WO2019209226A2

WO2019209226A2 - Polyimide matrix based multi functional neutron shielding materials and production method

Info

Publication number: WO2019209226A2
Application number: PCT/TR2018/050914
Authority: WO
Inventors: Oktay BAYKARA; Mahmut DOĞRU
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-12-27
Filing date: 2018-12-27
Publication date: 2019-10-31
Anticipated expiration: 2020-06-27
Also published as: WO2019209226A3

Abstract

The invention relates to the development of polyimide-based multifunctional neutron shielding nanocomposite material that have better mechanical properties and stability under thermal pressure at elevated temperatures (300 ° C). Nano-sized (80-100 nm) h-BN and Gd2O3 nanoparticles have been added to the polyimide (PI) matrix in different weight percentage to effectively absorb both neutron and gamma radiations. The mechanical and thermal properties of shielding materials containing different ratios of nanoparticles were determined. They were also exposed to neutron and gamma radiation to determine their radiation absorption ability. Hereby, in general and most importantly, optimum filling ratios and material thicknesses of the shielding material produced had been determined to achieve the highest shielding efficiency to neutron and gamma radiation shielding as high as possible. As a result of the experiments, it was found that the mechanical and thermal properties of the shielding materials were improved and the best neutron shielding (about 100%) was realized at 7wt% and 11wt% h-BN and 3wt% Gd2O3 filling rates. By adding Gd2O3 to the neutron absorber materials, it has been increased gamma absorption ability of polyimide-based materials manufactured in the range of 10-50%.

Description

POLYIMIDE MATRIX BASED MULTI FUNCTIONAL NEUTRON SHIELDING

MATERIALS AND PRODUCTION METHOD TECHNICAL FIELD

The invention relates to the development of polyimide-based multifunctional neutron shielding nanocomposite material that have better mechanical properties and stability under thermal pressure at elevated temperatures (-300 ° C). PRIOR ART

The application areas of nuclear technology are developing rapidly and with this development, the need for effective radiation shielding materials is increasing day by day. However, the shielding materials required should be diversified according to the type of radiation absorbed and its application area. High-performance radiation protection materials are needed in most of nuclear applications areas. The structure of the radiation shielding materials used shows alternation according to the type of radiation exposed and areas of use mentioned such as nuclear reactors, safety of nuclear waste, nuclear applications in health, aerospace technology etc. As a result, radiation-shielding materials must to function in environments where has increasingly difficult conditions (e.g. temperature, pressure, high radiation, etc.). So, in addition to the effective radiation absorption capabilities of the shield materials, it need to meet the required standards for material properties such as mechanical, thermal resistance, radiation damage, etc. as well. The used existing conventional shielding materials do not have the required structural features for using in harsh environments conditions. To overcome the problems encountered, new and advanced neutron shielding materials need to be explored and developed that have high thermal stability, excellent mechanical and most important radiation absorption properties. Besides, studies in this context have shown a large increase in recent years.

As well as conventional neutron shielding materials (e.g. concrete, water, neat polyethylene etc.), polyethylene and polyethylene-based micro and nano composites that are doped various micro and nano additives (e.g. Pb, Sm, B, Gd vb.) are used as shield materials in neutron shielding. However, the setting temperature of used polyethylene/polyethylene composites is limited to about 100 °C and also, polyethylene composites have very poor tensile strength and difficulties in using under high pressure and temperature. At the same time, good shield materials that used in neutron shielding should also have the ability to absorb gamma radiation. Because, the highly energetic gamma rays with energies in a very wide energy range (keV-MeV), named as accompanying radiations, occurs in neutron-matter interactions and the neutron absorber material used must be effectively absorbed/shielded against the accompanying or secondary gamma rays. The plastic-based neutron shield matters currently used are insensitive to these gamma radiations or have very small mass attenuation coefficient in high energies. Moreover, polyethylene-based neutron absorbers cannot function for a long time due to their weak resistance to neutron and gamma rays damage. High-density polyethylene has been used to overcome mentioned problems, but improvement in temperature and pressure performance was limited. Otherwise, metal-based composites have been produced and tried to overcome mentioned problems, but the relatively complex production process have caused the high production costs. As a result, detailed studies on the use of very different materials carry on without slowing for the production of new generation and multifunctional neutron shielding materials that can functions in all kinds of harsh environments.

Generally, the main material used in the design and development of hybrid neutron shielding materials were polymer and its derivatives thanks to the rapid development of polymer technology between 1960 and 1990,.

BRIEF DESCRIPTION OF THE INVENTION

The invention relates to manufacturing and developing of a new effective neutron shielding material, multi-functional and next-generation. For this purpose, the Polyimide (PI), is known to have stable mechanical and thermal properties under thermal pressure at high temperatures (-300 °C), was used as the main matrix. By doping of the nano-size (-80-100 nm) and different rates of h-BN and Gd203 additive materials to Polyimide matrix, ternary h-BN/Gd203/PI nanocomposites were manufactured. FIGURES LIST

Figure 1. Image of h-BN SEM

Figure 2. Image of h-BN SEM

Figure 3. Image of h-BN SEM

Figure 4. Image of h-BN TEM

Figure 5. Image of h-BN TEM

Figure 6. The EDX results of h-BN nano particles

Figure 7. Image of GCI2O3 SEM

Figure 8. Image of GCI2O3 SEM

Figure 9. Image of GchCb TEM

Figure 10. Image of GCI2O3 TEM

FigureH . Schematic view of the experimental setup used in neutron transmission experiments

DETAILED DESCRIPTION OF THE INVENTION

Polyimide (PI), a class of polymers, is known to have stable mechanical and thermal properties at high temperatures. In the invention, polyimide having high heat, mechanical and radiation resistance produced by polymer technology has been used. In addition, it has also high wear resistance, high stability under vacuum, anti-radiation and solvent resistance. The effects of the radiation damage that may occur the through neutron-matter (matrix and additives) in shield material in process of usage will be minimum with the use of polyimide in neutron shielding. In accordance with this information, a new generation multi-functional neutron shielding materials were developed. This h-BN/Gd203/PI nanocomposite having ternary structure was produced by adding different percentages h-BN and Gd203 nano-particles to polyimide main matrix. In the neutron absorption tests performed, it is observed that the ratios of shielding of neutron and gamma radiations are approximately 90-90% and 10-50%, respectively, in some additive rates and certain thickness. The h-BN and Gd203 nano particles that used in invention have been manufactured in our laboratory and improved shielding materials, which can be capable of effective absorption against neutron and gamma radiation, were fabricated by adding certain amounts of these nanoparticles to the polyimide matrix.

Characterization of Boron Nitride Nano Platelet (Nano Disk, h-BN)

The images of SEM, TEM, EDX results and the XRD peaks of the h-BN nano platelets (nano-disc) produced by arc-discharge were given Figure1 -5, Table 1 and Table 2, respectively. The images of SEM of h-BN nano particles were shown in Figurel -3 and the images of TEM were shown Figure 4 and 5. The Figure 6 has shown EDX image. When the images in Figures 1-6 are examined, it was observed that the nanoparticles consisted of nano- platelets and nano-rods of different sizes. From the SEM images, it was seen that the thickness and diameter of h-BN were changing 20 to 80 nm and 50 to 500 nm, respectively, and this nano particles have different morphology structure. It was rarely seen that some of the h-BN nanoparticles were in nano rods of different length and diameter possible due to the fluctuation in voltage during arc-discharge process.

Table 1. The EDX results of h-BN nano particles

Spectrum: Objects

Element Series unn . C norm. C Atom. C Error

[wt.%] [wt.%] [at.%] [%]

Nitrogen K-series 76.45 76.45 71.47 23.9

Boron K-series 23.55 23.55 28.53 7.5

Table 2. The XRD peaks of h-BN nano particles The according to EDX results, the weight percentages of B end N are 23,55 and 76,45, respectively. The crystallographic characteristics of the BN nano platelets synthesized by the arc discharge set-up were specified via XRD (X-ray diffraction) technique and the obtained this spectrum is given in Figure 3. When considering to XRD measurements, the watched characteristic is compatible with JCPDS (Card no.34-0421 ) and this is a sign that the structure obtained has hexagonal boron nitride structure. As that graph shows, the strongest peak is 27° which corresponds to (002) the lattice structure. The other peaks (001 ) and (101 ) are in lattice structure corresponding 42° and 45°, respectively, the peak of 30° belongs to the Ni particles that act as catalysts.

Characterization of Gadolinium Oxide (Gd203) Nano Particles

The micro-size and powder form of gadolinium oxide was used as electrode during production with arc-discharge whist the deionized water was utilized as medium. De-ionized water decomposing into hydrogen and oxygen pass to ion phase during the arc, the Gd and O atoms in ion phases combine to forming Gd203 compound and ultimately, in the nano-size (-20-100 nm) Gd203 particles are formed in the medium. Thus, the production of Gd203 in nano size had been performed. The results of SEM, TEM and XRD used to determining the properties of the Gd203 produced are shown in Figure 7, 8, 9, 10.

In SEM and TEM images, it seen that Gd203 nanoparticles are in various forms.

The irregularly dispersed Gd203 has a spherical structure and some of them are spherical, some are in the form of thick bars. The size and shape of the nanoparticles are dependent on the arc current and the dimensions of this nanostructure are changing between 20 and 100 nanometers. XRD peaks of Gd203 nanoparticles produced and studied in the literature (10-100nm) are shown in Tables 3 and 4, respectively.

According to the Gd203 XRD peaks compared with the literature in Table 3 and 4, it can be said that the nano structures of Gd203 produced is cubic (a=b=c=10.82311 A°). The lattice structures and peaks are (221 ), (222) and (400)... and 20=21 °, 28°, 33° ve 39°, respectively. Finally, Raman analysis of Gd203 nanoparticle produced by arc discharge and Raman peaks in literature are given in Table 5 and 6. At the end of the analysis, marked major peaks of 362, 445 and 570 pertain to Gd203. Similar major peaks have been found in the literature.

Table 3. The XRD peaks of produced Gd203 nano particles

Table 4: The XRD peaks of GCI2O3 nano particles in literature

Table 5: The Raman results of Gd203 fabricated

Table 6: The Raman results of Gd203 in the literature

Composite Production and Mechanical Testing The composites were mixed in a double-screw extruder (Xplore 15cc micro- compounder) at a speed of 80 rpm and a temperature of 180 °C for 5 minutes At the end of the mixing time, the melt was transferred directly to the injection molding machine at 10 bar pressure and pressed from 180 °C to 25 °C. In the first stage, the modified h-BN composites were produced with different concentrations 1 %, 5% and 10% (by mass) in h-BN/PI composites. In the second stage, Gd203/PI double composites and in the third stage nano-composites were prepared by adding nano- sized BN, B₄C and Gd203 in different proportions inside the PI main matrix. In the last step, in order to improve the interfacial bonding, 1 %, 2%, 3% PI-g-MAH and 3%, 5% and 7% h-BN composites were prepared. h-BN/Gd203/PI Characterization of Nano Composites.

Table 7 shows the change in tensile strength of h-BN/Gd203/Polyimide composites according to the type and amount of the fillers. The tensile strength of the particle-reinforced composites depends on the particle loading rate and the level of adhesion between the particle and the matrix. If the adhesion is strong, the stress applied to the composite during the tensile test is transferred from the matrix to the particle and rupture occurs at higher stresses and later. However, if the adhesion is not sufficient, the matrix and the surrounding medium are separated before the yield strength of the matrix is reached during the tensile test, and therefore, low stresses and rupture occur at an early stage. In Table 7, the tensile strength of the pure polyimide (PI) was measured about 76 MPa. When the pure polyimide nano Gd203 and h-BN are added together in the range of 1 % to 11 %, changes in tensile strength first increasing and then in a decreasing trend are observed. The highest tensile strength values are observed in 3% BN independently of Gd203 ratio. With the increasing in BN, the tensile strength values gradually decrease and it is even lower than the tensile strength of pure PI at the h-BN ratio of 11 %. Although no covalent interaction is expected between polyimide and nano-additives used in the composites, weak physical interactions may be an important factor because of the large surface area of nano-additives. With increasing BN ratio, possible agglomeration decreases the surface area and the tensile strength decreases after this point.

Table 8 shows the values of the elongation at break in h-BN/Gd203/PI composites according to the type and amount of the fillers. As a general behavior, when a rigid particle is added to a ductile matrix, it is seen that the composites are broken around the pour point of the matrix, especially due to defects occurring at the interface under the tensile load. This point can be delayed by interface modification. The elongation at break of PI is about of 16.5% and this value gradually decreases with the addition of fillers to the PI. With the increases in h-BN, the rate of decrease in the elongation at break is increasing. Table 9 shows the impact strength of h-BN/ Gd203/Polyimide composites with respect to the type and amount of the fillers. Although impact strength in particle-reinforced composites depends on a complex mechanism, in summary, if there is a good bond between the particle and the matrix or the particle has a homogeneous distribution, the impact resistance generally increases When BN is added at a rate of 3%, independent of Gd203, the values of impact strength are increased by 15%, but decreases at higher loading rates compared to PI with an average impact of 16.2 kJ / m² This can be attributed to the fact that the distribution of nano-additives, as in the case of tensile strength, is more uniform at lower loading rates. The increase in the loading rate can be related to the increase in the size of the dispersed fraction depending agglomeration.

Table 7. The values of tensile strength of h-BN/ Gd203/PI composites according to fill type and quantity

Table 8. The values of the elongation at break in h-BN/GcteOs/PI composites according to the type and amount of fillers

Table 9. The values of impact strength in h-BN/Gd203/PI composites according to fillers type and quantity The hardness changes according to filling type and quantity in h-BN/ Gd203/Polyimide composites was given in Table 9. The hardness in the composites is an increasing physical property along with the increase in the amount of filler. The reason is that the reduced ability of the polymer chains to move in the fillers. The increase in hardness was obtained at a concentration of 3% BN and it reached the maximum value with an increasing in the amount of Gd203 at this concentration. The increase in hardness was obtained at 3% BN concentration. This concentration increases with the amount of Gd203 reaches the maximum value of hardness. Thermal Properties of h-BN/GcbOs/PI Nano Composites

As can be seen in Table 10, the pure polyimide is amorphous and the glass transition temperature (Tg) starts at 169°Cto 177 °C. In addition, all composites in Table 10 are amorphous and only glassy transition temperature of the polyimide phase was observed in these composites. Although there are slight increases in glass transition temperature with the addition of fillers to polyimide, these increases can be considered as experimental errors in experimental error limits of DSC analysis. In other words, an increase in the glass transition temperature was not observed with the addition of reinforcements to the PI. This is due to the fact that there is no chemical compatibility of the reinforcements used with PI. In fact, this finding is compatible with the mechanical test results. The glass transition temperature in the polymers is one of the important factors determining the utilization area of the polymer. The polymers acts as soft and fluid characteristic over the glass transition temperature and below this temperature the polymer is glassy and hard. As can be seen in Table 11 , it has a pure polyimide amorphous character and a glass transition temperature (Tg) of 177°C. The addition of h-BN and/or Gd203 nano-particles to polyimide does not affect the glass transition temperature of polyimide within the experimental error limits. In other words, the high Tg value of PI is not influenced by the addition of inorganic fillers into PI and so, PI composites remains a potential material for high temperature applications such as pure PI.

Table 10. DSC thermograms of Pure PI and selected composites

Table 11. The values of glass transition temperature of h-BN/Gd203/Polyimide composites according to fillers type and quantity * Sample name PB3G1 : composite containing 3% BN and 1 % Gd203 Determination of Neutron and Gamma Absorption Characterizations of Nanocomposites

Neutron Absorption Tests Nano composites containing nanoparticles (h-BN and Gd203) with different percentages in the polyimide matrix, in order to determine the shielding characteristics against neutron and gamma irradiation, they were produced in thickness of 1 cm and diameter of 2,5 cm.

Neutron absorption/shielding experiments were performed using Howitzer containing ²³⁹Pu-Be neutron source having 2 Ci activity. The average energy of the neutrons released from the neutron source is about 4.5 MeV and the neutron flux is approximately 10⁶ ncnr²s^·1.

The neutron absorption/permeability characteristics of the nano composites produced were determined by measuring neutron permeability (l/lo) of composites in different thickness (in the range of 1-6 cm). Due to the nature of the neutron radiation, the slow response of the detector used in the experiments (Targetinside Identifinder Detector containing He-3 gas) and the fluctuations in the measurement results taken by regular intervals, duration of the neutron interaction (exposure) with the absorber were taken as 180 seconds (3 minutes) for each thickness. In order to obtain data more effectively in the absorption experiments and make analyzes of the results, the experimental processes were recorded with the video shooting. Thus, it was tried to determine how the neutron count (cps) and gamma dose rate (pR/h) in the detector changed every three minutes (180 seconds) and how this change occurred. Initially each of sample is exposed with neutrons (I, measuring) and then the neutron measurements have been continuously taken without any sample or shield between source and detector (lo, measuring). Nevertheless, before starting the experiment the bacground neutron and gamma dose levels (BKGR) from Pu-Be neutron source were also determined and these values were used to determe the net neutron flux and gamma dose amounts (Net count = count-BKGR). The detector used in the experiment has the ability that simultaneously detect both neutron (cps, particle/second) and gamma radiation (pR/hour) (the value of both radiation on the screen are shown). By means of this detector, the gamma rays of high energy that may occur with the interaction of thermal neutron with gadolinium existing in nanocomposites produced for the neutron shielding and subsequent possibility of increase in the gamma dose level had been observed during the experiments. No instantaneous increase in the measured gamma dose has been observed during the experiments performed. In the determination of the neutron absorbing abilities (thermal neutron capture) of the nano- composites fabricated, the following equation was used.

Where, /₀ and /(x) are respectively the intesity of the neutron from the source and transmitted through the absorber material, x is thickness of shielding material and å is total macroscopic neutron absorption cross-section (cm ¹).

Gamma Absorption Experiments

The gamma shielding characteristic of nano-composites manufactured with different percentages has been tried to determine in different gamma ray energies. The specifications of gamma shielding experiment such as the type of radioisotope, its gamma ray energies and exposure time of neutron flux are given in Table 12. Depending on the different gamma energies, the mass absorption coefficients were calculated using by Equation 2.

Where, /₀, for incident gamma rays, is intensity of gamma rays without shield material between detector and gamma source, /, for transmitted gamma rays, is the intensity of the gamma ray passing through the absorber material, p and t are density and thickness of gamma absorber material, respectively.

Table 12. The used radioisotopes and their energies

The detector used in neutron absorption experiments has a dual function which measures both neutron (cps) and gamma radiation (pR/h). While the neutron count was done in the experiments, the gamma radiation dose level in the set of surroundings was also measured. Thus, while the newly developed shielding material working in the real environment, absorption properties of that materials were also investigated the against of gamma radiations that exist in the environment (e.g. laboratory) and produced by neutron-matter interaction.

The neutron shielding properties of h-BN/Gd203/PI nano composites

Using polyimide (PI) as the matrix, polyimide based nano composites containing different percentage of h-BN and Gd203 were produced. The neutron transmission curves for (3 %-7%-11 % h-BN+1 %-2%-3% Gd203)/PI nano composites manufactured are shown in Tables 12-14. The first of these is (3%-7%-11 % h-BN+1 % Gd203)/PI and Table 13 shows the changes in the neutron permeability curves of these nano composites

Thickness of material (cm)

Tablo 13. The neutron transmission curves of (3%-7%-11 % h-BN+1 % Gd203)/PI nano composites In Table 13, although the pure polyimide has an exponentially decreasing and non-fluctuating neutron transmission curve, the composite doped of 3% BN has more stable and the transmission curves of composites doped of 7% and 11 % BN are decreasing to a certain thickness and then, they exhibited an increasing feature. While the neutron shielding of samples containing 3% and 11 % BN is better, the composite containing 7% BN showed poor performance. The whilst neutron absorption for pure polyimide occurs at 45%, this value for nano composites including 3% and 11 % BN reached at 60% in some thicknesses.

The neutron transmission curves of nano composites doped of 2% Gd203 are shown in Table 14. Although the transmission curves of each three nano composites show a fluctuation characteristic, in general, the neutron shielding ability of composite including 7% and 11 % BN have better and the shielding of neutrons was observed varying from 60% to 80% in particular thicknesses. As the thickness increases for all three shielding materials (>5 cm), the neutron permeability has increased.

The neutron transmission curves for (3%-7%-11 % h-BN+3% Gd203)/PI nano composites manufactured are shown in Table 15. It is seen that compared to 1 % and 2% Gd203 doped samples, clearer absorption curves are obtained in filler content of 3% Gd203. The samples including of 3% h-BN exhibit a more stable property, while 7% and 1 1 % BN doped samples show increasing and decreasing properties after a certain thickness. In the meantime, 1 1 % BN doped sample has been seen to absorb neutrons at an average level of 90-95% in thiskness of 3 cm, while the sample contaning 7% BN absorbs neutrons around an average of 70-75% in 4-6 cm thicknesses.

Table 14. The neutron transmission curves of (3%-7%-1 1 % h-BN+2% Gd203)/PI nano composites

Thickness of material (cm) Tablo 15. The neutron transmission curves of (3%-7%-1 1 % h-BN+3% Gd203)/PI nano composites As h-BN content in nanocomposites was fixed, how change the neutron absorption properties of nanomaterials produced were examined and neutron transmission of the nano composites depending on the amount of doped nano Gd203 is given in Table 16 to Table 18. It was clearly seen that, in Table 16, the same percentage of h-BN and Gd203 filler has a more stable transmission curve, different rates of neutron absorption occurred in 1 cm for three samples, but they have roughly the same transmission percentages while the increasing of thickness (except for sample doped of 2% Gd203 at 5 cm).

Table 16. The neutron transmission curves of composites doped 3% BN depending on Gd203 filler content

Table 17. The neutron transmission curves of composites doped 7% BN depending on Gd203 filler content It is observed that in Table 17, the sample doped 1 % Gd203 has a very poor absorption property, while the transmission curve of sample doped 2% Gd203 is more stable and finally, the charcteristic curve of last sampe (3% Gd203) is fluctuating but has better neutron absorption. After 2 cm, the transmission curve of the sample of 2% Gd203 has approached to the pure polyimide and the sample doped of 3% Gd203 has found to have a decreasing transmission curve as the increasing of thickness and the absorption reached up 100% in thicknes of 4 and 6 cm.

Table 18. The neutron transmission curves of composites doped 11 % BN depending on Gd203 filler content

The neutron transmission of nano composites containing 11 % BN based on the amount of doped Gd203 is given in Table 18. The transmission of sample of 1 % Gd203 increased after 2 cm and approaches to pure polyimide. It can seen that in Table 18, the samples, which contanied 2% and 3% Gd203, exhibited similar neutron permeability till 3 cm and shielding efficiency of nanocomposites doped of both 2% Gd203 and 3% Gd203 have reached up 90% and approximately 100% at thickness of 5 and 3 cm, respectively. As a result, it is evident from the above graphs that nano composites exhibit very different neutron permeability/absorption properties depending on the nanoparticle content and its thickness. When considering all of the neutron absorption experiments, variation of the shielding efficiency changes according to the filler ratios. The best transmission results were obtained in nano composite that have nano fillers content of 11wt% BN and 3wt% Gd203 with low (3 cm) material thickness. In order to mechanical and thermal properties of the produced material to be optimum interval, the total amount of nanoparticles in polymer matrix must not be high. The optimum BN ratio was 1 1 wt% and 7wt% in this study. Besides, the content of Gd203 that added in nanocomposite for absorbing gamma radiation should be as the lowest as possible. The using higher content of Gd203 nanoparticle in main matrix is not suitable for both absorption of the radiation and the material properties of the nano composite produced. Because, especially, prompt gamma rays that generated from interactions with neutron and Gd atoms are a big issue in case of neutron shielding. Because of these reasons, the highest filling rate of Gd203 was taken as 3%.

In addition, macroscopic absorption cross sections of nano composites fabricated were also calculated using Eq.1 . According to the results obtained, the pure polyimide macroscopic effect cross-section (å) was to be 0.1316 cm ¹, while the macroscopic effect cross-section of samples of 1 1 % BN and 3% Gd203 added nano composite was found to be 0,4052 cm ¹. The mass neutron absorption coefficient of doped nanocomposites increased about 210% of that of neat PI (Table 19). Among the nano composites produced at different fillers ratio, the highest neutron interaction cross-section was found in the material which containing higher h-BN and Gd203 nanoparticles. As a result, it was observed that the better neutron absorption (about 100%) was performed at higher h-BN and Gd203 nanoparticle content and low material thickness.

Table 19. The variations in the neutron and gamma absorption coefficients of the nano composites produced. To determine how the changes gamma absorption properties of the nanocomposite produced against gamma radiations that exist in the system during the neutron absorption experiments vary depending on the amount of added Gd203, the gamma shielding curves that were drawn by keeping the amount of h-BN are given in Table 20 (a), (b) and (c).

20 (b)

20 (c)

Table 20. The gamma radiation transmissions of nano composites depending on

Gd₂0₃ filler content (a) 3% BN, (b) 7% BN and (c) 11 % BN

As can be easly seen in all three graphs in Table 20, the gamma absorption abilities of the nanoparticles produced depending on the Gd203 filler content have increased compared to that of the pure polyimide. While the gamma absorption percentage of pure polyimide was approximately 35%, this ratio was increased to 45% by 3% Gd203 filled content. However, the effect of percentage contribution of Gd203 on gamma absorption cannot be clearly seen in higher BN content (see Fig. 20 c). As a result, very small changes in gamma absorption were observed while increasing the content of Gd203 as seen in Table 20 (a) and (b), but very close permeability values were obtained in all three samples. Gamma Shielding Properties of h-BN/Gd203/PI Nano Composites

The mass attenuation coefficient of (3%-7%-11 % B₄C+1 %-3% Gd203)/PI nano composites containing nano particles depending on different gamma radiation energy are shown in Table 21 (a), (b) and (c).

20 c

Table 21. The mass attenuation coefficient of nano composites (a) 3% BN, (b) 7%

BN and (c) 11 % BN In Table 21 , the gamma absorption coefficients of the all samples produced have generally increased as depending on the amount of Gd203 doped in the polyimide main matrix. The polyimide already had a high gamma absorption coefficient and the absorption coefficients of the samples fabricated have further increased as the addition of Gd203 nanoparticles. For example, the mass attenuaiton coefficcient of neat polyimide was found to be 0,0996 cm²g-¹ at gamma energy of 1332.5 keV, the highest mass attenuation coefficient was found to be 0.1506 cm²g ¹ in the nano composite contaning of 3% BN+3% Gd203 nano particles in this energy and the mass attenuation coefficient have increased by approximately 51 %. The mass attenuation coefficient of nanocomposite that best results in neutron shielding was found to be 0,1130 cm²g-¹. However, while the neutron absorption capability of the this nanocomposites developed was increased, the increase in gamma absorption ability (~ 14%) has remained limited. The absorption ability of plastic-based shield materials against gamma radiation is not very good especially at high energies. It was observed that the gamma absorption ability of the new neutron absorber material developed has been improved.

As a results, a poylimide-based neutron shielding material which can be used as an alternative to conventional neutron absorber materials and can effectively absorb in the gamma radiation as well as neutrons has been developed.

Claims

1. Polyimide based multifunctional neutron and gamma shielding material; it is characterized in that it is composed of h-BN/Gd203/Polyimide nano composite material with thickness of 4-6 cm containing h-BN and Gd203 nano particles (sized 80 to 100 nm) in 7wt% to 11wt% and 3wt%, respectively.