化学和树脂干胶的先驱体制备过程.docx

1、化学和树脂干胶的先驱体制备过程Chemistry and Applications of Polymeric Gel PrecursorsPolymerizable complex (PC) method seems to be the easiest and the most elegant variation of solgel methods. The simplest implementation of this technique requires only inexpensive chemicals, a beaker, a stirrer, a hot plate and a f

2、urnace. It is not sensitive to the presence of water (for the rare exceptions); it does not require inert atmosphere, and even without careful control of gel processing time and conditions, one should be still able to obtain samples of outstanding homogeneity. It is the combination of these factors

3、that explains the growing popularity of the polymerizable complex method for the synthesis of ferroelectric, superconducting, ferromagnetic, CMR, photocatalytic, fuel cell, electrode, catalytic and other complex oxide materials.The origin of the polymerizable complex method is dated back to the Pech

4、ini patent (Pechini, 1967) on fabrication of thin film capacitors by utilizing multifunctional organic acids capable of chelating metal ions into the stable complexes and a diol, which serves as a solvent during complex formation step and later participates in the polyesterification reaction to form

5、 three-dimensional polymer network with incorporated metal complexes mixed, as assumed, on the atomic scale. The essential steps of the polymerizable complex method are presented in Figure 4-1. Suitable metal salts are introduced into the ethylene glycol (EG) after dissolution of citric acid (CA), w

6、hich is added in a large excess to form metal-citrate complex. Then the temperature should be increased to 100130C to speed up the formation of polyester due to the reaction between free citric acid and ethylene glycol. After the formation of plastic-like gel, the temperature is increased further to

7、 remove the excess of ethylene glycol. The obtained relatively hard substance should be treated at 450600C to oxidize organic compounds. Precursor powders thus obtained contain a mixture of homogeneously distributed metal oxides, carbonates or sometimes an intermediate single phase compound with the

8、 proper stoichiometry of the metal ions.The basic idea of this approach is to use in situ polymerization of monomers specially introduced into solution in addition to the required metal cations. This technique is known as Pechini method (Pechini, 1967). The background of the proposed approach is est

9、erification reaction between glycol and multifunctional carboxylic acid. For continuous growth of the polymer chain the existence of at least two functional groups in a monomer is important. The viscosity of the solution drastically increases during the polymer chain augmentation (Fig. 4-2). As one

10、could notice form Figure 4-2, constructed from the reported data (Arima, 1996; Kakihana, 1997), the nature of the cations affects the rate of viscosity increase, which indicates that metal ions play an important role in cross-linking the polymer chains by means of complex formation. This viscosity i

11、ncrease is often referred to as a “gelation”. Probably it would be more accurate to use the word “inspissation” to reflect the nature and conditions of the polyesterification reaction; however, in this chapter we will follow the established terminology. During the first stage of the polymer growth t

12、he solution provides necessary environment to prevent cation segregation, and later, the relatively rigid polymer network traps cations and preserves the initial homogeneity of the solution. The original Pechini patent states the importance of the carboxylic acid, which can form a stable chelate com

13、plex such as citric acid and the polyhydroxy alcohol, which is liquid below 100C and may serve simultaneously as a monomer and as a solvent. After the polymerization is completed and the excess of solvent is evaporated, the polymer matrix of gel is to be oxidized yielding aerogel which is composed o

14、f oxides and carbonates.Figure 4-1. Scheme of polymeric gel method.Figure 4-2. Viscosity increase of the metal-citric acid-ethylene glycol system as a function of reaction time.Since the publication of the original patent, the polymerizable complex method was intensively studied, sometimes remarkabl

15、y modified to meet the particular synthetic needs or to overcome the obstacles that may arise during the synthesis. In the past few years it became more common to refer to polymerizable complex method as “Pechini method”, “modified Pechini method”, “liquid mix technique”, etc. In this chapter we wil

16、l also follow the well-established custom whenever we will discuss synthesis employing citric acid and ethylene glycol as essential reagents. In addition to that we will discuss the synthesis techniques developed from Pechini method that can be classified as polymerizable complex method (Kakihana, 1

17、996, 1999a) since they possess two distinctive features: formation of stable metal complex in the solution and preserving atomic scale homogeneity achieved in the solution by use of polymer to hinder the ions mobility and prevent segregation.ChemistryCompared to the knowledge base on the practical u

18、se of Pechini method for synthesis of different oxide materials, relatively little is known about chemistry of this technique. Most of the fundamental studies on citric acid and its metal complexes were carried out in 60 and 70 seither before Pechini publication or right after it when polymerizable

19、complex method has not deserved much attention yet. Therefore most of the results are not directly related to polymerizable complex method. More recent studies on the chemistry of Pechini method often were carried out by material scientists with necessary chemistry background and focused on very spe

20、cific questions or narrow set of particular problems. We will attempt to summarize the available information and create a general (and hopefully useful) picture of the chemistry involved in this method.PrecursorsIn the typical synthesis of oxide materials by Pechini-type process, soluble metal nitra

21、tes, acetates, chlorides, carbonates, isopropoxides or other suitable metal compounds are dissolved in the CAEG solution. Citric acid is relatively strong multifunctional organic acid. The acidity of the middle carboxylic group (mCOOH) is enhanced by OH group attached to ternary carbon atom. Methyle

22、ne groups, on the other hand, provide a destabilizing effect; however it is reduced by the neighboring terminal carboxylic groups (tCOOH). Therefore middle carboxylic group in water solution loses H+ very easily (pK1 = 2.91) (Harris, 1976) and pH of citric acid aqueous solution is usually in the ran

23、ge of 02 depending on concentration. The terminal carboxylic groups are less acidic (pK2 = 4.36 and pK3 = 5.74) (Harris, 1976) and the dissociation becomes substantial at higher pH. At very high pH the hydroxy group of citric acid may become deprotonated (pK4 = 10.96) (Grigoreva, 1975). One may also

24、 notice that location of OH group is favorable for the formation of hydrogen bonds between carboxylic groups and hydroxy group, which should stabilize the carboxylic ion. What is important, is that such a skeleton is responsible for the formation of stable five- and six-member rings in metal citrate

25、 complexes.Citric acid is well soluble in ethylene glycol, which provides a wide range of CA:EG ratios for the Pechini method, and makes it possible to tune the conditions of synthesis for each particular system. Chemical interaction between citric acid and ethylene glycol occurs at room temperature

26、 without any treatment of the solution. Of course, this interaction is not complete and chemical equilibrium is strongly shifted toward free citric acid and ethylene glycol, however 13C-NMR spectrum unambiguously proves ester formation (Kakihana, 1999a; Fang, 2001). The quality of the final powder d

27、epends on the extent to which the molecular-scale homogeneity achieved in the solution may be preserved in the process of polymer formation and pyrolysis of the polymer resin. For the purpose of oxide powders synthesis citric acid and ethylene glycol ratio in the Pechini process seems to be far from

28、 the optimal value. Figure 4-3 summarizes the results of the investigation of CAEG system behavior during gelation, solvent removal and thermal decomposition of organic matrix conducted by Tai and Lessing (1992a, 1992b). The range of CA concentrations from 50 to 60% seems to be the most appropriate

29、for the complex oxide powders preparation since it provides the maximum viscosity of the obtained gel. Strong foaming is an additional factor that prevents segregation during thermal decomposition of the polymer. Mild burning would guarantee relatively low temperature inside of the precursor and, co

30、nsequently, relatively slow grain growth. One should keep in mind, however, that ester formation is a reversible process.The equilibrium in this reaction can be shifted toward polyester by either increase of a starting reagent concentration or by a product removal from the reaction medium. On the ci

31、tric acid rich side one would be restricted by the CA solubility in ethylene glycol (Fig. 4-3). From practical viewpoint, high viscosity of the concentrated CA solution will slow down the dissolution of metal salts and, in addition, the precipitation of citric acid might occur while pH, temperature

32、or metal salt concentration (ionic strength) will change during the processing. Another reason to use excess of EG is connected with a need to remove water from the reaction mixture. In this case, the vapor will contain ethylene glycol as a major fraction that will be progressively removed from the

33、reaction. However, the complete removal of the excess of EG rarely occurs during polyesterification and the use of 20% citric acid concentration requires relatively long heat treatment to eliminate unreacted glycol. The boiling point of EG is the lowest among dials, so the choice of ethylene glycol as a solvent and as a monomer is the most convenien