Journal of Non-Crystalline Solids 352 (2006) 4166–4173 www.elsevier.com/locate/jnoncrysol
Negative dielectric loss phenomenon in porous sol–gel glasses Ekaterina Axelrod a, Alexander Puzenko a, Yaara Haruvy b, Renata Reisfeld c, Yuri Feldman a,* a
c
The Hebrew University of Jerusalem, Department of Applied Physics, Jerusalem 91904, Israel b Soreq NRC, Div. of Radiation Protection, Yavne 81800, Israel The Hebrew University of Jerusalem, Department of Inorganic and Analytical Chemistry, Jerusalem 91904, Israel Available online 18 September 2006
Abstract The broadband dielectric spectroscopy method was employed to investigate glasses of a fine porosity produced via regular and fast sol–gel routes with different catalysts. The study was carried out in the frequency range 20 Hz to 1 MHz and temperature interval 100 to +300 C. The process discussed in the paper demonstrates unusual dielectric behavior. Dielectric losses of this process are negative, in certain regions of frequency and temperature, and the corresponding real part of the dielectric permittivity increases with increasing frequencies, according to Kramers–Kronig relationships. This exceptional process is located in different temperature regions for each sample. The concentration and type of acids, used as catalysts, influence the amplitude and the temperature–frequency ranges of the negative losses process (NLP). In order to decipher the physical nature of the process, the experimental study has been accompanied by a theoretical one. It was shown that the NLP in the porous sol–gel samples can be attributed to the local non-compensated matrix-anchored charges, which accumulate both inside the glassy matrix bulk and on the interface of the pores therein. 2006 Elsevier B.V. All rights reserved. PACS: 61.43.Gt; 81.20.Fw; 77.22.d; 77.22.Ej Keywords: Dielectric properties; Relaxation, electric modulus; Sol–gels (xerogels)
1. Introduction The sol–gel process is a method for the production of ceramic media, which provides a convenient way to incorporate a wide variety of materials, including organic molecules and active proteins, in porous ceramic matrices. The term sol–gel denotes a process in which a solution or a sol undergoes a sol-to-gel transition [1–3]. At this transition, the solution becomes a rigid, porous mass. Most sol–gel techniques use water and low molecular weight alkoxysilanes such as tetramethoxysilane (TMOS), tetraethoxysilane, or an equivalent organometallic alkoxide as sol–gel precursors. Since alkoxysilanes are immiscible in water, a mediator solvent is often used for homogenization. The *
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[email protected] (Y. Feldman).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.07.008
polycondensation of alkoxysilanes can be summarized in terms of two reactions: (1) hydrolysis of the ester, and (2) condensation via either silanol–silanol or silanol–ester reaction. The hydrolysis stage is most rapid and reaches completion in the presence of catalysts. Mineral acids or ammonia are the most common catalysts for the sol–gel processing. Catalysts such as acetic acid, KOH, amines, KF, HF, titanium and vanadium alkoxides and oxides are employed as well. As shown in previous articles [1,4] mineral acids are more effective catalysts than bases, at equivalent concentrations. It has been discussed [4] that sol–gels which were catalyzed using strong acids (e.g. HCl, HNO3) result in transparent high-density glassy structures with small pores (called micropore structures, with a pore size
