T-precursor

In principle these compounds offer access to materials with AliCh-SiCL and Al203 2Si02 stoichiometries. The latter stoichiometry is equivalent to the Al[OSi(OBu-t)3 (OBu-t)] precursor. The major drawbacks with these materials are their air and moisture sensitivity, and the cost of the starting materials. Although the idealized stoichiometries of the above ceramics products are not those of crystalline aluminosilicates, amorphous aluminosilicate glasses are often important in optical applications or in scratch-resistant coatings. Furthermore, they may offer potential for CVD-type applications. There still remains considerable need for simple precursors to crystalline aluminosilicates, especially for structural applications. Dense, phase pure crystalline ceramic materials are desired for optimal mechanical properties, e.g. ceramic fibers for composite manufacture. 

B. The polarizations of the reaction products are related to the modes of pair formation. They are opposite for the cases of pair formation from S and T precursors and are equal in signs for T and F precursors. An example of this relationship will be given in Sect. 4.1. 

Precursor Method T precursor (°C) Substrate ("C) Pressure (Torr) Atmo- sphere Substrate Ref- erence... 

Let us consider chemical reactions through radical pairs as shown in Fig. 3-1. Radical pairs are usually produced from singlet and triplet excited states. These reaction precursors are called S- and T-precursors, respectively. Radical reactions also occur from free radicals, which is called F-precursors. Because the dynamic behavior of radical pairs from F-precursors is similar to that from T-ones, we omit discussion of reactions from F-precursors for simplicity s sake in this book. 

Eq. (5-9) shows that the Pi(/ ) value is too small to give appreciable CEDEP from the following reasons (1) Because Cj(0)Q(0) and c (0)Q(0) are zero for reactions from S-and T-precursors, the first and second terms of Eq. (5-9) become zero. (2) Because the 3 value is nearly zero during t = 0 tthe third term of Eq. (5-9) becomes also zero. 

Fig. 5-2. Phase pattern of CIDEP spectra through the S-T. mixing (7 < OJ) for the following simple example (1) radical a in a radical pair has one large HF interaction with A, > 0 J and /j = 1/2. (2) radical b in the pair has no HF interaction. (3) The pair is generated from (b) a T-precursor and (c) an S-precursor, respectively. The number of circles schematically shows the population of each sub-level and the black circle indicates the transfer of the population through the S-T. mixing.

There are many other mechanisms for generating CINEP. In this section, the Triplet Mechanism (TM), which is the oldest among them, will be explained. CIDEP due to the TM appears in the reactions which proceed through triplet-excited states. Thus, the reactions through T-precursors often show CIDEP spectra which have contributions due to both the PRM and the TM. The spin-Hamiltonian (Ht) of a triplet state can be represented as follows ... 

The observed E/A pattern means that Fm is negative. Because x is positive for a T-precursor, the J value should be negative. This negative J value has often been observed for neutral radical pairs. From Problem 5-11, the HF intensity ratio of the CEDEP spectmm of the 2-hydroxy-2-propyl radical in this reaction is obtained to be -1 -2 /3 -5 /3 0 sVs 2>/3 1, which is -l -3.46 -8.66 0 8.66 3.46 1. This ratio is quite different from that for the thermal equilibrium, 1 6 15 20 15 6 1, as shown in Problem 2-3. 

Fig. 5-11. The STo mixing of a radical pair with gi g2 and A, / gfig = 8Ms = 0 T and its possible ESR transitions whose signal intensities (Ip ) can be calculated from Eq. (5-43). Here, the radical pair is assumed to be produced from a T-precursor and the initial population of each sub-level is schematically represented by a circle. (Reproduced from Ref. [27] by permission from CRC Press)...

Second, let us consider the case when a radical pair is produced from a T-precursor. In this case, a triplet radical pair is initially generated. From Fig. 3-1, we can obtain the following MFEs on the yield of reaction products Because the T-S conversion rate increases with increasing B due to the AgM as shown in Fig. 6-1(a), the yield of the cage product from the singlet pair (Tc ) also increases with increasing B from Eqs. (3-42) and (3-38). 

Fig. 6-2. Theoretical prediction of the magnetic field dependence on the product yield (7(5)) in the reactions through radical pairs (a) the Ag mechanism (AgM), (b) the HFC mechanism (HFCM), (c) the mixed effect of the AgM and the HFCM, and (d) the LCM. The full curves indicate the magnetic field dependence of cage (escape) products produced from S-(T-) precursors. The broken curves indicate the dependence of escape (cage) products produced from S-(T-) precursors. In this figure, cage products mean those produced from singlet radical pairs. The curves for triplet states are omitted for simplicity, but they show similar dependence as those of escape products. (Reproduced from Ref. [34] by permission from The Chinese Chemical Society)...

In this case, the S-T conversion suddenly occurs at the level-crossing field (5lc) as shown in Fig. 6-1(c). When a reaction occur from an S-precursor, the S-T conversion rate is increased by a magnetic field at Blc. Thus, the Yc (B) value is decreased by Blc and the Ye (B) value is increased by it. When a reaction occur from a T-precursor, the T-S... 

In the presence of a magnetic field, the radical pair decay can be represented by a combination of two exponential functions as shown by Eq. (7-15). The fast component can not often be observed because the transient absorbance due to radicals is often overlapped with that of their T-precursor. Thus, the slow component can usually be observed. From Eqs. (7-15) and (7-16), the kp + kp value was found to decrease gradually with increasing B from 0 T to 1.34 T [2]. This means that the Trp value should... 

When a reaction occurs from a T-precursor, only the real part of p,2(0) appears in Eq. (11-69) and the right-hand side of Eq. (11-69) gives... 

Kecskes, L. J., Niiler, A., and Kottke, T., Precursor morphology effects in combustion-synthesized and dynamically consolidated titanium carbide and titanium boride. J. Am. Ceram. Soc., 76, 2961 (1993). 

A combination of both methods offers the possibility to produce stoichiometric WC. The WCi t precursor is first prepared by the entrainment method, after which the... 

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