achieving the final qualityof the welded joints. Wide range of shielding gases varying from the pure gasesto complex mixtures based on helium, argon, nitrogen and carbon dioxide are commercially available. These gas mixtures should be considered in terms of their suitability during laser welding of aluminium alloys to produce quality welds. The main objective of the present work is to study the effect of the shielding gascomposition during laser welding of aluminium alloys. Aluminium alloy A15754 was welded using 3kW Nd-YAG laser (continuous wave mode). The alloy samples were butt welded with different shielding gases (pure and mixture of gases) so that high quality welds with high joint efficiencies could be produced. It was observed that the chemical composition of the gases influenced the final weld quality and properties. In general, the mixture gases, in correct proportions, enabled better utilisation of the properties of the mixing gases, stabilized the welding process and produced better weld quality compared to the pure shielding gases.

Seostamattoman hitsausteräksen leimuhitsaus

Valssilangan valmistuksessa päättymättömällä valssausprosessilla voidaan parantaa tuotannon tuottavuutta ja joustavuutta. Menetelmän ansiosta myös valssilankakieppien kokoa voidaan vaihdella asiakkaan tarpeiden mukaan. Prosessin joustavuus ja tuottavuus perustuvat liikkuvaan leimuhitsauslaitteistoon, jolla austeniittialueelle kuumennetut teelmät hitsataan toisiinsa ennen valssausta. Koska leimuhitsausliitoksen hyvä laatu on valmistusprosessin kannalta erityisen tärkeää, tutkittiin tässä työssä hitsausparametrien vaikutusta liitoksen laatuun. Hitsausparametreista tärkeimpiä ovat leimutusaika ja -jännite sekä tyssäyksen voima ja määrä. Parametrien ollessa liian suuria muodostuvat liitospinnat liian plastisiksi ja sopivan tyssäysvoiman ylläpitäminen on vaikeaa. Liian pienillä parametreilla liitospintojen plastisoituminen jää kesken ja tyssäys jää puutteelliseksi. Leimuhitsauksen laatuun vaikuttaa myös liitospinta-alan koko. Kaikki nämä tekijät heikentävät liitoksia, joten parametrien optimaalisuus on liitoksen hyvän laadun kannalta välttämätön. Parametrikokeina hitsattujen koekappaleiden rakennetta ja ominaisuuksiatutkittiin ainetta rikkovilla kokeilla. Lyhyemmillä leimutusajoilla hitsatuistakoekappaleista löytyi virheitä, jotka heikensivät huomattavasti liitoksien laatua. Menetelmäkokeiden perusteella koekappaleet eivät kaikilta osin täyttäneet liitosten laatuvaatimuksia.

In situ spectroellipsometric study of the nucleation and growth of amorphous silicon

A detailed in situ spectroellipsometric analysis of the nucleation and growth of hydrogenated amorphous silicon (a:Si:H) is presented. Photoelectronic quality a‐Si:H films are deposited by plasma‐enhanced chemical vapor deposition on smooth metal (NiCr alloy) and crystalline silicon (c‐Si) substrates. The deposition of a‐Si:H is analyzed from the first monolayer up to a final thickness of 1.2 μm. In order to perform an improved analysis, real time ellipsometric trajectories are recorded, using fixed preparation conditions, at various photon energies ranging from 2.2 to 3.6 eV. The advantage of using such a spectroscopic experimental procedure is underlined. New insights into the nucleation and growth mechanisms of a‐Si:H are obtained. The nucleation mechanism on metal and c‐Si substrates is very accurately described assuming a columnar microstructural development during the early stage of the growth. Then, as a consequence of the incomplete coalescence of the initial nuclei, a surface roughness at the 10-15 Å scale is identified during the further growth of a‐Si:H on both substrates. The bulk a‐Si:H grows homogeneously beneath the surface roughness. Finally, an increase of the surface roughness is evidenced during the long term growth of a‐Si:H. However, the nature of the substrate influenced the film growth. In particular, the film thickness involved in the nucleation‐coalescence phase is found lower in the case of c‐Si (67±8 Å) as compared to NiCr (118±22 Å). Likewise films deposited on c‐Si present a smaller surface roughness even if thick samples are considered (>1 μm). More generally, the present study illustrates the capability of in situ spectroellipsometry to precisely analyze fundamental processes in thin‐film growth, but also to monitor the preparation of complex structures on a few monolayers scale.

Keraamisten teräpalojen käyttö niukkaseosteisten terästen lastuamiseen normaaleissa toimitustiloissa

Tämän tutkimuksen tavoitteena oli tutkia keraamisten kääntöterien soveltuvuus niukkaseosteräksille ja M-käsittelyn vaikutus teränkestoaikoihin. Lisäksi tavoitteena oli tutkia keraamiseosten erityisominaisuudet ja muodostaa käsitys keraamisilla kääntöterillä sorvaamisen erityisvaatimuksista. Kirjallisuusosassa selvitettiin tutkimushetkellä saatavissa olleet keraamiset teräaineseokset ja niiden ominaisuudet sekä selvitettiin keraamisten kääntöterien viimeaikaiset kehitystrendit. Lastuamistutkimus toteutettiin standardin ISO 3685:1993 mukaisesti, minkä lisäksi mitattiin lastuamisvoimat ja tehtiin kokeessa käytetyille terille lastunmurtokoe. Koemateriaalit olivat Imatra Steelin GreenCut, Hydax 25, M-käsittelemätön 42CrMo4 ja M-käsitelty MoC410M. Kokeissa käytetyt kääntöterät olivat eri valmistajien alumiinioksiditeriä, titaanikarbidiseostettuja alumiinioksiditeriä, kuituvahvisteisia ja yksi puhdas titaanikarbidiseosteinen terä. Koetulosten perusteella hyvän teränkestoajan saavuttamiseksi on tärkeää valita oikea keraamiseos ja sille soveltuvat parametrit. Oikein valituilla parametreilla M-terästä sorvattaessa tulokset ylittävät kovametalleilla saavutettavissa olevat arvot. Tulosten perusteella niukkaseostettujen terästen sorvaus onnistuu hyvin keraamisilla kääntöterillä, mutta kovametallisorvaukseen verrattuna syöttöä on laskettava ja lastuamisnopeutta lisättävä. Tämän työn tuloksia voidaan soveltaa sopivissa olosuhteissa toteutettavassa kappaletavaratuotannossa. Sovelluksen onnistuminen vaatii tukevat työstöolosuhteet ja riittävien lastuamisnopeuksien saavuttamisen.

Application of the N-alkane molecular alloys to thermally protected containers for catering

A thermally controlled transport device was designed and tested. As hot food needs to be transported at temperatures between 60 and 70ºC in order to avoid contamination by microorganisms, the use of Molecular Alloy Phase Change Materials (MAPCM) can lead to improvements in this field of application. A heat transfer numerical simulation of the box used for transporting the food was conducted. Despite obvious simplifications, a good agreement between numerical simulation and experimental results was obtained. Furthermore, we compared our experimental results with those from other experiments related to the transport of hot food. Here, pizza is taken as the example, and it is shown that delivering time can be increased three-fold.