KR102348835B1 - Non Pb based piezoelectric ceramics production process through the atmosphere control - Google Patents

Abstract

The present invention provides a non-lead piezoelectric ceramic manufacturing method through the atmosphere _{control, Li (Nb 0.8 Ta 0.2)} O 3 powder, Na (Nb _{0 0 .8 .2} Ta) O ₃ powder, K (Nb _{0 .8} Ta preparing a _{0 .2)} O ₃ powder, each with; mixing the powders with Li ₂ CO _{3 ;} It is a technical summary to include the step of sintering the mixed powders and the Li ₂ CO ₃ at 1000 to 1100° C. under nitrogen gas or hydrogen-nitrogen mixed gas atmosphere to obtain a piezoelectric ceramic. Accordingly, the lead-free piezoelectric ceramic composition may be sintered under nitrogen gas or nitrogen and hydrogen mixed gas atmosphere to have excellent piezoelectric properties.

Description

Method for manufacturing lead-free piezoelectric ceramics through atmosphere control {Non Pb based piezoelectric ceramics production process through the atmosphere control}

The present invention relates to a method for manufacturing a lead-free piezoelectric ceramic through atmosphere control, and more particularly, an atmosphere in which a lead-free piezoelectric ceramic composition is sintered under nitrogen gas or nitrogen and hydrogen mixed gas atmosphere to have excellent piezoelectric properties. It relates to a method for manufacturing lead-free piezoelectric ceramics through control.

With the recent development of precision machinery and information industries, piezoelectric actuators that control micro-displacements or control vibrations have been widely applied to precision optical devices, semiconductor equipment, gas flow control pumps, and valves. This is because the piezoelectric actuator has the advantages of being able to miniaturize and precisely control the piezoelectric actuator compared to the conventional mechanical driving element, and the response speed is fast. Therefore, along with the development of mechatronics, micro-displacement control parts are being converted from a method using a conventional step motor to a method using a piezoelectric actuator. Therefore, in the application of piezoelectric actuators of piezoelectric ceramics, a material that generates high displacement is required.

Among piezoelectric ceramics, perovskite lead zirconate titanate (PZT) is currently used in many applications as a piezoelectric material with the best piezoelectric properties. In _{a solid solution of PbTiO 3} and PbZrO ₃ , a PZT solid solution having a Curie temperature of 390°C while having strong piezoelectric properties at the tetragonal-trigonal phase boundary (MPB: Morphotropic Phase Boundary) was found. Effects and inverse piezoelectric effects are being actively studied to be applied to piezoelectric actuators, piezoelectric transducers, sensors, resonators, and the like.

However, since most ceramics with excellent piezoelectric properties have a composition containing lead (Pb) like PZT, PbO rapidly volatilizes above 1000° C. In order to prevent this, the piezoelectric ceramics are manufactured by adding PbO excessively to the composition. Since this causes environmental pollution as well as a problem in terms of price competitiveness, a lot of research on the composition of lead-free ceramics has been recently conducted.

Of the lead-free piezoelectric ceramic (Na _{0 .5 0 .5} K) NbO ₃ has got the characteristics such as high phase transition temperature (420 ℃), a low coercive field (5kV / cm), high remnant polarization (30μC / cm ²⁾ lead It is considered as one of the representative materials that can replace piezoelectric ceramics with a basic composition. In particular, when lithium (Li) and tantalum (Ta) ions are dissolved in _{(Na 0.5} K _0.5 )NbO _{3 , they form a phase boundary and are known to exhibit high piezoelectric properties.} In addition, it is known that it is difficult to manufacture a sintered body having high properties by a conventional sintering method due to the high hygroscopicity of raw materials such as _{Na 2} CO ₃ , K ₂ CO _{3 and the like and volatilization during sintering.} Therefore, until now, sintering has been performed using expensive manufacturing processes such as hot press and spark plasma sintering.

As a prior art for sintering piezoelectric ceramics, 'a lead-free piezoelectric ceramic composition for sensors and actuators and a method for manufacturing the same' is known in Korean Patent Office Registration No. 10-1012143. This prior art is characterized in that it is possible to manufacture the piezoelectric ceramics even at a low firing temperature by improving the problems of the high firing temperature and low piezoelectric constant of the lead-based piezoelectric ceramics. Since oxide piezoelectric ceramics containing oxygen (O) are common for piezoelectric ceramics, heat treatment is mainly performed in air containing oxygen during the manufacturing process. However, to manufacture the piezoelectric ceramic, non-lead piezoelectric ceramic, especially as in the prior art (Nb _{0 0 .8 .2} Ta) O ₃ piezoelectric ceramics containing large particles are grown to be difficult to be used as a piezoelectric material in an oxygen atmosphere disadvantages There is this.

Korean Intellectual Property Office Registered Patent No. 10-1012143

Accordingly, it is an object of the present invention to provide a method for manufacturing a lead-free piezoelectric ceramic through atmosphere control in which a lead-free piezoelectric ceramic composition is sintered in a nitrogen gas or nitrogen-hydrogen mixed gas atmosphere to have excellent piezoelectric properties.

The above object is, the Li (Nb _{0 0 .8 .2} Ta) O ₃ powder, Na (Nb _{0 0 .8 .2} Ta) O ₃ powder, K (Nb _{0 0 .8 .2} Ta) O ₃ powder manufacturing each; mixing the powders with Li ₂ CO _{3 ;} Preparation of lead-free piezoelectric ceramics through atmosphere control, characterized in that it comprises the step of sintering the mixed powders and the Li ₂ CO _{3 at 1000 to 1100° C. under a nitrogen gas or hydrogen-nitrogen mixed gas atmosphere to obtain a piezoelectric ceramic} achieved by the method.

Here, mixing the said powder and Li ₂ CO ₃ is, and the powders are mixed to scale such that (Nb _0.8 Ta _0.2) O ₃ powder _{(Na x K 0 .98-x} Li 0.02), the ( _{Na x K 0 .98-x Li} 0.02) (Nb 0.8 Ta 0.2) O 3 for powder 1mol the Li ₂ CO ₃ is preferably from 0.005 to 0.015mol mixture. (Here, x is 0.47 to 0.53.)

In addition, in the step of obtaining the piezoelectric ceramic by sintering, the heat treatment temperature is 1000 to 1050° C., the nitrogen gas is a gas in which the entire amount is nitrogen, and the nitrogen and hydrogen mixed gas is 5% or less of hydrogen gas and the remaining amount It is preferable to consist of this nitrogen gas.

After the step of sintering to obtain the piezoelectric ceramic, it is preferable to further include the step of post-heating the piezoelectric ceramic at 800 to 1000 ℃ in an oxygen gas atmosphere.

According to the above-described configuration of the present invention, the lead-free piezoelectric ceramic composition may be sintered under nitrogen gas or nitrogen and hydrogen mixed gas atmosphere to have excellent piezoelectric properties.

1 is a flowchart of a lead-free piezoelectric ceramic manufacturing method through atmosphere control according to an embodiment of the present invention;
2 is an SEM photograph showing the results of sintering the piezoelectric ceramic at various temperatures under an _{oxygen (O 2 ) atmosphere;}
3 is an SEM photograph showing the results of sintering the piezoelectric ceramic at various temperatures in an air (200O _{2 -80N 2 ) atmosphere,}
4 is a SEM photograph showing the results of sintering the piezoelectric ceramic at various temperatures under a _{nitrogen (N 2 ) atmosphere,}
5 is a SEM photograph showing the results of sintering the piezoelectric ceramic at various temperatures under a hydrogen gas mixture (5H ₂ -95N _{2 ) atmosphere;}
6 is a graph showing the piezoelectric constant according to temperature.

Hereinafter, a method for manufacturing a lead-free piezoelectric ceramic through atmosphere control according to the present invention will be described in detail with reference to the drawings.

First, the prepared Li (Nb _{0 0 .8 .2} Ta) O ₃ powder, Na (Nb _{0 0 .8 .2} Ta) O ₃ powder, K (Nb _{0 0 .8 .2} Ta) O ₃ powder, each (S1).

_{Li 2 CO 3, Nb 2 O} 5, a sample with a Ta ₂ O ₅ as starting materials and were weighed and mixed according to the proportion of O ₃ to prepare a powder composition of _{Li (Nb 0 .8 Ta 0 .2} ). _{Na 2 CO 3, Nb 2 O} 5, Ta 2 O ₅ and weighed sample of the starting material according to the ratio, and mixed to prepare the Na (Nb _{0 0 .8 .2} Ta) O ₃ powder of the composition, and K ₂ CO ₃ , Nb ₂ O ₅ , Ta ₂ O _{5 Using} a sample as a starting material, weighing and mixing according to a ratio, _{a powder of K(Nb 0.8} Ta _0.2 )O ₃ composition is prepared. As described above, the piezoelectric ceramic of the present invention becomes a lead-free piezoelectric ceramic that does not contain lead.

Each of the powders and Li ₂ CO ₃ are mixed (S2).

Li (Nb _{0 0 .8 .2} Ta) O ₃ powder, Na (Nb _{0 0 .8 .2} Ta) O ₃ powder, K (Nb _{0 0 .8 .2} Ta) O ₃ powder (Na _x K _{0 .98-x} Li _0.02 )(Nb _0.8 Ta _0.2 )O ₃ Mix in proportion to form a powder. Here, x is preferably 0.47 to 0.53. If x is less than 0.47, only a tetragonal crystal structure is formed and the properties of the piezoelectric ceramic are deteriorated. There is a downside to being On the other hand, when x is within the range of 0.47 to 0.53, two orthorhombic and tetragonal phases coexist to form a morphotropic phase boundary (MPB). MPB means a boundary that changes continuously in shape, and it has been reported that piezoelectric ceramics exhibiting MPB characteristics have a large dielectric constant and excellent piezoelectric properties. Therefore, since it is advantageous for the piezoelectric ceramic to form MPB, x is preferably 0.47 to 0.53.

Are mixed in a (K _x Na _x Li _{0 .98- 0 .02)} (Nb _{0 0 .8 .2} Ta) O ₃ powder of Li ₂ CO ₃ for a 0.005 to 1mol 0.015mol mixed in the desired proportions. Li ₂ CO ₃ increases the density of the piezoelectric ceramic and compensates for volatilization by sintering. When Li ₂ CO ₃ is mixed, the piezoelectric properties increase. Here, when Li ₂ CO ₃ is mixed at less than 0.005 mol, the mixed amount is small, so that it does not function properly as a sintering aid, and when it exceeds 0.015 mol, a secondary phase is formed, which adversely affects electrical properties. Therefore, Li ₂ CO ₃ It is preferable to mix 0.05 to 0.015 mol.

In particular, the most preferred ratio is (Na _x K _x Li _{0 .98- 0 .02)} (Nb _{0 0 .8 .2} Ta) O ₃ in the powder x is 0.51, _{and, (Na x K 0.98-x} Li 0.02) (Nb _0.8 Ta _0.2 )O _{3 For} 1 mol of powder, Li ₂ CO ₃ is 0.01 mol.

The mixture (Na _x K _x Li _{0 .98- 0 .02)} (Nb _{0 0 .8 .2} Ta) O ₃ and by sintering the powder, and Li ₂ CO ₃ to form a piezoelectric ceramic (S3).

The mixture (Na _x K _x Li _{0 .98- 0 .02)} (Nb _{0 0 .8 .2} Ta) O ₃ powder and Li ₂ CO ₃ and then molded into a shape 950 to facilitate the sintering temperature of 1100 ℃ Sintering is carried out in If the sintering temperature is less than 950 ℃, smooth sintering is not made, and if it exceeds 1100 ℃, there is a fear that other unwanted phases are formed.

In general, the oxide piezoelectric ceramic containing oxygen (O) is sintered in a gas atmosphere containing oxygen. However, in the present invention, sintering is performed under a _{100% nitrogen gas (N 2} ) atmosphere or a mixed atmosphere of nitrogen gas (N ₂ ) and hydrogen gas (H _{2 ) without oxygen.} In the case of sintering through the method of the present invention, the properties of the piezoelectric ceramic are better than that of sintering in an atmosphere containing oxygen gas as in the prior art.

The 100% nitrogen gas atmosphere does not affect the composition ratio of the piezoelectric ceramic because it does not contain oxygen gas or hydrogen gas, and rather shows better piezoelectric properties than the oxygen gas atmosphere.

When sintering is performed in a mixed atmosphere of nitrogen gas and hydrogen gas, the oxide piezoelectric ceramic is partially reduced by hydrogen gas. At this time, if the mixing ratio of hydrogen exceeds 5% of the total, since there is a risk of explosion by hydrogen, it is preferable that hydrogen is mixed in 5% or less of the total gas. The most preferred ratio is hydrogen : nitrogen = 5 : 95 ratio.

In this case, the piezoelectric ceramic is reoxidized through a separate post heat treatment process. Since the piezoelectric ceramic heat treated in a hydrogen atmosphere does not have piezoelectric performance unless it is oxidized, it is made to have piezoelectric performance through post heat treatment. In this case, it is preferable that the post-heat treatment condition is performed at 800 to 1000° C. for about 4 hours in a gas atmosphere containing oxygen.

Hereinafter, embodiments of the present invention will be described in more detail.

<Example>

_{Li 2 CO 3, Nb 2 O} 5, and a sample with a Ta ₂ O ₅ as starting materials was prepared the powder of O ₃ composition of _{Li (Nb 0 .8 Ta 0 .2} ). The starting materials were pulverized and dried for 24 hours using ethanol and zirconia balls, and then calcined at 850° C. for 5 hours using an alumina crucible. At this time, the grinding, drying and calcining steps were repeated twice for more complete phase synthesis.

In the same manner as described above, a powder of _{Na(Nb 0.8} Ta _0.2 )O ₃ composition was prepared using a sample _{of Na 2} CO ₃ , Nb ₂ O ₅ , and Ta ₂ O _{5 as a starting material, and K 2} CO ₃ , Nb ₂ O ₅ , Ta ₂ O _{5 Using} a sample as a starting material, a powder having a composition of _{K(Nb 0.8} Ta _0.2 )O _{3 was prepared.}

It is manufactured by a method such as the Li (Nb _{0 0 .8 .2} Ta) O _3, Na (Nb _{0 0 .8 .2} Ta) O _3, K (Nb _0.8 Ta _0.2) O ₃ powder the xNa (Nb _{0 .8 Ta 0 .2) O 3} - (0.98-x) K (Nb 0 .8 Ta 0 .2) O 3 - 0.02Li (Nb 0.8 Ta 0.2) mixed at a ratio of O _3, and the mixed powder 1 _{Li 2} CO ₃ is mixed in a ratio of 0.01 mol to mol.

The final mixed powder was molded in the form of a disk using a mold and CIP, and then heat-treated at 950° C., 1000° C., 1050° C., and 1100° C. for 4 hours using an alumina crucible, respectively. The heat treatment atmosphere is 1) oxygen (O ₂ ), 2) air (20O _{2 -80N 2} ), 3) nitrogen (N ₂ ), 4) hydrogen-nitrogen mixed gas (5H ₂ -95N ₂ ) Using each, heat treatment Changes according to the atmosphere were observed. At this time, since the raw material powder has high hygroscopicity, contact with moisture was suppressed as much as possible in all processes.

The phase of the final powder and the sintered specimen was confirmed through XRD analysis, and the microstructure was observed using SEM. 2 is an SEM photograph showing the results of sintering the piezoelectric ceramic at various temperatures in an oxygen (O _{2 ) atmosphere.} 2a is the result of sintering at 950°C, FIG. 2b is 1000°C, FIG. 2c is 1050°C, and FIG. 2d is 1100°C. In the case of FIG. 2a, the interface is stabilized with the oxide ceramic material. It can be seen that large and small particles coexist irregularly. In addition, as the sintering temperature increases, as shown in FIG. 2B , the size of the particles increases and the particles grow abnormally, and the large particles collide with each other. Also, in the case of FIGS. 2C and 2D , only large particles exist, and pores are formed according to the cubic particle shape, so that the density is reduced.

3 is an SEM photograph showing the results of sintering the piezoelectric ceramic at various temperatures in an air (200O _{2 -80N 2 ) atmosphere.} 3a is the result of sintering at 950°C, FIG. 3b is 1000°C, FIG. 3c is 1050°C, and FIG. 3d is 1100°C. can check that In addition, as the sintering temperature increases, as shown in FIG. 3b , the size of the particles increases and the particles grow abnormally. Also, in the case of FIG. 3D , only large particles exist, and pores are formed according to the formation of an equilibrium particle shape, resulting in a decrease in density.

The factor that most affects the piezoelectric properties is the density of the piezoelectric ceramic. However, as shown in FIGS. 2 and 3 , when the particles of the piezoelectric ceramic are formed in an angular shape such as a cubic shape, an empty space is formed between the particles to decrease the density. Therefore, there is a disadvantage in that the piezoelectric properties are lowered when the density is lowered and the piezoelectric ceramic is used.

4 is a SEM photograph showing the results of sintering the piezoelectric ceramic at various temperatures under a _{nitrogen (N 2 ) atmosphere.} 4A is a result of sintering at 950°C, FIG. 4B is 1000°C, FIG. 4C is 1050°C, and FIG. 4D is 1100°C, and it can be seen that the grain growth behavior changes according to the increase in oxide interface energy. When sintering is performed under a nitrogen atmosphere as shown in FIG. 4A , small and uniform piezoelectric ceramic particles are obtained, and FIG. 4B also obtains small and uniform particles as a whole. 4c and 4d show that the piezoelectric ceramic particles are grown as the nucleus, and the piezoelectric ceramic particles grown by the uniform, small diameter and round particles are also grown in a round shape without being angled as shown in FIGS. 4a and 4b. do. In particular, in the case of FIG. 4d , the grown particles collide with each other, and it can be seen that the density of the particles is higher than when sintering in an oxygen or air atmosphere.

5 is a SEM photograph showing the results of sintering piezoelectric ceramics at various temperatures under a hydrogen-nitrogen mixed gas (5H ₂ -95N ₂ ) atmosphere. The interface energy is further increased according to the hydrogen reduction atmosphere, and the grain growth pattern is changed. . 5a is the result of sintering at 950°C, FIG. 5b is 1000°C, FIG. 5c is 1050°C, and FIG. 5d is 1100°C. Normal grain growth can be confirmed. Also, it can be seen that the density of the piezoelectric ceramic is the highest among the four gas atmospheres.

The piezoelectric ceramic of the present invention is an oxide piezoelectric ceramic containing oxygen, and it is generally known that the oxide piezoelectric ceramic is preferably sintered in an atmosphere containing oxygen. However, since the piezoelectric properties increase as the density of the piezoelectric ceramic increases, as shown in FIGS. 4 and 5 , sintering in a nitrogen or hydrogen-nitrogen atmosphere is more preferable than in an air or oxygen atmosphere.

In order to measure the electrical properties of the piezoelectric ceramic according to the embodiment, a silver (Ag) electrode was applied to the polished specimen to a thickness of 1 mm and heat-treated, followed by polarization treatment at 130° C. for 30 minutes at 2.8 kV/cm. _{The electromechanical coupling coefficient (k p} ) and dielectric constant (ε _r ) of the piezoelectric ceramic were measured using the resonance-anti-resonance method with an impedance analyzer (HP4194A), and the piezoelectric constant (d ₃₃ ) was measured by a Berlincourt dielectric constant meter (Berlincourt d ₃₃ meter) was used. The measured values can be confirmed through Table 1.

number atmosphere Temperature (℃) d ₃₃ (pC/N) k _p ε _r relative density
(%) One 5H ₂ -95N ₂ 950 153 0.28 814 93.8 2 1000 223 0.38 855 98.1 3 1050 261 0.41 902 98.6 4 1100 255 0.36 895 98.9 5 N ₂ 950 167 0.36 823 93.5 6 1000 270 0.45 966 97.9 7 1050 324 0.52 992 98.4 8 1100 255 0.38 878 98.2 9 20O ₂ -80N ₂ 950 197 0.36 843 94.1 10 1000 290 0.46 955 97.5 11 1050 282 0.45 956 97.8 12 1100 233 0.32 833 96.5 13 O ₂ 950 220 0.41 854 96.1 14 1000 265 0.43 920 95.5 15 1050 234 0.39 910 93.7 16 1100 205 0.31 874 89.5

The piezoelectric constant among the contents described in Table 1 can also be confirmed through the graph of FIG. 6 .

When sintering in a hydrogen mixture gas atmosphere, some piezoelectricity is lost as oxygen vacancies are formed. However, after a heat treatment process such as annealing is performed at 900°C for 4 hours, the piezoelectricity is restored. As the temperature increases to 950°C, 1000°C, and 1050°C, the physical properties are gradually improved. This means that the physical properties are improved as normal grain growth and densification progress, and although the properties are slightly decreased at 1100° C., they are not significantly different from other temperatures. It is predicted that the density is highest at 1100°C, but the decrease in physical properties is due to a change in composition due to volatilization.

In the case of heat treatment in a nitrogen atmosphere, some piezoelectricity is lost as oxygen vacancies are formed. However, as in the hydrogen mixture gas atmosphere, the piezoelectricity is restored after a post-annealing process such as annealing is performed at 900°C for 4 hours. It was confirmed that there was no difference in the microstructure even when annealing was performed. In the case of piezoelectric properties, it comes out lower than air at 950°C because of the low degree of grain growth and densification. However, at 1000°C, the difference decreases, and at 1050°C, it shows very good characteristics. In addition, it can be seen that the temperature decreases slightly at 1100 °C.

In the case of sintering in an air atmosphere, the highest characteristics were shown at 1000°C, where large and small particles coexist, and it was confirmed that the characteristics were also high at 1050°C. However, at 1100°C, the piezoelectric properties decreased, which is judged to be a change in composition due to a decrease in density and volatilization. It can be seen that the piezoelectric ceramic produced under an air atmosphere has lower properties than that of the piezoelectric ceramic sintered under a hydrogen gas atmosphere or a nitrogen atmosphere.

When sintering is performed in an oxygen atmosphere, it is expected that electrical properties will be improved because it is generally an oxide piezoelectric ceramic. As a result, it was confirmed that the physical properties were improved at 950°C than in the air atmosphere, but at 1000°C, the properties were worse than that of air, and in particular, it was confirmed that the properties were worse than that of nitrogen or hydrogen gas mixture. It is predicted that this is due to the formation of pores and a decrease in density due to the maintenance of the cubic particle shape. In addition, at 1050°C and 1100°C, it was found that the density was further lowered and the physical properties were deteriorated, which proved that the microstructure had a great influence on the electrical properties.

Summarizing this, it was confirmed that the physical properties of the piezoelectric ceramic were excellent when sintered through heat treatment at 1000 to 1050 ° C. After 1100 ° C, it is expected that the phase decomposition due to volatilization will have more influence than the microstructure. In addition, it is confirmed that the piezoelectric properties are better at 1000 to 1050°C than at 1100°C. When the relative densities were compared, it was confirmed that sintering in nitrogen or hydrogen-nitrogen mixed gas atmosphere had higher density than sintering in oxygen or air atmosphere. This is because the density is lowered by cubic-shaped particles generated when sintering in an oxygen or air atmosphere.

Claims (5)

In a lead-free piezoelectric ceramic manufacturing method through atmosphere control,
preparing Li(Nb _0.8 Ta _0.2 )O ₃ powder, Na(Nb _0.8 Ta _0.2 )O ₃ powder, and K(Nb _0.8 Ta _0.2 )O ₃ powder, respectively;
mixing the powders with Li ₂ CO _{3 ;}
sintering the mixed powders and the Li ₂ CO ₃ at 1000 to 1100° C. under a nitrogen gas or hydrogen-nitrogen mixed gas atmosphere to obtain a piezoelectric ceramic having a relative density of 97.9 to 99.0%; and
and post-heating at 800 to 1000° C. under an oxygen gas atmosphere to recover the piezoelectric properties of the sintered piezoelectric ceramic. The method of claim 1,
Mixing the powders and Li ₂ CO ₃ is,
The powder will (Na _x K _x Li _{0 .98- 0 .02)} (Nb _{0 0 .8 .2} Ta) O ₃ mixed to scale so that the powder and the (Na _x K _x Li _{0 .98- 0 .02)} (Nb _{0 0 .8 .2} Ta) O ₃ powder for the Li ₂ CO ₃ 1mol 0.005 to 0.015mol non-lead piezoelectric ceramic manufacturing method through the controlled atmosphere characterized in that the mixture.
(Here, x is 0.47 to 0.53.) The method of claim 1,
In the step of obtaining the piezoelectric ceramic by sintering, the heat treatment temperature is a lead-free piezoelectric ceramic manufacturing method through atmosphere control, characterized in that 1000 to 1050 ℃. The method of claim 1,
The nitrogen gas is a gas wholly composed of nitrogen,
The nitrogen-hydrogen mixed gas is a lead-free piezoelectric ceramic manufacturing method through atmosphere control, characterized in that the total amount of hydrogen gas or less and nitrogen gas in the remaining amount of 5% or less. delete

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