Foam-Glas Crystal Materials

Kazmina О.V.1, Semukhin B.S.2, Mukhortova А.V.1 1National Research Tomsk Polytechnic University, Russia

2Institute of Strength Physics and Materials Science of the Siberian Branch of RAS, Russia Corresponding - kazmina@tpu.ru

Abstract - The paper describes the foam-glass crystal material production technology via the intermediate product comprising residual crystalline phase. Different factors impact (macroscopic structure, quantity and size of the crystalline phase) on the foam material strength properties has been searched. It has been stated that the material with a hexagonal shape of pores 1-1.5 mm in size and partitions 40-50 µm thick is characterized by the maximum strength. When nanospheroids are formed in the noncrystalline matrix of the foamed material partition, its strength reaches 4.5 MPa which is close to the values of calculated strength (5 MPa).

Keywords - glass foam; interpore partition; nanospheroid; noncrystalline matrix; strength

I. INTRODUCTION

A two-stage method for producing foamglass via the intermediate product (quenched cullet) synthesized by thermal treatment of the mixture of the certain composition was developed at Tomsk Polytechnic University. This product acts as the raw material for the following sponging and obtaining foam-glass-crystal products with the pre-set characteristics. According to the phase composition the quenched cullet represents a vitrified product with residual crystal inclusions which define the density and strength of the end items.

The idea of the technology developed lies in the following principles (fig.1.):

• Special preparation of raw materials allows synthesizing a glass ceramic at temperatures not over 950оС which is the raw material for the foam glass;

• Controlling the formation of nano and microstructure of the interpore partition allows regulating the strength and density of the end item.

Relatively low temperatures of the glass phase synthesis carried out according to the designed mode, promotes not only energy consumption decrease, but also that of the carbonic acid release. The working operation of the mixture compaction makes possible to lower the air pollution of the working area as well as the general dust emission to the atmosphere.

Mechanical properties of porous materials similar to the glass foam are defined by their structure, i.e. size, form, homogeneity of pore distribution, as well as the thickness of the interpore partition and the composition of a noncrystalline component [1, 2, 3]. It is known that mechanical strength of

amorphous phase significantly increases in presence of the crystal phase particles of micro- and nano-sizes, i.e. without the concentration of stress at the phase interface that results in destruction [4, 5, 6]. The particles of this size can be obtained in glass by means of partial crystallization or using the phenomenon of microliquation delamination [7, 8, 9].

The objective of the paper is to show the impact of glass foam material macrostructure on the its mechanical strength, as well as the quantity and the size of crystalline material particles present in the non-crystalline matrix of an interpore partition.

II. EXPERIMENTAL

The foamed material was produced according to the two-stage technology developed by the authors [10, 11]. At the first stage the quenched culled was synthesized at temperatures of 850 – 950оС that are significantly lower as compared to that of glass melting (1400 – 1500оС) used in the conventional glass foam technology. Such crystal and noncrystalline silica-containing rocks as silica sand, diatomite, flint, silica clay, and perlite were sampled as a feedstock. Low-temperature synthesis of quenched cullet results in the formation of residual silica in the vitrified material. At the second stage, a foam-forming mixture was prepared from the quenched cullet powder and gas developing agent in sponging modes individual for each composition. Studies conducted on Na2O-CaO-SiO2 system glasses and containing Na2O – 23%, CaO – 5%, SiO2 – 72%, showed that the quenched cullet contained a residual crystalline material in the silica state. The number and size of residual silica particles in the finished material are controlled by the composition of the initial mixture and technological modes of the quenched cullet synthesis and sponging.

By changing the technological mode and using various feedstocks the foamed material samples were produced containing the residual crystalline material from 5 to 20% in bulk, which is supported by the results of X-ray structural analysis.

To determine the structure of the obtained samples the optical and electron microscopy was used. The elemental composition of interpore partitions was determined with the use of high resolution scanning electron microscope JSM-7500FA equipped with X-ray microanalyzer. Before

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recording, the samples were covered with a thin platinum layer.

Figure 1. The principal procedure for obtaining foam-glass-crystal materials

The X-ray structural analysis of the phase composition of quenched cullet and foamed material was carried out using DRON-3M diffractometer at CuKα radiation with the monochromatization of the diffracted beam by a pyrolitic graphite crystal.

To analyze mechanical properties and define ultimate resistance of the samples, tests were carried out using TestMachine ‘Instron1185’ with the load range from 0-100 N to 0-100 kN. Precision of load measurements is ± 0.25% of the load scale used. According to the recommendations of the Federal standard РЕН 826-2008 (Test method of compression properties), the tests were carried out at the rate of 2 mm/min.

III. RESULTS AND DISCUSSION

Optical investigations of the noncrystalline matrix of the foamed material partition showed the presence of particles with the strong interface (fig. 2). The particles formation can be explained by silica dissolution in the amorphous melt during sponging.

Figure 2. Electronic pictures of interpore partition of foamed material obtained from the low-temperature quenched cullet

It has been stated, that the durability of the experimental samples increased from 1.8 to 3 MPa in changing the size of particles from 1000 to 300 nm in a crystalline material. On the basis of the experimental results, an isogram was designed for foamed material durability and the number of micro and nanoscale crystalline material dependence (fig. 3). The results of calculations obtained by an interpolation method showed the maximum displacement of the foamed sample durability with the crystalline material particles of small sizes (300 nm) into the area of lower concentrations (5-7%).

Figure 3. Isogram of foamed material strength dependent from the size of particles of crystalline material: 1 – 3 МPа; 2 – 2.3 МPа; 3 – 1.8 МPа

For the samples with the fixed amount of residual

crystalline material (5% on the average) and different sizes of its particles, the experimental dependence has been established which shows that the correlation between the strength and the size can be described by the simple exponential relationship y = yo + a1×e-x/t (correlation coefficient R2=0.98). Based upon the assumption that the minimum particle size of the crystalline material corresponds to the size (10 nm) of heterogeneity areas that are present in the glass structure, the maximum theoretical strength of the foamed material was calculated by this dependence, and its value amounts to 5

MPa.

Controlling the technological mode of the quenched cullet synthesis and the sponging process so as to obtain a noncrystalline matrix in the interpore partition of residual silica not more than 5-7% in concentration and up to 100 nm in size, it is expected that the mechanical strength of the foamed material can reach the values over 5 MPa. In case of the glass foam with no crystalline material in its noncrystalline matrix this value does not exceed 1.5 MPa.

The foamed material samples selected for further investigations possess higher strength properties (fig. 4). The macrostructure of these samples corresponds to the following parameters: hexagonal shape of pores 1-1.5 mm in size; interpore partition 40-50 µm thick; a high degree of pore distribution homogeneity in the bulk of material; the crystalline material content not more than 5%, with its particles size not exceeding 200 nm.

Figure 4. Foamed material pores shape with various strength: a – 3.41 MPa; b – 3.67 MPa

Electronic Microscope Image of the foamed material interpore partition clearly shows spherical elements ranging in size from 60 to 160 nm. Such structural elements (spheroids) have not been found in the interpore partition of the glass foam. According to the bar chart of the spheroid size distribution, the average size value is 89 ± 10 nm with the distribution maximum of 60 nm (fig. 5).

Figure 5. Size distribution of the spheroids in the interpore partition

It has been stated, that the value of ultimate strength for the samples containing nanoscale spheroids is consid-erably higher (2-3 times) in comparison with that of the samples without such structural elements. It should be noted that during compressive testing of the foamed material, there is no abrupt destruction observed which is typical for brittle materials. With the load increase the sample starts to deform. The resultant glass powder is pressed into the newly destroyed cells with the following gradual destruction of the sample.

IV. CONCLUSIONS

As a result of this research the factors affecting the glass foam mechanical properties have been determined. When producing foamed materials with the increased mechanical properties the following factors should be taken into account:

1. Structural factor that determines the strength of the foamed material due to the crystalline material nanoparticles present in the noncrystalline matrix. Maximum strength is observed in the samples with 5-7% concentration and the particle size of not more than 300 nm. When nanoscale spheroids are formed in the noncrystalline matrix of the foamed material partition, its strength reaches 4.5 MPa which is close to the calculated strength values.

2. Technological factor which is determined by the mode of foamed material production. In terms of strength, the optimum mode is the one that allows producing the material structure with pore size of 1-1.5 mm and hexagonal shape; interpore partition 40-50 µm thick and the equilibrium distribution of pores in bulk.

a

b

ACKNOWLEDGMENTS

We are grateful to the Ministry of Education and Science of the Russian Federation (grant N02.740.1 1.0855) and the European Commission (grant N 228536; NMP4-SL-2009-228536; Project NEPHH) for financial support. Research was conducted using the equipment of the Nano-Centre of the National Research Tomsk Polytechnic University.

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