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51.
52.
Using the SiO2 and Al2O3 components of the amorphous phase in coal fly ash (Fa), Fa was converted to Na-X zeolites in NaOH-NaAlO2 solutions by stirring at 35°C for 72 hr and then aging at 85°C for a given period. The molar ratio SiO2/Al2O3 of the starting materials was controlled from 2.0 to 13.2. The resulting materials were characterized by various means. Increasing the SiO2/Al2O3 ratio of the starting material increased the degree of crystallinity of faujasite, exhibiting a maximum at SiO2/Al2O3 = 8.0. The faujasite formed was identified as Na-X zeolite with Si/Al = 1.20. The amorphous phase in Fa was dissolved during the stirring to form a precursor of zeolite, such as amorphous aluminosilicate. The Na-X zeolite was formed by aging for 24 hr, and the degree of crystallinity of this material was increased with the increasing aging period. The cation exchange capacity and specific surface area were increased with the increasing degree of crystallinity of the Na-X zeolites. 相似文献
53.
54.
55.
56.
Morten
rregaard Nielsen 《时间序列分析杂志》2005,26(2):279-304
Abstract. We consider semiparametric estimation in time‐series regression in the presence of long‐range dependence in both the errors and the stochastic regressors. A central limit theorem is established for a class of semiparametric frequency domain‐weighted least squares estimates, which includes both narrow‐band ordinary least squares and narrow‐band generalized least squares as special cases. The estimates are semiparametric in the sense that focus is on the neighbourhood of the origin, and only periodogram ordinates in a degenerating band around the origin are used. This setting differs from earlier studies on time‐series regression with long‐range dependence, where a fully parametric approach has been employed. The generalized least squares estimate is infeasible when the degree of long‐range dependence is unknown and must be estimated in an initial step. In that case, we show that a feasible estimate which has the same asymptotic properties as the infeasible estimate, exists. By Monte Carlo simulation, we evaluate the finite‐sample performance of the generalized least squares estimate and the feasible estimate. 相似文献
57.
58.
59.
李静 《制冷与空调(北京)》2007,7(2):86-88
介绍新型节流装置——线性浮阀的结构、工作原理、特点及其在大型冷水机组中的应用以及实际使用中可能存在的问题,为在设计具有较高能效比的中大型冷水机组中,在节流机构选择应用上提供更大的选择空间。 相似文献
60.
This study is concerned with the stability of a two-dimensional incompressible conducting liquid film surrounded by a passive
conducting medium, when an electric field is applied in a direction parallel to the initially flat bounding fluid interfaces.
Currents generate charges at the bounding interfaces which in turn affect the stress balances there. In the absence of an
electric field, the viscous liquid film is stable (instability can be induced by the inclusion of van der Waals forces for
ultra thin films). A complete model is presented, at arbitrary Reynolds number, which accounts for conductivity and permittivity
contrasts between the fluid and surrounding medium, as well as surface tension. The linear stability of the system is considered
for arbitrary Reynolds numbers and it is shown that the stable film can become unstable if, (i) σ
R
ɛ
p
>1, or (ii) σ
R
ɛ
p
<1 and (σ
R
−1)(1−ɛ
p
)<0, where σ
R
is the ratio of outer to inner conductivity and ε
p
is the ratio of inner to outer permittivity. Instability is possible only if the electric field is non-zero and the scalings
near bifurcation points that can be used to construct nonlinear theories are calculated. Several asymptotic limits are also
considered including zero Reynolds numbers and short or long waves. The instability criteria given above are constructed explicitly
in the case of Stokes flow. 相似文献