JPS61219069A - Developing device - Google Patents

Abstract

PURPOSE:To obtain a developing device which is superior in the gradations of dot images and the sharpness of linear images and has the excellent reproducibility of solid black parts and uses a magnetic or nonmagnetic single- component developer by forming a toner carrier of resin which has a specific resistivity value. CONSTITUTION:A hopper 2 which contains single-component magnetic or nonmagnetic toner, an agitator 6, and a fixed magnet roll 3 are provided, and a toner carrier 4 is pivoted freely, rotatably. The toner carrier 4 uses phenol resin and its resistivity value is 10<6>-10<12>OMEGA-cm. A toner quantity control member 5 is constituted by bonding silicone rubber to a nonmagnetic spring material by thermocompression and a power source 7 applied a high-frequency alternating electric field to a developing electrode and the conductive base of a photosensitive body 1. A toner layer is formed by a toner layer forming member 5 on the toner carrier 4, which is also charged electrostatically to hold toner on the carrier 4, so that the toner is conveyed to a developing area.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a developing device that visualizes a static image, and more specifically, it relates to a developing device that is excellent in the gradation of halftone images and the sharpness of line images. The present invention relates to a developing device that uses a magnetic or non-magnetic component regulating agent that is highly reproducible in black areas.

[Conventional technology]

The electrophotographic method disclosed in Carlson, U.S. Pat. For example, this is visualized or developed with a substance such as electrostatic powder, and then transferred and fixed onto paper. Cascade development, magnetic brush development, and liquid development are well known for developing this electromagnetic material. On the other hand, another important developing method is U.S. Patent No. 2.895.8.
There is a transfer development method using a toner carrying member called a donor, which is disclosed in No. 47. The transfer development described in this patent is (1) when the toner layer is not in contact with the photoconductor and the toner flies through this gap, (2)) when the toner layer is in rotational contact with the photoconductor, (3) ) one layer comes into contact with the photoreceptor,
When the image surface is glided, it is a general term for ``touchdown development.''

On the other hand, the problem with the thick surface of transfer development is fogging in the background area, and to improve this problem, U.S. Patent No. 2, 289.40
Non-contact transfer development was proposed in No. 0. However, in order to develop by flying toner across the gap between the photoreceptor and the donor, the gap must be less than 0.05μ, preferably less than 0.031W. The mechanical precision of the photoreceptor and donor was extremely difficult to control. To solve this problem, US Pat.
No. 866, 574 J3, 890, 929-j No. 3,
No. 893.41 discloses a method of applying an alternating electric field between a photoreceptor and a donor.

In particular, US Pat. No. 3,866,574 describes the relationship between the development gap and the alternating electric field, and the development gap is 0.
05■≦D, ≦0.18■, the frequency f of the alternating electric field is 1.
5KHz≦f≦10KHz, the amplitude VP-P of the alternating electric field is D' mf that satisfies the relationship vp-p≦800 Zelt
It is stated that svP-1' provides the best line development and minimizes background fog. On the other hand, even if the toner is manufactured and formulated according to a certain recipe, it will have a certain distribution depending on the particle size and the physical properties of each toner, but it will be distributed narrowly around a constant value. There is. Therefore, in the non-contact transfer development described in U.S. Pat. The binary current gI111?, in which toner adhesion occurs at the surface potential below this value, and toner adhesion does not occur at the surface potential below this value. It has an extremely high so-called r (gamma - the slope of the characteristic curve of image density with respect to static image potential).
This method has the disadvantage that it results in an image with poor gradation. Furthermore, even if the charge distribution of the toner is wide, if the amplitude v pp of the alternating electric field is less than 800 z, only a portion of the toner will fly, and as a result, only an image with a high r value can be obtained.

On the other hand, Japanese Patent Publication No. 58-3237 proposed a development method that improves the binary development characteristics caused by the presence of a threshold value in the flight of toner, that is, the development characteristics with high γ and poor gradation.
It is disclosed in No. 5. In order to overcome the above-mentioned problems of non-contact transfer development, the invention described in this publication applies a low-frequency alternating electric field to the development gap, transfers the toner from the toner carrier to the photoreceptor, and transfers the toner from the photoreceptor to the toner support. It is characterized by alternately repeating the process of retroposition to the body. Furthermore, the effects of dislocation and reverse transposition shown here are almost absent when the frequency of the applied 6-Iass voltage is 2 KHz or higher, and IKH'z
It is stated below that the results are very good.

Development in which this low-frequency alternating electric field is applied to the development gap is effective in causing toner adhesion faithfully to the surface potential when the toner charge distribution is narrow and there is a clear threshold for flight in the development gap. it is conceivable that.

However, in the case of non-contact transfer development, the development gap is 0.1
m or more, in the case of an electrostatic latent image with a high spatial frequency, the lines of electric force are not resolved on the toner carrier, and the electric field becomes the same in the image area and the non-image area, in other words, extremely thin lines. Or, a problem occurred where an image made up of points "collapsed".

This phenomenon will be explained in detail below. Explanation will be given using the M value defined by the formula shown below as an index of "7 collapse".

ΔD here is the image density difference between the image area and the non-image area on the developed image. FIG. 2 shows the relationship between this M value and the spatial frequency of the latent image. This is the result, 5j!・It is resolved up to p (line pair)/-, but 6jI-97
It can be seen that at a value of 1111 or more, the image area and the non-image area cannot be distinguished at all. Furthermore, when the image was observed under a microscope, it was found that the cause of the decrease in the M value was that the developed image K was "collapsed". On the other hand, the development characteristics of a line point image are as shown in FIG. 3, and when the line point image exceeds 65 lines/inch, the image portion becomes distorted and a deviation occurs between the image input range and the developed area. As a result, there are major problems in that the developed image of halftone printed matter with a high number of lines is generally dark and the image is unclear with no contrast in details. In order to overcome this problem, we tried the method of applying a low frequency alternating electric field disclosed in the above-mentioned Japanese Patent Publication No. 58-32375, and the gradation reproducibility was certainly improved and the surface potential of the photoreceptor was relatively high. Now it is faithfully developed. however,
This effect was only visible below 65 lines/inch, and had no effect at all at high lines/inch.

The reason why this low-frequency alternating electric field is not effective is that the collapse of the high halftone dot image is not due to the binary high r characteristic due to the flight threshold, but because the electric field generated by the electrostatic latent image is not faithful to the latent image, and the toner This is particularly caused by the fact that there is no difference in the electric field between the image area and the non-image area on the carrier, in other words, there is no contrast in the electric field.

Further, when the toner carrier does not have an appropriate resistance value and thickness, for example, when a normal metal sleeve is used, no reverse electric field is generated even near the photoreceptor (10 to 20 μm). For this reason, the toner that flies into the image area and the non-image area gains kinetic energy in the development gap and does not fly faithfully along the lines of electric force, causing toner adhesion to the non-image area as well.

The two points mentioned above are (1) that there is no difference in the electric field generated by the electrostatic latent image on the toner carrier between the image area and the non-image area, and (2) that the toner does not fly along the lines of electric force. As a result, the aforementioned problems of blurred image areas and low resolution of high halftone dot images occur.

That is, in other words, a problem arises in that minute and faithful reproducibility is poor.

[Problem that the invention seeks to solve]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to improve the problems of the conventional one-component developing device described above, and to provide a one-component developing device that can be used with both magnetic and non-magnetic one-component toners.

[Means and effects of rounding to solve the problem] The one-component current device according to the present invention includes a hopper for storing toner and a non-magnetic semiconducting material rotatably fixed to the periphery of a fixed magnet. The toner carrier is composed of a toner carrier made of resin, and a means for pressing at least a soft elastic body against the toner carrier, placing a photoreceptor and the toner carrier facing each other with a gap therebetween, and applying an alternating current voltage to a developing electrode. ing.

The first feature of the present invention is that a peripheral electric field is generated in the electrostatic latent image area in order to achieve faithful reproduction of halftone dot images and faithful reproduction of line images. The distance between the developing electrode and the photoreceptor is minute (
0.1 to 0.5 .mu.), no peripheral electric field is generated in the electrostatic latent image area or it hardly droops, so it is necessary to maintain a certain distance between the developing electrode and the photoreceptor. However, if the developing electrode is simply moved away from the photoreceptor, discharge will occur between the developing electrode and the photoreceptor, and since the kinetic energy of the moving toner is large, the toner will not move along the lines of electric force and will not move between the images. To prevent the problem of toner adhesion, the distance between the developing electrode and the photoreceptor is large, and the distance between the toner and the photoreceptor is very small, by providing a resistor layer on the developing electrode. A peripheral electric field is generated in the image area.

The resistor layer on this developing electrode has a specific resistance of 106 to 1
012Ω sub is desirable. The reason for this is that if the conductivity is high, no peripheral electric field will be generated, and if the insulation is high, the voltage contrast at the center of the image will become smaller and the density at the center of the image will decrease.

The second feature of the present invention is that a high frequency alternating electric field is applied to the developing electrode. The reason for this is that when the developing electrode is moved away from the photoreceptor, it becomes difficult for toner to move across the gap between the developing electrode and the photoreceptor, so it is necessary to apply a high-frequency alternating electric field to the rounding that facilitates movement. This high-frequency alternating electric field preferably has a frequency di 1 to 10 KHz and an amplitude t 1400 to 4500 kHz. More preferably, the frequency is 1 to 3 kHz and the amplitude is 80 kHz.
It is 0 to 2500 Zelto.

The sixth feature of the present invention is that the distribution of the amount of charge on the toner is widened. °For conventional one-component developers, the amount of charge was distributed in a relatively narrow range. Therefore, when non-contact transfer development is performed using this developer, the toner flying threshold value is not known, which causes binary development characteristics to be exhibited.

Therefore, in the present invention, it was decided to improve the gradation reproducibility by widening the distribution of the toner charge amount and providing a wide range of the threshold value of the toner that can fly.

According to the present invention, the distribution of the charge amount of the B toner is preferably ±15 μc/g from the median value.

A fourth feature of the present invention is that, in order to be able to use both magnetic and non-magnetic one-component toner, the toner carrier is fixed inside with a semi-conductive resin toner carrier rotatably fixed. A magnetic roll was installed. Therefore, when reproducing a black image, it is possible to use a magnetic toner, and when reproducing a bright color, it is possible to use a highly transparent non-magnetic toner.

The present invention will be explained in detail below with reference to the drawings. First, with reference to FIG. 4, the formation of a peripheral electric field in the electrostatic latent image area will be described.

Toner carrier layer 1 formed from photoreceptor 10 and resistor layer
A high frequency alternating electric field is applied by a power source 18' across the developing electrode 14 and the conductive substrate 16 of the photoreceptor. In the configuration shown in FIG. 4, the electric field formed by the electrostatic latent image on the photoconductor is determined by the resistance value and thickness of the developer carrier layer 12, its dielectric constant, and the relationship between the photoconductor 10 and the developer carrier layer 12. A peripheral electric field 20 is applied to the electrostatic latent image by controlling the gap between the layer 12 and the electrostatic latent image.
The effects of various control factors will be explained below regarding the reproducibility of halftone dot images and the detailed and faithful reproduction of line images.

Referring to Figure 5, this figure shows the halftone image reproducibility of 175 lines/inch, and the horizontal axis is the original density DIN.
, the vertical axis is the copy density DOU? It is.

It is desirable that this characteristic be a straight line with a slope of 1.

The case where the regulating agent layer carrier 12 has a thickness of 11 m+1 and a dielectric constant e of 20 will be described with reference to FIG. If the specific resistance ρ of the carrier 12 is 1060・a or less, DX
At high N, the reproduction curve is curved and the image area becomes distorted, resulting in a so-called "dark" image. When ρ becomes 1070/sub, the reproduction curve becomes relatively linear and the slope becomes close to 1.

Furthermore, if ρ is 108Ω and on a tilted board, DIN, DQU
The relationship between T was a straight line with a slope of 1, and the image showed excellent fine and faithful halftone dot reproducibility without distortion. In FIG. 5, the thickness L of the toner carrier 12 is 1 mm, but as is well known, the electrical thickness, so-called dielectric thickness (- is more commonly expressed as -). , toner carrier 1 in FIG.
2 is! ! -=5×10-'.

On the other hand, if the thickness L of the toner carrier is too large, the Frinde electric field of the latent image is strengthened, causing a problem that the uniformity of the black area is impaired. This will be explained with reference to FIG. The thickness of one carrier is 3mm (or the thickness is 1.5x1
4) If the specific resistance ρ of the supported ε body is 106 to 101! The uniformity of the peta was within an acceptable range within the range of Ω/country (above the 0 point in Figure 6). On the other hand, the carrier thickness 2 is 5-=1 (or each other is 2.5X1
O-4), ρ is less than 1010 Ω・ω, which indicates peta uniformity within the allowable range, and when k is 8-1 (or - is 4.0×ε 1O-4), ρ is 10 “It was below Ω・Confucian.

As a result of the above various experiments, the specific resistance ρ of the toner carrier layer that satisfies both the halftone image reproducibility and the uniformity of the black area is 10.
6 to 1013 Ω, equipped, dielectric thickness C is 4.0X10-
It turns out that it needs to be less than 4.

One of the major features of the non-contact transfer developing device of the present invention is that the electric field in the development gap is not only generated by an electrostatic latent image, but also an externally directed electric field is applied. This feature will be explained in detail using FIGS. 7 and 8. 7th
The figure shows the relationship between the surface potential of the photoconductor and the amount of developed toner.
This is an example in which the photoreceptor has a background potential of 250 V. Ll voltage applied to development gap 1dDC300V or dDC300
V + AC20007, the amplitude of the alternating electric field is IKHz
The experiment was conducted by changing the frequency from 3KHz to 3KHz. As shown in the line (d), the toner cannot fly through the development gap only by applying DC noise of 300 V to the background potential portion (2507) to suppress the toner from flying. This D
The case where a high voltage of 2000 VAC is superimposed and applied to C and IA is shown by the straight lines from ) to (C). As shown in FIG. 7, when an AC alternating electric field is applied superimposed on a DC electric field, the toner flies through the development gap, and development characteristics faithful to the photoreceptor potential can be obtained. The r value of this development characteristic depends on the frequency of the applied AC, etc., and the frequency is IK.
Good gap flight occurs in the range of Hz or higher. However, A
If the frequency of C/Ais is 10KHz or higher, the toner movement will not respond and the upper limit of the frequency of AC, Ais is 10KHz.
It is considered to be KHz. Figure 8 shows the relationship between the peak-to-peak voltage v'p-p of the AC bias voltage required to separate the toner from the carrier and make it fly to the photoreceptor and the carrier thickness (d). .Similar to Fig. 7, the specific resistance of the toner carrier ρ = 1Q10Ω, the relative permittivity ε = 20,
The background potential of the photoreceptor was 250 V, and the frequency of the applied AC noise was 2 KHz. For example, m special thickness Jy%80pm (also h'=1x10-'), current f
When ε[d is 620 μm, the flight start ACA insulation pressure VP-P is required to be 400 V or more as seen from the figure. Also, if jl+d2>11 sm(7), Ktf, Vp-
pH must be 1000v or higher.1. + d Ka3
Ka Vp-ptlj: 300 [177 or more was required. This "P-P" changes depending on the specific resistance ρ, relative permittivity t, AC) and frequency f of the carrier, but in the normal case, it is sufficient if it is in the range of 400v≦v p-p≦45007. It is possible to make toner fly.Furthermore,
Desirably, 800v≦vgl-P≦2500V.

Next, another feature of the present invention, which is to improve the gradation reproducibility of an image by widening the distribution of the toner charge amount, will be explained. Referring to FIG. 9(J)=(b), these figures show the relationship between the photoreceptor surface potential, the force acting on the toner (FIG. 9(a)), and the amount of developed toner (FIG. 9(a)).
The horizontal axis represents the photoconductor surface potential;
The force acting on the toner (FIG. 9(a)) and the amount of developed toner (FIG. 9(b)) are plotted on the vertical axis. The problem with the conventional non-contact transfer development method as described in U.S. Pat. This will be explained using 9 (to).

Now, when the toner charge amount is 91, the surface potential of the four-photon body (to)
The electric force acting on the toner is the product of Qs and V, Qs
x Proportional to V. On the other hand, the force with which the toner is attracted to the carrier (force opposite to the developing direction, ie, development resistance) is proportional to the square of the toner charge amount Q1. At least 9 points, where the electric force acting on the toner and the force that attracts the toner to the carrier are equal, that is, the threshold photoreceptor surface potential (vO
) At surface potentials higher than ), toner flying occurs equally, and so-called binary development characteristics with high r are exhibited. That is, as shown in FIG. 9 (if F is the force by which toner having a charge amount of 91 is attracted to the toner carrier, then when the photoreceptor surface potential becomes larger than the flight start threshold potential To, the charge amount is Q). For the toner of Q2, which has a larger charge amount than Ql, the flying start threshold potential Vo2 becomes larger than Vol.The charge amount Q of the conventional one-component developer falls within a relatively narrow range. Since the distribution was within A well-known method is described in Japanese Patent Publication No. 58-32375, in which a low-frequency alternating electric field is used to alternately repeat the steps of transferring the toner from the toner carrier to the photoreceptor and back-transferring the toner from the photoreceptor to the toner carrier. - In the Riki invention, since the developing electric field is controlled by the resistance value, thickness, dielectric constant, and developing gap of the developer layer carrier, a high-frequency alternating electric field is required). It can't be improved. Therefore, in the present invention, the amount of charge on the toner is given a certain appropriate distribution, and FIGS. 9(a) and 9(b)
), a width is generated in the development threshold potential vO shown in
Therefore, it is an attempt to improve the binary development characteristics. Referring to Figure 10, the curve in this figure (the barrel shows the case where the toner charge amount is distributed with a width of ±3 μc/g with respect to the average charge amount Q, and shows a so-called high r characteristic). (b) shows the case where the distribution width is +15 μc/g, and shows extremely excellent tone reproducibility.

On the other hand, in the curve (Q), the width of the distribution of toner charge amount is +20 μ
In the case of c/g, the development start potential extends to a negative voltage and causes fog in the background area, making it unusable. The cause of this fog is (for convenience of explanation, the explanation will focus on positively charged photosensitive materials, but the same argument can be made for negatively charged photosensitive materials), which is caused by reverse polarity toner (positively charged toner). Even if reverse polarity toner up to g is mixed in, no major fogging will occur in the background,
Experiments have shown that the fog level becomes unacceptable if it exceeds this level. Therefore, the toner charge amount distribution considered desirable in the present invention is +15 from the average value.
μc/g.

Next, FIG. 1 shows a schematic diagram of the basic configuration of an i-component developing device to which the present invention is applied.

The developing device includes a hot groove 2 for containing component-based magnetic or non-magnetic toner, an agitator 6 for agitating and conveying the toner, and a plurality of magnetic poles fixed so as not to rotate inside, which are arranged alternately. A toner carrier 4 is rotatably fixed around the magnet roll 3. The toner carrier 4 is made of phenolic resin, and the surface roughness of the toner carrier 4 is JIS 10-point average roughness in order to hold the necessary amount of toner on the cylindrical resin carrier 4, which has a thickness of 1.2 ms, for example. The toner carrier 12 is polished in the longitudinal direction to a roughness of 1 μ to 15 μ, preferably 1 μ to 5 μ in Rz. The toner amount regulating member 5 is made of, for example, a non-magnetic spring material with a thickness of 0.1 m and a soft elastic material with a thickness of 1 m, which is a thermo-pressure layer, and is suitable for a soft elastic material! A member, preferably a silicon beam, is selected, the hardness of which is from 605 to 7001, preferably from 40°
Use a 60° angle. This toner amount regulating member 5 comes into pressure contact with the toner carrier 12 to regulate the amount of toner, and is arranged so as to face the position of the trimming magnetic pole on the upstream side of the developing magnetic pole of the magnet roll 6.

The toner amount regulating member 7 is normally 50g/cm to 200g/cm.
Welded together at a pressure of an. A power source 7 opposes the photoreceptor 1 and the toner carrier layer 4 formed of a resistor layer, and applies a high frequency alternating electric field across the developing electrode and the conductive substrate of the photoreceptor.

Embodiment In a one-component developing device as shown in FIG.
The toner was used so that the specific resistance ρ was 1010 Ω/guard, the thickness of the carrier was 1 dew, the luer constant ε was 20, there was an electrode on the back side, and the surface roughness was 5 μ, the ten-point average roughness Rz of JIS. A book prepared by polishing the carrier 4 in the longitudinal direction was used. A fixed magnet roll 6 is disposed inside. The toner single-layer forming member 5 is made of silicone gym, has a hardness of 50°, has a thickness of 1 cm, and a width of 10 m, and has a pressure of 15087 cm.
It was pressed onto the Menar carrier 4 at a pressure of 1.5 cm.

The magnetic toner contains 55 wt% of magnetic powder, 22.5 wt% of dimethylaminomethyl methacrylate as the main binder, 22.5 wt% of styrene-butadiene and polyethylene wax as the sub-binder.
%, and as a non-magnetic (non-magnetic) material, a material consisting of styrene-buta-citric and polyethylene wax was used.

A layer of toner was formed and charged on the toner carrier 4 by the toner layer forming member 5, and then the toner was held on the carrier by magnetic force, image force, van der Waals force, etc., and conveyed to the developing area. The charge amount Q of the toner at this time is -5 μc/g for both magnetic toner and non-magnetic toner.
It had a wide distribution of ≦Q≦+25 μc/g.

On the other hand, the development gap was 200 μm, and the photosensitive material used was a negatively charged organic photosensitive material. The initial surface potential is -900v
In addition, the ground potential of Nokku was set to -200V. The developing bias to be applied is a frequency of 2.4 KHz, a peak-to-peak voltage of 25007 x a DC voltage v c = -250
A copy was collected at V.

Next, a copy test was performed in the same manner using this developing machine after replacing the toner with non-magnetic toner (red). The potential conditions of the photosensitive materials were exactly the same. The applied development noise is frequency 2.
．． 4KHz, peak-to-peak voltage 2500V, DCt
Niapi was collected at pressure Vpa = -350V.

As a result, the original image input area and copy image area are approximately 1:1 for both magnetic toner and non-magnetic toner, as shown in Figure 11.
The ideal halftone reproducibility corresponding to the above was obtained.

Effects of the Invention According to the developing device of the present invention, it is possible to obtain copies that are excellent in halftone image reproducibility and fine and faithful reproducibility of line images without deteriorating other image characteristics such as a flat black image. can. Further, electrostatic latent images can be developed using the same developing device for magnetic toner and non-magnetic toner, and it can be used as a developing device for black and bright colors.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of an embodiment of the developing device of the present invention, Fig. 2 is a graph showing the relationship between the M value, which is an index of image collapse, and spatial frequency, and Fig. 3 is a graph showing the image input area and the image A graph showing the correspondence relationship with the output area, FIG. 4 is a schematic diagram showing the arrangement of the photoreceptor and toner carrier in the present invention, and FIG. 5 is a graph showing the network when the specific resistance of the toner carrier layer is changed. A graph of point image reproducibility, Figure 6 is a graph showing the relationship between specific resistance of the toner carrier layer and peta black uniformity when the thickness 1 of the toner carrier layer is changed, and Figure 7 is a graph of the photosensitive A graph showing the relationship between the body surface potential and the amount of developed toner, Fig. 8 shows the toner carrier thickness (R,) and the regular gap (d), that is, the distance from the developing electrode to the surface of the photoreceptor and the flight start AC. A graph showing the relationship between the voltage and the voltage, Figure 9 ((b) shows the relationship between the photoreceptor surface potential and the amount of toner to be developed. Graph, 1st
Figure 0 is a graph showing the relationship between the photoreceptor surface potential and the amount of developed toner when the amount of charge on the toner is changed, and Figure 11 is a graph showing halftone image reproducibility according to an embodiment of the present invention. . Reference numeral 1 in the figure: Photoconductor; 2: Hot spring; 6: Magnet roll S4: Toner carrier i5: Toner layer 11111 Member 16: Agitator S7: Power supply ;10...Photoreceptor s12...Toner carrier layer s14
...Development electrode s16...conductive tube 1 Figure 2 Figure 2 East space freezing ()p/mm) 3rd Figure A Force cut into the peak ('/,) :/Qs %, “E-go) 1!. 711 Figure 8 Country 9

Claims (1)

[Claims] a hopper for accommodating one-component toner; a toner carrier made of semiconductive resin that has a magnet roll made of a plurality of magnetic poles fixed therein and is rotatably fixed around the roll; A developing device comprising a toner amount regulating member for regulating the amount of toner by press-contacting the toner carrier, wherein the toner carrier is 10^6 to 10^
A developing device characterized in that it is formed of a resin having a specific resistance value of 1^2 Ωcm.