Aster

It is produced artificially for use as a gemstone, but it is relatively soft. Star sapphires and rubies exhibit their asterism as a result of the presence of Ti02. 

P. Rothsteia, ia Proceedings of the Biopharma Conference 91, Aster Publishing Corp., Eugene, Oreg., 1991, pp. 95—118. 

A.lkyl Sulfosuccinate Half Asters. These detergents are prepared by reaction of maleic anhydride and a primary fatty alcohol, followed by sulfonation with sodium bisulfite. A typical member of this group is disodium lauryl sulfosucciaate [26838-05-1]. Although not known as effective foamers, these surfactants can boost foams and act as stabilizers when used ia combination with other anionic surfactants. In combination with alkyl sulfates, they are said to reduce the irritation effects of the latter (6). 

Cycloocta l,5-dlona-2 -(or 3 )earboxyllc acid mattiyl aster (3) (4), A solution d eyclohe)(a-1,3-dlone 1 (1.00 g, 8.9 mmol) in methyl acrylate 2 (100 g. 1.16 mmol) was Irradiated with a X 450 W medkjm pressure lamp under Na through a pyrex (Iter (or 5 h. The semicrystalliM residue obtained after removal oi the solvent, was crystallized from MeOH, the n ther liquor, separated by preparative TLC (PhH Et20) and the main fraction oonMned vrilh the crystals to gM 1.06 g of 3 (60%), mp 104-105°C (MeOH), The second minor fraction (from TLC) gave 4, mp 91 C (MeOH). 

B. Addition of Sodium Cyanide to Ethyl ct-Cyano-fi-phenyl-acrylate —Twenty grams of cyanophenylacrylic aster is treated with 40 cc. of 50 per cent alcohol and 10 g. of finely pow-... 

Vitamin C Phytopharma Aster C (Corvi) Gels (Carlson)... 

Abrahamson, W. G. and Solbrig, O. T. 1970. Soil preferences and variation in flavonoid pigments in species of Aster. Rhodora 72 251-263. 

FIG. 2. The dependence of the carbon fraction. v = Cl/(lSi] + [C ) on the gas-flow ratio r = [CH4]/([SiH4] -I- ICH4I) for films deposited in the ASTER system [ASTI (filled circles) and AST2 (filled triangles)] and for films deposited in a similar system (ATLAS) [ATLI (open circles) and ATL2 (open triangles)]. (From R, A. C. M. M. van Swaaij, Ph.D. Thesis, Universiteit Utrecht, Utrecht, the Netherlands, 1994. with permission.)... 

FIG. 5. Schematic representation of the ASTER deposition system. Indicated are (I) load lock. (2) plasma reactor for intrinsic layers. (3) plasma reactor for />-type layers. (4) plasma reactor for t -type layers, (5) metal-evaporation chamber (see text). (6) central transport chamber. (7) robot arm. (8) reaction chamber, (9) gate valve, (10) gas supply. (11) bypass. (12) measuring devices, and (13) tur-bomolecular pump. 

This section treats the plasma physics and plasma chemistry of the typical silane-hydrogen RF discharge, with occasional examples that employ a somewhat higher excitation frequency. Electrical characterization of the discharge is followed by an analysis of the silane chemistry. An appropriate set of gas phase species is presented, which are then used in the modeling of the plasma. A comparison is made between modeling results and experimental work in ASTER. Extension to 2D modeling is presented as well. 

In the ASTER system (see Section 1.2.4), experiments were performed in order to test this scaling law. To this end, a newly designed RF electrode assembly was retrofitted to a deposition chamber. With this electrode setup, it was possible to change the electrode distance from the outside, without breaking the vacuum. A large data set was taken, consisting of 420 data points [162] at three values of the pressure (0.1 < p < 0.45 mbar), five of the RF power (5 < P < 25 W), seven of the electrode distance (12 < L < 30 mm), and four of the RF frequency (13.56 < o)/2n < 50 MHz). 

Comparing published data on the relation between the dc self-bias and RF voltage between Rauf and Kushner [180] and our group [151] shows that the proportionality constant (slope) in the GECRC is nearly twice the one observed in the ASTER reactor. In other words, the ASTER reactor is less asymmetric than the... 

The ID fluid discharge model has been applied to the ASTER deposition system (see Section 1.2.4). The deposition reactor has an inner volume of 10 1 and an inner diameter of 20 cm. The upper electrode is grounded (see Fig. 4a), and the powered electrode is located 2.7 cm lower. Other typical silane-hydrogen discharge parameters are summarized in Table IV. 

Typical Settings for a Silanf.-Hydroghn Discharge in the ASTER Sy.stem... 

In this sub-subsection the results of the ID model are shown together with data taken from experiments in the ASTER system. Both the partial pressures of the... 

In the ASTER reactor deposition experiments were performed in order to compare with the 2D model results. Normalized deposition rates are plotted in Figure 22 as a function of radial position for data taken at 25 and 18 Pa. The deposition takes place on a square glass plate. For each pressure two profile measurements were performed, each profile perpendicular to the other (a and b in Fig. 22). A clear discrepancy is present. The use of the simplified deposition model is an explanation for this. Another recent 2D fluid model also shows discrepancies between the measured and calculated deposition rate [257], which are attributed to the relative simplicity of the deposition model. 

The SiHa H2 ratio was 20% 80% in all simulations. As most experiments in the ASTER deposition system are performed at a constant total power, the... 

In the ASTER system a data series is measured for an argon and a hydrogen plasma running at 13.56 MHz, in which the power (5-30 W) and pressure (5-50 Pa) are varied [265,295], The probe tip is positioned exactly between powered and grounded electrode, at the center of the discharge. 

Probe measurements in silane discharges have been reported [296,297]. Apparently, no difficulties were experienced, as the deposited amorphous silicon layer on the tip was sufficiently photoconductive. For typical silane discharge conditions values for are found to be between 2 and 2.5 eV. Electron densities are around 1 x 10 cm - [296]. Probe measurement in the ASTER system failed due to strong distortions of the probe current, even after following cleaning procedures. 

The partial pressures of the stable neutral molecules in the discharge (silane, hydrogen, disilane, trisilane) can be measured by a quadrupole mass spectrometer (QMS). The QMS usually is mounted in a differentially pumped chamber, which is connected to the reactor via a small extraction port [286]. In the ASTER system a QMS is mounted on the reactor that is used for intrinsic material deposition. The QMS background pressure (after proper bake-out) is between 10 and 10 mbar. The controllable diameter in the extraction port is adjusted so that during discharge operation the background pressure never exceeds 10"" mbar. 

FIG. 40. The influence of deposition temperature on (a) the hydrogen concentration, (b) the microstructure parameter, and (c) the Raman half width P/2. The labels A and P refer to the ASTER and the PASTA deposition system. Series A1 was prepared from a SiH4 H2 mixture at 0.12 mbar. Series A2 and A3 were deposited from undiluted SiHa at 0.08 and 0.12 mbar. respectively. Series PI was deposited from undiluted SiHa. (From A. J. M. Bemtsen. Ph.D. Thesis. Universiteit Utrecht. Utrecht, the Netherlands, 1998, with permission.)... 

A systematic study of the role of the ions in the deposition process and their influence on the quality of the layers has been performed by Hamers et al. [163, 301, 332] in the ASTER deposition system. More specifically, a study has been made on the relation between the plasma parameters and the material properties in both the a- and the y -regime at typical deposition conditions. Here, the... 

In the ASTER deposition system, experiments have been carried out in which the excitation frequency was varied between 13.56 and 65 MHz [169]. The other process conditions were kept constant at a power of 10 W, a pressure of 0.16 mbar, gas flows of 30 seem SiHa and 30 seem H2, and a substrate temperature of 250°C. As in Section 1.6.2.3, plasma properties that are deduced from lED measurements are compared with material properties in Figure 63. The lEDs of SiH at four frequencies are shown in Figure 64. 

The plasma potential is about 25 V (Figure 63a). This value of the plasma potential is typical for the silane plasmas in the asymmetric capacitively coupled RF reactors as used in the ASTER deposition system, and is also commonly found in argon or hydrogen plasmas [170, 280, 327]. From the considerable decrease of the dc self-bias with increasing frequency (Figure 63a) it is inferred that the... 

---