Microstructure properties of lead silicate glasses

In the present work, lead silicate glasses have been prepared withdifferent amount of lead oxide content. Structure properties such asX-ray diffraction, AFM, and FTIR analyses have been done. Theexceeding of PbO content more than 25wt% revealed a decreasing indensity. The X- ray revealed that the strongest peak related toHexagonal silica dioxide and the other crystal phases formed wererelated to silica oxide (SiO2) and lead oxide (PbO). Growth anddecayed phases in X-ray have been observed with changing leadoxide content. Homogeneous surface was obtained using AFManalyzer with an average diameter around 100 nm. Infrared spectrumis characterized by the presence of large absorption band between1200 and 900cm-1 and have its maximum at 1080 cm-1 which istypical to stretching vibrations of Si–O–Si bonds, another bandsnoticed were attributed to Pb–O–Pb, Pb–O–Si, [AlO4]-tetrahedron,and to Si–O–Al bond.


Introduction
As for ceramics, inorganic glasses, glass-ceramic and glaze materials have long gone beyond their traditional uses to address a wide array of modern technological challenges in fields such as electrical engineering / electronics / sensors, solar energy, protective and decorative coatings, optics / optical telecommunications, structural mechanics, medical, nuclear technology, superconductors and microfluidics [1].
Glasses find wide applications, among others, in process technologies, vacuum science and technology etc. because of their desired thermomechanical properties and chemical durability. PbO-based glasses are technologically important due to their high refractive indices and relatively low melting temperatures. Among the various lead oxide containing glasses, PbO-SiO 2 system has been found to form glass over a wide composition range and has got many applications in making ultrasonic delay cables, electron multiplier, TV picture tubes and glass-to-metal (GM) seals [2].
Oxide glasses based on PbO are of great interest in material filed because they can form amorphous materials without traditional network formers like SiO 2 , and they can be used to produce glass ceramics, optical and optoelectronic devices, thermal and mechanical sensors, and reflecting windows. [3][4]. Lead oxide is not a glass-forming oxide by itself, but it can be incorporated in substantial quantities into other glass-forming oxide systems such as silicates, borates and phosphates [5,6]. In pure PbO lead atoms are bonded to four oxygen atoms, giving rise to a puckered layer structure with large Vander Waals (Pb-Pb) interactions. However, few studies have been reported on the PbO-based glasses. It was pointed out that Pb +2 cations are highly polarizable and the asymmetry of their polyhedra inhibits crystallization processes in the melts in which they participate [7]. In many oxide glasses, PbO is known to play a dual structural role both as a network former [8] and as a network modifier [9] depending upon its concentration in the glass. When PbO enters the network, it is expected to form PbO 4 units in which lead is coordinated to four oxygen's in a covalently bonded configuration [10]. In the present work, the incorporation of different PbO content in silica feldspar glass (with a fixed alkali oxides NaO 4wt%, K 2 O 9wt% ,and 2wt% of alumina ) have been prepared using conventional glass technology. The glass structure were discussed using X-ray. AFM and FTIR techniques.

Experimental work
The starting materials used in the preparation of the lead silicate glasses were red lead (Pb 3 O 4 ), alumina (Al 2 O 3 ), and feldspar (containing some amount of sodium and potassium) as a source of quartz (SiO 2 ). It was well known that Pb 3 O 4 decomposes to lead oxide (PbO) completely at 500 ˚C. Pb 3 O 4 (2PbO.PbO 2 , minium, red lead) -red powder, good oxidizer, at 500 ˚C decomposes to PbO and oxygen.
Lead glass samples were synthesized using a powder technology technique. The starting material was prepared by mixing and grinding of pre-determined of appropriate amounts of mentioned material. 100g from each glass batches were melted in platinum crucible using an electrically heated furnace at 1150 ˚C in air for a period of time 2h followed by normal cooling of molten liquids onto room temperature. Re-grinding process in a planetary mono mill ( Retzh, Model PM100) using zirconia jar and ball to obtain final glass powders has been done for homogenization product. Two gram from each batches were weighted and pressed to a disk samples have a diameter 1.5 cm by subjected uniaxial pressure holder of 5 ton/cm 2 to produce clear homogenous discs. Prepared samples have sintered at 650˚C for 2h. Fig. 1 shows the variation of green and bulk density as a function of PbO wt% content. Glass density measurements were determined at room temperature using the standard Archimedes method with water as the immersion fluid. A mass of glass disc was weighed in air (W ob ) and then immersed in water and reweighed (W w ), water density is 1 g/cm 3 . Thus the density of the object is calculated by the following relation [11]:

Results and discussion 1. Green and bulk density
(1) As it can be seen that the green and bulk density have the same behavior for all prepared samples. The increase of PbO wt% content leads to increasing in both densities till it reach the highest value for green density ~2.1 gm/cm 3 and ~2.4 gm/cm 3 for bulk density of the specific PbO 25wt%. Additions of PbO wt% higher than this percentage producing an obvious decreasing in densities for these compositions. However, increasing in bulk density is very conceivable in contrast with green density. This can be attributed to the effective role of heat treatment on the bulk density leading to removing of the bubbles content in the samples and generation of liquid phases formed from dissolving of the constituents. This situation leads to a homogeneous of sample compositions to produce more dense material. Besides, it is well known the fact that inclusion of PbO in most glass structure must leads to increasing of density because Pb has a high density 9.53 gm/cm 3 [12], in other words, the addition of Pb 2+ atoms, will modify the glass structure by creating a non-bridging oxygen (NBOs) in the network [28]. The NBOs created were believed to alter the glass structure in a way that the packing of the molecules becomes denser as more network modifier ions (in this case Pb 2+ ), attempt to occupy the interstices within the network [13]. The results are consistent with several studies by authors on the phosphate-based glasses [14,15].
But in this study, the exceeding of PbO content more than 25wt% revealed a decreasing in density. This can be explaining to, as the PbO content increase, sample may reach a saturation state which means that the liquid phases can consumption the contents then producing inrich PbO content samples. The exceeding of PbO content will segregates and retains bubbles leads to phase separation occurs then increasing in volume and consequently less dense.

X-Ray analysis
Samples that have 15 wt% PbO and 30 wt% PbO content were chosen to subjected to X-Ray diffraction analysis. The results were shown in Figs.2 and 3 respectively. PCPDFWIN program was used to estimate the presence phases. It can see clearly the appearance of several peaks related to the formation of crystal structures, Fig.2. The strongest peak related to silica oxide which has a position 2θ = 27.020 deg. The other crystal phases formed were related to silica oxide(SiO 2 ) and lead oxide (PbO). The formation of these phases is due to the heat treatment that use in this work which helpful the glass composition to react and produce crystal structure of glass-ceramic with Hexagonal SiO 2 a=4.867 c=5.331. The other important information were illustrated in Table 1.    Fig. 3 show the X-ray diffraction of sample contain 30 wt% PbO by subject the same heat treatment condition. This pattern provide Hexagonal SiO 2 a=4.884 c= 5.4000 with the same phases produce early for 15 wt% PbO with attention to appearance of K 2 O and decay of all crystal phases that obtained before.
This decay can be explain as the presence of large quantity of PbO in the composition of lead silicate at the same condition of heat treatment are unable to produce growth process of crystal structure. Important information for prepared sample contain 30wt% of PbO were illustrated in Table 2.

AFM analysis
The microstructure of selected sample (has 15 wt% PbO) was analysed using AFM (type scanning probe microscope CSPM) Fig. 4. The surface morphology revealed the presence of homogeneous crystal growth tend to have a certain direction Fig. 4a . This state raises attention to the growth tendency of specimen to a favorite direction. The important feature is, although, the method used in the manufacture of the specimen is the traditional way of glass, it was obtained a sample with an average diameter: 104.33 nm which mean a good result. The granularity normal distribution chart was shown in Fig.4b.

FTIR analy
Fourier TIR) techniq ght spectro be the struc ss matrix. T y depends des, but also sent in the IR vibration sses were  [16]. The formation of SiO 4 groups proceeds at the rate of two Tetrahedral oxygen. This formation of tetrahedral is reduced with increasing the content of PbO above 20% because some of the lead atoms will participate in the network as PbO 4 pyramids with the Pb atom forming the apex of the pyramid [17].
The peaks corresponding to the infrared bands observed in these glasses are given in these figures. The background spectrum includes the contribution from any ambient water (these bands lies in the range from 3200 to 3640cm −1 , and about 1600 cm -1 ) This band is ascribed to OH stretching of the adsorbed water, related to the bridged hydroxyl group with stretching vibrations of adsorbed water molecules or with other OH groups, and to isolated OH groups [18] and carbon dioxide doublet at 2360 cm -1 [19]. The comparison between these figures illustrate the broadening absorption peak at 3, 450cm −1 . It can conclude that the increasing of PbO content to 30wt% causes an increase in intensity of this broad. This increases may be due to the moisture adsorbed during the test or its related to formation of more adsorbed structure with increasing PbO percentage, as well as the presence of K 2 O which is characterize as highly absorbance material as demonstrated in XRD Fig.3. Infrared spectrum is characterized by the presence of large absorption band between 1200 and 900cm -1 and have its maximum at 1080 cm -1 which is typical to stretching vibrations of Si-O-Si bonds [20,21]. For comparison, Verweij (1979b) observed a broad depolarized band near 1080 cm -1 for K 2 Si 2 O 5 and Na 2 Si 2 O 5 glasses and Furukawa et al. (1981) observed a depolarized band near 1080 cm -1 between silica and the disilicate composition for the Na-series, whose depolarized band maximum decreased sharply to near 1050 cm -1 between the disilicate and metasilicate [22]. The broad bands in the range 800-1, 300 cm -1 indicate different structural orderings of the glass network due to overlapping of stretching vibrations of various silicate units. This behavior can be influenced by quantities of PbO and Na 2 O in the glass composition. The bands noticed in the range 400-500 cm -1were attributed to Pb-O-Pb and Pb-O-Si bonds according to previous observations [23,24]. There are also absorption bands in the area of 900-850cm-1 typical for [AlO 4 ]-tetrahedron ,and the maximum at 867 cm -1 pertinent to Si-O-Al bond. The characteristic band of the cordierite phase with absorption maximum at 779 cm-1 pertinent to [Si 5 AlO 18 ] is a peculiarity of refractory IR-spectrum [20]. weak, sharp peak which also appear polarized is observed at 600 cm -1 . The origin of these weak peaks is controversial. It was originally attributed to structural defects in vitreous silica associated with broken Si-O-Si bonds [22]. the peak around 470 cm -1 was related to Pb-O bond vibrations [25], and around 462 cm -1 for Al-O-Al bending vibration [26].