Comportement rheologique de sols d'acide polysilicique. II. Etude de la gélification

Résumé L'analyse du comportement rhéologique de sols d'acide polysilicique en solution aqueuse de concentration supérieure à 7 g 1^–1 de silice en fonction du pH entre 6 et 11 et de la température entre 25 et 45 °C a permis de situer la gélification dans un domaine de pH compris entre 8 et 10. Ce processus prend une intensité maximale à 30°C. Le comportement rhéofluidifiant à bas gradients de cisaillement et antithixotrope donne naissance sur les rhéogrammes à un point d'inversion à pH = 9. L'ionisation des groupes silanols à la surface des particules colloïdales permet la réorganisation du milieu après cisaillement. Un état d'ionisation trop élevé est responsable d'un effet electro-répulsif à pH = 10. L'influence de la concentration et de la température sur ces processus est discutée.

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The rheological behavior of polysilicic acid in aqueous solutions at higher concentrations than 7 g 1^–1 of silice is studied for pH between 6 and 11 with temperature varying from 25° to 45°C. This study indicates that gelification takes place between pH 8 and 10. The process of gel formation attains the highest intensity at 30°C. The shear-thinning behavior at low shear rate gives rise to an inversion point at pH = 9 in shear stress—shear rate curves. The state of ionization at the surface of colloïdal silice particles after shear involve shear thickening observed at pH = 9. A high ionization degree causes an electrostatic repulsion at pH = 10. The concentration and temperature effects upon the organization of network are discussed.

     

Low-lying resonances in F and B

In a simple potential model, we have computed energies and widths of low-lying resonances of ¹⁴F and have compared them with recent experimental results. Consequences for ¹⁴B are summarized.     

The energy dependence of 18O(π+, π−)18Ne(gs)

Eight angular distributions measured for ¹⁸(π⁺, π^?)¹⁸Ne(gs) across the Δ₃₃ resonance show the details of the energy dependence of the reaction. At higher energies, the maximum is at q ? 0.85 fm^?1, in agreement with simple sequential scattering models, but a lower energies it is at q ? 0.5 fm^?1, with the transition occuring over a narrow energy range.     

Structure of 14C from 12C(t,p)

The reaction ¹²C(t, p)¹⁴C has been investigated with an 18 MeV triton beam. Twenty energy levels of ¹⁴C were identified up to 13 MeV excitation. Angular distributions were measured and compared with DWBA calculations. A previously unreported 0⁺ level at 9.75 MeV was observed; it undoubtedly corresponds to the second predominantly sd shell 0⁺ state in ¹⁴C. Additional spin and parity assignments have been made in the present work: 9.81 MeV, (1^?); 10.43 MeV, 2⁺; 10.50 MeV, (3^?); 10.74 MeV, 4⁺; 11.40 MeV, 1^?; 11.67 MeV, (1^?); 11.73 MeV, (5^?); 12.58 MeV, (2⁺, 3^?); 12.87 MeV. 2⁺, 3^?; and 12.96 MeV, (1^?); none of which had a definite spin and parity assignment previously. Our results confirm the previous information on the level structure of ¹⁴C below 8.5 MeV. The cross section for the unnatural parity state at 7.34 MeV, J^π = 2^?, is well reproduced by a two-step reaction calculation. The results are compared with the predictions of a weak coupling model.     

Investigation of 16O excited states via the 14N(3He,p) reaction

The reaction ¹⁴N(³He, p)¹⁶O has been investigated at a bombarding energy of 15.0 MeV, using a differentially pumped gas target. Angular distributions were measured for 30 levels below 16.5 MeV in excitation. Data were compared with shell-model calculations of Zuker, Buck and McGrory for states whose correspondence with theory is established. Many states are found to possess a large compound-nucleus reaction component. Several previously unreported levels are observed at high excitation. Angular distributions for all except the weakest levels have been compared with DWBA calculations.     

The 27Al(t,p)29Al reaction at Et = 15 MeV

The ²⁷Al(t, p)²⁹Al reaction was studied at an incident energy of E_t = 15 MeV. Proton angular distributions were measured for the first 23 low-lying states. Comparisons of the data to DWBA calculations based on cluster-model and pure-configuration form factors were made which provided decompositions of the angular distributions into contributing L-values. Comparisons were also made to DWBA predictions based on microscopic amplitudes from a complete, sd-basis shell-model calculation.