Stoichiometric samples, preparation

Sample Preparation. The sample series was prepared by varying the ratio of Epon 828 to the methylene dlaniline (MDA) curing agent. The compositions tested contained 0%, 23.1%, 50.0% and 100% excess MDA over the stoichiometric amount. The MDA and diepoxide were heated to 100°C and mixed. The mixture was cast onto one side of a sheet of aluminum 5.5 cm. x 24.5 cm. x 5 mils, which had first been wiped with acetone to degrease the bonding surface. The final coating thickness was controlled at approximately 60 mils. 

In Chapter 3, four examples of non-stoichiometric compounds used as practical materials are described from a chemical point of view. The sections on ionic conducting materials and hydrogen-absorbing alloys concentrate on how to utilize the characteristic properties of these compounds, in relation to their non-stoichiometry. In the section on magnetic and electrical materials, methods of sample preparation, focusing on the control of non-stoichiometry, and the relation between non-stoichiometry and the properties of the compounds are presented. 

The Th4H15 samples were prepared by Cameron Satterthwaite and coworkers (1,2) and consisted of two polycrystalline samples that were determined to be within 1% of the stoichiometric composition, Tn4Hi5 o.i5 The samples differed primarily in the pressure and temperature used in their synthesis. The sample hydrided under lower pressure ana temperature conditions (1 atm of H2 and a temperature cycle initiating at 800° K and dropping to 450° K before removing the H2) is labeled the LP sample, and the one hydrided under higher pressure and temperature (1100° K ana 10,000 psi of H2) is labeled HP. The sample preparation and characterization of this stoichiometric compound is apparently critical since the results of the present study differ significantly from the previous NMR studies (12,13,14), and some difference was detected between the LP and HP samples themselves. The carbonyl hydride samples were furnished kindly by John R. Shapley (20). 

Silver oxalate is a colourless, crystalline substance which on heating undergoes an exothermic decomposition. The reaction begins at a little over 100 °C and easily becomes explosive. It was noticed quite early that samples prepared in the presence of an excess of oxalate were less stable thermally than those prepared using stoichiometric amounts of oxalate and silver ions. The thermal decomposition of silver oxalate into silver and C02 has subsequently been studied under varying conditions of preparation, decomposition environment and preirradiation.258,259... 

Following this step there is continued dissolution, which removes whatever hyperfine particles may have resulted during sample preparation. After removing these, further dissolution breaks down the outer surface of the residual layer at the same rate that alkalis are replaced by hydrogen at the interface between fresh mineral surfaces and the residual layer. This releases all constituents to the solution. Release is now stoichiometric, based on solution chemistry and surface morphological results. Thus, the reaction is surface-controlled (Velbel, 1985). 

Method II (Sample Preparation) Transfer approximately 0.2 g of sample, accurately weighed, into the titration flask. The stoichiometric factor (Fs) for Allura Red is 8.06. Arsenic Determine as directed under Arsenic Limit Test, Appendix MIR, using a Sample Solution prepared as directed for organic compounds. 

Sample Preparation. The samples were prepared by the direct oxidation of stoichiometric uranium dioxide obtained by the thermal decomposition of uranyl iodide (17). 

According to Fig. 6.26, spinels, prepared by mechanical activation followed by heat treatment, are characterized by very low lattice parameter a, which increases as temperature rises. It is well known that the radius of Mu ions is lower than that of Mn . Mn ions are more stable at lower temperatures. Thus, low values of a parameter for the samples with molar ratio Li/Mn=l/2 point to the increased amount of Mn ions comparing with that of stoichiometric spinel [93]. One can see that in the samples prepared from Li2C0 by mechanical activation and heating at 450°C, the value of a and the amount of Mn ions are higher than in the case of LiOH samples because of reducing character of CO2, eliminating in the course of interaction [98]. 

Furukawa et al. (2) measured C (15-370 K) using a sample prepared by stoichiometric proportions of LIF and AlFg in graphite. X-ray diffraction and petrographic examination of separate portions of the sample indicated a single phase identified as 0-LigAlFg. The authors tabulated values of C and S based on their data and the extrapolation S (15 K) = 0.042 cal K" mol". These values are adopted. 

Stoichiometric samples of CogSg can best be prepared by heating cobalt sulfate in a stream of H2/H2S at 525°C. Whereas it takes almost two weeks to obtain CogSg by the direct combination of the elements, pure single-phase products can be obtained from the sulfate in six hours. 

PSC samples were prepared in galvanostatic regime at current density (/) = 4-120 mA cm-2 in dilute hydrofluoric acid without illumination. PSC samples prepared at / 4-80 mA cm-2 remained stoichiometric (Si/C ratio = 1), so below we refer to this as SPSC. 

The use of organometallic compounds is efficient in the case of Li. Solvent is not cointercalated which is an advantage compared to the liq NHj case, but a disadvantage concerning the diffusion of the alkali-metal ions, and a complete intercalation may not be achieved (the NH3 molecule separates the sheets and favors A ion diffusion). Solid-state high-T techniques are well adapted to preparing the stoichiometric samples, each time mixtures of sulfides or oxides are used. Well-crystallized products are obtained. 

Figure 1 shows the diffuse background maximum of liquid mercury. The diffuse background maxima of Figs. 1 and 2 were observed at the same ai le. This Indicated that the method of preparing stoichiometric samples, in which the vapor pressure of mercury was r ulated by varying the amount of excess mercury in the melt, could give rise to defective samples, which would have poor electrical properties (especially at low temperatures). 

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