Nanocomposites and nanomaterials

Formation of a conducting phase in Porous Glasses
Doycho I.1, Lepikh Ya.1, Filevska L.1, Rysiakiewicz-Pasek E.2, Grinevych V.2
1Odessa I.I. Mechnikov National University, Dvoryanska str., 2, Odesa 65082, Ukraine. E-mail: lfilevska@gmail.com
2Department of Experimental Physics, Wrocław University of Science and Technology, Poland
Porous silicate glasses may be used as matrices for the formation of nanoparticles’ ensembles for sensors due to the peculiarities of their structure. At the same time, rather considerable intrinsic electrical resistance of such systems, causes their usage mainly for luminescent-type sensors. At the present work it is suggested to form inside the pores the nanoparticles’ ensemble of a conducting substance for significant reduce of the sample resistance. This attitude will make it applicable as an active element in resistive type sensors. Such substance is a carbon, which in the form of graphite exhibits excellent conductive properties. To create a carbon conductive phase, the porous glass of any type should be saturated with aqueous solution of glucose. At the consequent heating to 150 ºС, it is easily reduced directly in the matrix cavities to carbon in the form of highly dispersed graphite with the release of water. This process, described by the equation
C6H12O6 –(t)→ 6C+6H2O,
lasts about 24 hours. Its completion can be fixed visually by sample blackening. After this treatment, the initial resistance of standard size samples 1×0.5×0.1 cm3, having several teraOhms is reduced by several orders of magnitude.
Water vapor, always present in the atmosphere, saturates the pores to a certain extent thus reducing the electrical resistance of the system. This makes it possible to use porous glass as an active element of a resistive type humidity sensor. The presence of a wide range of pore sizes (from about 10 to 100 nm in different types of glasses) corresponds to the hydrophilicity of the system in a wide range of humidity. A significant decrease in the intrinsic resistance of the samples after carbon treatment, due to the conductive properties of graphite, eliminates the initial   disadvantage, since it makes the fluctuations in the resistance of the system due to changes in humidity noticeable. This treatment also significantly reduces the sample cellularity at the ends, making it possible to create sufficiently reliable contacts to such a system using silicone or indium-gallium paste. Humidity sensors constructed according to this principle remain operational at any temperatures at which the system is not destroyed, including negative ones.
Such partial carbon treatment of the sample’s ends using specially designed holders will allow the formation of ohmic contacts at the ends of an arbitrary porous system for further applications, such as possible correlation studies between luminescent and electrical properties.