Hydrogen Pore Research Articles

Critical assessment of reduced pressure test. Part 1: Porosity phenomena

During the production of aluminium ingots and castings, the surface oxide on the liquid may be folded into the bulk liquid to produce crack-like defects (bifilms) that are extremely thin, but can be extensive, and so constitute seriously detrimental defects. In this work, it has been found that bifilms are the initiator and hydrogen is only a contributor to the porosity formation process. For the first time, evidence is presented for the contribution of air (or perhaps more strictly, residual nitrogen from air) as an additional gas, adding to hydrogen in pores in cast Al alloys. The discriminating use of the RPT clearly reveals the existence of bifilms, and the effect of hydrogen on porosity formation. However, it seems that the RPT is of little use to evaluate the hydrogen content of the alloy. To investigate these effects, two alloys were studied in laboratory experiments LM0 (99.5%Al), LM4 (Al– 5Si–3Cu) and one in an industrial environment LM24 (Al–8Si–3Cu–Fe).

Mechanism of nucleation and growth of hydrogen porosity in solidifying A356 aluminum alloy: an analytical solution

This study derives an analytical solution for the mechanism of nucleation and growth of hydrogen pore in the solidifying A356 aluminum alloy. A model of initial transient hydrogen redistribution in the growing dendritic grain is used to modify the lever rule for the mechanism of nucleation of pore. The model predicts the fraction of solid at nucleation, the temperature range of nucleation, the radius of hydrogen diffusion cell, and the supersaturation of hydrogen needed for nucleation. The role of solidus velocity in nucleation is explained. The parameters calculated from the model of nucleation are used for analyzing the mechanism of kinetic diffusion-controlled growth of pore, in which the mathematical transformations of variables are introduced. With the transformations, it is argued that the diffusion problem involving the liquid and solid phases during solidification could be treated as a classic problem of precipitation in the single-phase medium treated by Ham or Avrami. The analytical solution for the nucleation of pore is compared with the mechanism of macrosegregation. The predicted volume percent of porosity and radius of pore based on the mechanism of growth of pore is discussed with respect to the thermodynamic solution, the published experimental data, the numerical solutions, and the role of interdendritic fluid flow governed by Darcy’s law.

Low-Temperature-Programmed Replacement of Nitrogen by Hydrogen in Pores of Mordenite

Nitrogen molecules were filled in pores of NaMOR by calcination at Tc = 773 K under nitrogen atmosphere and then replaced by hydrogen molecules. The replacement was performed in a specially designed low-temperature routine operated between 173 and 423 K with a flow of 10% hydrogen in nitrogen. The obtained replacement profile comprised four kinds of phenomena, i.e., rapid replacement (Rr) of gaseous phase nitrogen in pores by hydrogen, hindered replacement (Rh) of nitrogen condensed around sodium ions, activated chemisorption of (C) of nitrogen molecules on sodium ions, and desorption (D) of chemisorbed nitrogen from Na+ site. The Rr exhibited two major peaks at ∼185 and ∼240 K for nitrogen in 12-MR channel (Rrc) and in 8-MR side pocket (Rrp). The Rh was delayed up to 350 K due to hindrance by sodium ions. Both the C and the D exhibited three sharp peaks for sodium ions at three kinds of environments in MOR structure.

A three-phase model of hydrogen pore formation during the equiaxed dendritic solidification of aluminum-silicon alloys

A computational model for the prediction of porosity due to dissolved hydrogen in binary aluminum-silicon alloys has been developed. The model combines the cellular automata technique for the simulation of the growth of the solid phase, the finite-difference technique for the simulation of diffusion of the dissolved species, and a quasi-equilibrium model for the growth of individual bubbles. The growth of the solid and gas phases is initiated by a stochastic nucleation model, depending upon the undercooling (for the solid) or the supersaturation ratio (for the gas). The results agree favorably with experiments. The low supersaturation values needed to simulate the experimental results are consistent with a nucleation mechanism of gas pockets entrained within the melt.

Modelling and simulation of heating/air-conditioning systems using the multi-hydride-thermal-wave concept

A computer code describing a transient model was developed to analyse the performance of a metal hydride heating/air-conditioning system. The model considers the effects of heat transfer by conduction in the metal hydride reaction beds, conduction–convection in the heat transfer fluid, hydrogen flow governed by Darcy's law and gas consumption by sorption kinetics. The model was applied to simulate two different concepts of fast reaction beds in the heating/air-conditioning device, using a novel capillary tube bundle reactor (CTBR) for improved heat transfer characteristics and the established metal foam tube reactor (MFTR), respectively. The reactors are equipped with cascades of different metal hydrides thus implying a moving thermal wave during operation. Numerical techniques were used to predict the liquid temperatures, solid phase temperatures, specific gas flow rates, hydrogen pore pressures and metal hydride compositions. A high performance of the devices was obtained by choosing optimized physicochemical and thermodynamic parameters of the employed metal hydrides. The devices allow cooling temperatures of about 3°C and a COP c (coefficient of performance for cooling) over 0.6 for the CTBR system and 0.5 for the MFTR system with the following input conditions: driving heat temperature 213°C, lowest heat rejection temperature 40°C and maximal cooling temperature 20°C. The calculated specific cooling power is about 30 W/kg hydride for the CTBR system and 23 W/kg hydride for the MFTR system.

微細粒超塑性７４７５アルミニウム合金の空洞問題に及ぼす予備加熱の影響

The effects of the conditions for high temperature annealing prior to deformation has been investigated for superplastic 7475 aluminum alloy on the microstructural features involved in the initiation stages of cavity formation. The superplastic elongation to failure, deformed at 793K with a strain rate of 2×10-4s-1, increases from 550% up to 800% with prior annealing time at high temperatures. This increment in elongation may be led by decrease in the volume of cavities forming with the superplastic strain. The microstructural observations made concerning the relation between superplastic cavitation and an initial formation of cavity suggest that numerous pre-existing spherical voids, probably formed during the thermomechanical processing, appear to be associated with hydrogen due to the peritectic reaction of an Al-Fe-Si phase→Al-Fe-Cu phase+liquid in Al-Zn-Mg-Cu type alloys. These voids seem to be obviously pre-existing nuclei of superplastic cavities. Hydrogen pore formation assisted superplastic cavity growth can be eliminated by high temperature diffusion prior to superplastic forming.

Effect of pore diffusion on the triacylglycerol distribution of partially hydrogenated trioleoylglycerol

Almost pure trioleoylglycerol (O 3) was hydrogenated using nickel-on-carrier catalysts of various pore structure under similar conditions. The triacylglycerol distribution of partially hydrogenated products was analysed by various chromatographic techniques (e.g. thin-layer chromatography, TLC). The results indicated clearly that the pore structure affected product composition (e.g. lower apparent selectivity for a medium-pore compared to a wide-pore catalyst). The hydrogenation of trioleoylglycerol (O 3) to tristearoylglycerol (S 3) occurs according to the following consecutive steps: O 3 → H 2 k 3 S O 2 → H 2 k 2 S 2 O → H 2 k 1 S 3 This reaction is assumed to be first-order with respect to local triacylglycerol and local hydrogen concentrations. The hydrogenation process is described by a pore diffusion model which takes into account reaction kinetics, triacylglycerol and hydrogen pore diffusion and hydrogen gas-liquid mass transfer. The hydrogenation is an intrinsically random process. Hence the ratios between the reaction coefficients are given by: k 1: k 2: k 3= 1:2:3 Key parameters in the model are the triacylglycerol Thiele modulus, based on k 1: Φ TAG= R·( k 1·H 2 bulk/D eff) 1/2, the hydrogen Thiele modulus: ΦH 2= R·( k 1·(3 O 3+2 SO 2+S 2O) bulk/D eff) 1/2 and the volumetric gas—liquid mass transfer for hydrogen, k L a L. Fitting of the experimental data revealed that under the conditions applied, the intraparticle hydrogen transport limitations were negligible. Quantification of the triacylglycerol pore diffusion limitations resulted in a significant difference in effective diffusivities of the triacylglycerols for the medium-pore and wide-pore catalyst respectively D eff= 4·10 −12m 2/s vs. D eff= 2·10 −11m 2/s Hence, a significant difference in triacylglycerol Thiele modulus (Φ TAG= 4 vs. Φ TAG= 1) was observed. Optimal selectivity was obtained for a low value for the Thiele modulus by application of a wide-pore catalyst and/or by limitation of the hydrogen concentration in the oil by optimization of process conditions like gas-liquid mass transfer or hydrogen pressure. To establish the performance of a hydrogenation catalyst it should be tested at a high hydrogen concentration.

アルミニウムの凝固界面近傍における気孔の生成と成長機構

Molten aluminum in which excess hydrogen I.3 to I.5 cc/Ioo g had dissolved was solidified unidirectionally and water quenched in the course of solidification. A (liquid + hydrogen pore) region is formed in the liquid ahead of the solid-liquid interface. Pores are formed before end of solidification. Pores in the region are coarse near the interface and become finer with distance from the interface. A coarse and elongated pore near the interface is that incorporated with two or three pores. Pores are formed on oxides or inclusions in the melt and trapped by the advancing interface before floating away. The trapped pores incorporate with pores in the region ahead of the interface and grow into elongated cylindrical ones. Molecular hydrogen is condensed at the interface, while atomic hydrogen in solution shows a contrary tendency.

LIQUID-FLOW DENSIFICATION IN THE TUNGSTEN CARBIDE–COPPER SYSTEM

AbstractThe fluid-flow stage of densification in two-phase sintering, with minimum contribution from intersolubility effects and change of particle shape, has been studied by selecting the insoluble tungsten carbide-copper system. Density determinations, photography of specimen shrinkage, and microscopic examination were carried out over a range of copper contents, with two carbide particle sizes, for sintering in a hydrogen atmosphere.When the copper melts it flows into regions of high carbide density to form carbide/copper colonies. If these occupy a minor proportion of the compact, densification is limited and determined by the larger, “rigid” carbide part of the compact, but if the colonies predominate there is massive shrinkage on fluid flow. Overall densification subsequent to fluid flow is unaffected by the presence of the copper. The copper may, however, be redistributed within the compact as hydrogen in pores near the surface diffuses out and the pores shrink, drawing copper from central regions ...