rus
Issue #2/2015
V.Lysenko, V.Mali, A.Anisimov, D.Trufanov
The study of nanoporous ceramics created by spark plasma sintering
The study of nanoporous ceramics created by spark plasma sintering
Obtain ceramics with open porosity of the Tarkosil silica nano-powder by spark plasma sintering (SPS) is a promising technology, which in the future
may find use in the manufacture of filters
for industrial gas separation.
may find use in the manufacture of filters
for industrial gas separation.
For the separation of fractions of liquids and gases, such as oil and natural gas, filtering is used – passing through the porous medium, i.e. the body, filled with a system of interconnected pores. Filtering can be performed using membranes that are selectively permeable to certain substances and allow to separate the components in mixtures.
Membrane methods are environmentally friendly and most cost-effective in comparison with competing technologies for the separation of substances. For example, helium in the industry is produced from natural and petroleum gases, where its concentration is very low, therefore cryogenic methods are ineffective. The use of membrane methods for producing helium concentrate can significantly improve process economics.
The most important characteristic of porous solids is porosity, defined as the proportion of body volume attributable to pores, or their volume per volume unit of the material. Usually the closed isolated cavities are ignored and only the interconnected flow-through pores, which form a complex branched and irregular network (a pore space) are considered. The porosity of most materials is in the range of 0.1–0.4.
If the membrane has a relatively large pores, the separation occurs mainly due to differences in molecular masses of the components. When the mean free path of gas molecules is much larger than the diameter of the pores (effuse, Knudsen flow), the separation factor is a power-law function of the ratio of their molecular masses. In membranes with small pore size, there are other mechanisms of separation, including an important role of the interaction of components with the walls of the pores in the membrane (sieve effect, adsorption).
For gas separation membrane from silicones, tetrafluoroethylene, polyetherimides, cellulose acetate, ceramic, glass are used [1]. For the membrane separation of helium and methane typically membrane of organic-silicon composite or cellulose acetate are used (for example, [2]). For the industrial separation of such gas mixtures as CO2/CH4, CO2/N2 and
O2/N2, recently often ceramic membranes are used. For example, in [3] a mixture of CO2/N2 were separated using nanoporous ceramics of silicon dioxide.
In [4] on the basis of the Tarkosil silicon dioxide nano-powder ceramics with open porosity was created, which can be used for filtration and separation of gases, in particular, for the enrichment of the mixture with helium. Ceramics was produced by the following method: the powder was moistened with distilled water and was dried within 2 months at room temperature. The result is a pretty solid porous workpieces, which were baked at a maximum temperature of 600 to 1100ºC. To avoid the destruction of the samples at the removal of bound water was used a low heating rate of 4ºC/min. After 180 minute exposure at a maximum temperature, the furnace was turned off and cooled down to room temperature for 12 hours. The study of filtration and separation properties of the filters obtained from such ceramics showed the possibility of enrichment of helium in its mixtures with nitrogen and methane (enrichment ratio of 0.7 and 0.6), which suggests the possibility of application of such filters for separation of helium from natural gas.
In General, [4] has shown the possibility of application of ceramics created by the authors for gas separation, however, used synthesis process is very long and does not allow to set and control the filtration and separation properties of filters. Therefore, the aim of the new study was to determine the possibility of obtaining ceramics with desired filtration and separation properties using spark plasma sintering (SPS), which was previously successfully applied for obtaining of dense (non-porous) ceramics [5].
The method of obtaining nanoporous ceramics
The ceramics was created on the basis of the Tarkosil silicon dioxide nanosized powder with a specific surface area of 140 m2/g and an average size of primary particles of about 20 nm. The nano-powder was obtained in Khristianovich Institute of Theoretical and Applied Mechanics and Budker Institute of Nuclear Physics by evaporation of pure oxide quartz under the influence of beam of high-energy electrons created by an electron accelerator, after which the saturated vapors of the substance were rapidly cooled in a stream and coagulated, forming agglomerates of solid nano-particles [6].
As in [5], spark plasma sintering was performed at the Lavrent'ev Institute of Hydrodynamics using Sinter Land Labox 1575 facility. The main difference of SPS in comparison with traditional pressing in powder metallurgy is an application of electrical impulses directly to the sample, which facilitates rapid heating and preservation of the microstructural parameters of the powder in the consolidated material [7-12].
In [5] to obtain of dense ceramics from Tarkosil pressing at heating rate 50°C/min with a maximum temperature of 1000, 1100 and 1200°C and a maximum pressure of 50 Pa was carried out. To obtain a porous ceramics the nano-powder sintered without pressing, using different amounts of powder in the same volume of the mold to obtain samples of different density and porosity. The heating rate for all samples was 60°C/min with a maximum temperature of 900 and 1000°C. At such temperatures, the samples were aged for 1 minute, and then the current was turned off.
The specific surface area of the obtained samples was measured by adsorption and desorption of the gas with the use of automatic instrument Sorbi-M: the sample was placed in liquid nitrogen, through which a gaseous mixture of nitrogen and helium was passed, the output composition of the gas was measured, and then the sample was heated to release nitrogen bound to the surface nitrogen, and the gas composition was measured again.
Study of microhardness obtained porous ceramics was carried out by means of microdurometer PMT-3.
The study of samples
of nanoporous ceramics
Of particular interest was the comparison of the filtration characteristics of ceramic filters, obtained at the same temperature, but at significantly different porosity of the samples, and the comparison of these data with the results of [4], which used a different method for the synthesis of ceramics. In all the graphs below, the experimental points superimposed on the dependencies documented in [4].
In the study of samples of ceramics, their specific surface area SS was first measured. When the sintering temperature of 1000°C for the sample with a porosity of 48%, it was equal to 108 m2/g, while for the sample with a porosity of 70% – 119 m2/g. In fig.1 these experimental data are compared with the dependence of the specific surface of ceramics on the maximum sintering temperature TС [4]. It is evident that the difference in the values of SS for both samples and their difference from data of [4] are small, and closer to this dependence is the characteristic of the sample with a porosity of 48%.
Fig.2 shows the dependence of the volumetric porosity m of samples of ceramics on the sintering temperature TС. The porosity was determined by the ratio of weight of dry and wet (aged in water for about 10 minutes) samples. The value m=0.48 for the first sample is close to the value m=0.45 for 1000°C [4], and a value of m=0.70 is significantly different. The average density of the first tablet-filter (m=0.48) amounted to 1.17 g/cm3, and the second (m=0.70) – 0.79 g/cm3.
Fig.3 shows the dependence of TC on approximate size (diameter) of the pores in the samples of ceramics, calculated from the values of specific surface area and porosity (fig.1 and 2) with a rough assumption of the presence in the samples of only uniformly distributed cylindrical pores. Fig.3 shows that the value d=18 nm for the first sample (m=0.48) is close to the value d=15 nm for 1000°C [4], and a value of d=30 nm for the second sample (m=0.70) differs significantly.
This was followed by a gas-dynamic tests of the samples of ceramics for filtration of helium and nitrogen. Purity of both gases exceed 99.9%. Diameter of the tablets of tested filters was approximately 10 mm, the thickness ranged from 1.8 to 2.3 mm. The following scheme of tests was used: working gas under pressure of 3 atm was continuously supplying to one side of the tablet filter, the gas, which passed through the filter, filled the control capacity and the time of filling was fixed. The experiments were conducted for control volumes 15 and 300 cm3.
Fig.4 shows the dependence of relative time of filling t of control volume on the pore size for nitrogen and helium. Dotted line shows the value of 2.65 corresponding to the relative time for this pair of gases when the flow through the major channels (the experiment was conducted for the channel with a diameter of 0.7 mm). The graph shows that with decreasing pore size (at least when it is less than 20 nm) relative time of the filling of control volume grows. This is because helium atoms, having a radius of about 0.12 nm, slow down in the pores of small size smaller than larger molecules of nitrogen, having a radius of about 0.18 to 0.19 nm.
Fig.4 shows that the value of t=2.75 for the first sample (m=0.48) is close to the value of d=2.7 for 1000°C in [4]. At the same time, it exceeds the value of 2.65 for large channels. This suggests the possibility of partial separation of gases from mixtures using the obtained ceramics, in particular, from a mixture of helium and nitrogen, or, for example, of separation of helium from natural gas consisting mainly of methane.
Microhardness obtained porous ceramics was quite high – 4-6 GPa.
Fig.1-4 shows that characteristics of the first sample (m=0.48) are similar to porous ceramics obtained in [4], and the second sample (m=0,70) is very different. It can be concluded that by using the SPS method it is possible to produce in a relatively short time the ceramics samples with predetermined and controllable characteristics (porosity and pore size) and, respectively, specified coefficients of the separation and enrichment of gases.
Thus, the conducted researches included approbation of producing of ceramics with open porosity using Tarkosil silicon dioxide nano-powder and method of spark plasma sintering, the study of the possibilities and advantages of this method compared to the traditional sintering of ceramics, the study of the filtration properties of the obtained ceramics for helium and nitrogen, determination of specific surface area, porosity, relative time of the filling of control volume and microhardness of obtained filters, allow to draw a conclusion about the possibility of obtaining using SPS of ceramics with desired filtration and separation properties.
The study is done in the framework of the project of fundamental research of SB RAS III.23.4.1.
Membrane methods are environmentally friendly and most cost-effective in comparison with competing technologies for the separation of substances. For example, helium in the industry is produced from natural and petroleum gases, where its concentration is very low, therefore cryogenic methods are ineffective. The use of membrane methods for producing helium concentrate can significantly improve process economics.
The most important characteristic of porous solids is porosity, defined as the proportion of body volume attributable to pores, or their volume per volume unit of the material. Usually the closed isolated cavities are ignored and only the interconnected flow-through pores, which form a complex branched and irregular network (a pore space) are considered. The porosity of most materials is in the range of 0.1–0.4.
If the membrane has a relatively large pores, the separation occurs mainly due to differences in molecular masses of the components. When the mean free path of gas molecules is much larger than the diameter of the pores (effuse, Knudsen flow), the separation factor is a power-law function of the ratio of their molecular masses. In membranes with small pore size, there are other mechanisms of separation, including an important role of the interaction of components with the walls of the pores in the membrane (sieve effect, adsorption).
For gas separation membrane from silicones, tetrafluoroethylene, polyetherimides, cellulose acetate, ceramic, glass are used [1]. For the membrane separation of helium and methane typically membrane of organic-silicon composite or cellulose acetate are used (for example, [2]). For the industrial separation of such gas mixtures as CO2/CH4, CO2/N2 and
O2/N2, recently often ceramic membranes are used. For example, in [3] a mixture of CO2/N2 were separated using nanoporous ceramics of silicon dioxide.
In [4] on the basis of the Tarkosil silicon dioxide nano-powder ceramics with open porosity was created, which can be used for filtration and separation of gases, in particular, for the enrichment of the mixture with helium. Ceramics was produced by the following method: the powder was moistened with distilled water and was dried within 2 months at room temperature. The result is a pretty solid porous workpieces, which were baked at a maximum temperature of 600 to 1100ºC. To avoid the destruction of the samples at the removal of bound water was used a low heating rate of 4ºC/min. After 180 minute exposure at a maximum temperature, the furnace was turned off and cooled down to room temperature for 12 hours. The study of filtration and separation properties of the filters obtained from such ceramics showed the possibility of enrichment of helium in its mixtures with nitrogen and methane (enrichment ratio of 0.7 and 0.6), which suggests the possibility of application of such filters for separation of helium from natural gas.
In General, [4] has shown the possibility of application of ceramics created by the authors for gas separation, however, used synthesis process is very long and does not allow to set and control the filtration and separation properties of filters. Therefore, the aim of the new study was to determine the possibility of obtaining ceramics with desired filtration and separation properties using spark plasma sintering (SPS), which was previously successfully applied for obtaining of dense (non-porous) ceramics [5].
The method of obtaining nanoporous ceramics
The ceramics was created on the basis of the Tarkosil silicon dioxide nanosized powder with a specific surface area of 140 m2/g and an average size of primary particles of about 20 nm. The nano-powder was obtained in Khristianovich Institute of Theoretical and Applied Mechanics and Budker Institute of Nuclear Physics by evaporation of pure oxide quartz under the influence of beam of high-energy electrons created by an electron accelerator, after which the saturated vapors of the substance were rapidly cooled in a stream and coagulated, forming agglomerates of solid nano-particles [6].
As in [5], spark plasma sintering was performed at the Lavrent'ev Institute of Hydrodynamics using Sinter Land Labox 1575 facility. The main difference of SPS in comparison with traditional pressing in powder metallurgy is an application of electrical impulses directly to the sample, which facilitates rapid heating and preservation of the microstructural parameters of the powder in the consolidated material [7-12].
In [5] to obtain of dense ceramics from Tarkosil pressing at heating rate 50°C/min with a maximum temperature of 1000, 1100 and 1200°C and a maximum pressure of 50 Pa was carried out. To obtain a porous ceramics the nano-powder sintered without pressing, using different amounts of powder in the same volume of the mold to obtain samples of different density and porosity. The heating rate for all samples was 60°C/min with a maximum temperature of 900 and 1000°C. At such temperatures, the samples were aged for 1 minute, and then the current was turned off.
The specific surface area of the obtained samples was measured by adsorption and desorption of the gas with the use of automatic instrument Sorbi-M: the sample was placed in liquid nitrogen, through which a gaseous mixture of nitrogen and helium was passed, the output composition of the gas was measured, and then the sample was heated to release nitrogen bound to the surface nitrogen, and the gas composition was measured again.
Study of microhardness obtained porous ceramics was carried out by means of microdurometer PMT-3.
The study of samples
of nanoporous ceramics
Of particular interest was the comparison of the filtration characteristics of ceramic filters, obtained at the same temperature, but at significantly different porosity of the samples, and the comparison of these data with the results of [4], which used a different method for the synthesis of ceramics. In all the graphs below, the experimental points superimposed on the dependencies documented in [4].
In the study of samples of ceramics, their specific surface area SS was first measured. When the sintering temperature of 1000°C for the sample with a porosity of 48%, it was equal to 108 m2/g, while for the sample with a porosity of 70% – 119 m2/g. In fig.1 these experimental data are compared with the dependence of the specific surface of ceramics on the maximum sintering temperature TС [4]. It is evident that the difference in the values of SS for both samples and their difference from data of [4] are small, and closer to this dependence is the characteristic of the sample with a porosity of 48%.
Fig.2 shows the dependence of the volumetric porosity m of samples of ceramics on the sintering temperature TС. The porosity was determined by the ratio of weight of dry and wet (aged in water for about 10 minutes) samples. The value m=0.48 for the first sample is close to the value m=0.45 for 1000°C [4], and a value of m=0.70 is significantly different. The average density of the first tablet-filter (m=0.48) amounted to 1.17 g/cm3, and the second (m=0.70) – 0.79 g/cm3.
Fig.3 shows the dependence of TC on approximate size (diameter) of the pores in the samples of ceramics, calculated from the values of specific surface area and porosity (fig.1 and 2) with a rough assumption of the presence in the samples of only uniformly distributed cylindrical pores. Fig.3 shows that the value d=18 nm for the first sample (m=0.48) is close to the value d=15 nm for 1000°C [4], and a value of d=30 nm for the second sample (m=0.70) differs significantly.
This was followed by a gas-dynamic tests of the samples of ceramics for filtration of helium and nitrogen. Purity of both gases exceed 99.9%. Diameter of the tablets of tested filters was approximately 10 mm, the thickness ranged from 1.8 to 2.3 mm. The following scheme of tests was used: working gas under pressure of 3 atm was continuously supplying to one side of the tablet filter, the gas, which passed through the filter, filled the control capacity and the time of filling was fixed. The experiments were conducted for control volumes 15 and 300 cm3.
Fig.4 shows the dependence of relative time of filling t of control volume on the pore size for nitrogen and helium. Dotted line shows the value of 2.65 corresponding to the relative time for this pair of gases when the flow through the major channels (the experiment was conducted for the channel with a diameter of 0.7 mm). The graph shows that with decreasing pore size (at least when it is less than 20 nm) relative time of the filling of control volume grows. This is because helium atoms, having a radius of about 0.12 nm, slow down in the pores of small size smaller than larger molecules of nitrogen, having a radius of about 0.18 to 0.19 nm.
Fig.4 shows that the value of t=2.75 for the first sample (m=0.48) is close to the value of d=2.7 for 1000°C in [4]. At the same time, it exceeds the value of 2.65 for large channels. This suggests the possibility of partial separation of gases from mixtures using the obtained ceramics, in particular, from a mixture of helium and nitrogen, or, for example, of separation of helium from natural gas consisting mainly of methane.
Microhardness obtained porous ceramics was quite high – 4-6 GPa.
Fig.1-4 shows that characteristics of the first sample (m=0.48) are similar to porous ceramics obtained in [4], and the second sample (m=0,70) is very different. It can be concluded that by using the SPS method it is possible to produce in a relatively short time the ceramics samples with predetermined and controllable characteristics (porosity and pore size) and, respectively, specified coefficients of the separation and enrichment of gases.
Thus, the conducted researches included approbation of producing of ceramics with open porosity using Tarkosil silicon dioxide nano-powder and method of spark plasma sintering, the study of the possibilities and advantages of this method compared to the traditional sintering of ceramics, the study of the filtration properties of the obtained ceramics for helium and nitrogen, determination of specific surface area, porosity, relative time of the filling of control volume and microhardness of obtained filters, allow to draw a conclusion about the possibility of obtaining using SPS of ceramics with desired filtration and separation properties.
The study is done in the framework of the project of fundamental research of SB RAS III.23.4.1.
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