TWI550647B - Non-linear impendance material - Google Patents

Description

Nonlinear impedance material

The present invention relates to a nonlinear impedance material, and more particularly to a high resistivity powder, the nonlinear impedance material of the present invention can suppress transient high voltage surges and improve the reliability of electronic devices sensitive to voltage surges. .

The application of integrated circuits is becoming more and more important in today's technology industry, especially in the mobile and communications industries. The mobile device pays attention to how to save energy, because the power of the mobile device depends on the battery system. Today's battery technology is limited in space on the mobile device, so the size of the battery is also limited, and the size needs to be improved. Battery capacity is the development direction of the battery industry today. Unfortunately, the requirements for mobile devices on touch-screens are getting higher and higher, so that power consumption is not falling, so system chip manufacturers are required to reduce power consumption and reduce production costs. The design and process of integrated circuits continue to find ways to reduce power consumption and increase the output of single-chip wafers. This demand has made system wafer manufacturers also require wafer foundries to improve process capability (such as 28 nm, The 20 nm, 15 nm process), the thickness of the gate at the input and output of the integrated circuit is getting thinner and thinner, so that the system chip is more susceptible to damage by EOS and ESD.

Voltage surges (such as static electricity) are in a real-world environment. There are many data (or signal) transmission interfaces available today that allow users to hot insert and remove. The probability of adding a voltage surge (such as static electricity) into the system wafer. How to reduce the voltage surge on the integrated circuit has always been the direction of system manufacturers and chip manufacturers. It is one of the factors that damage the reliability of electronic products. How to use appropriate design to achieve the protection function and reduce the cost. Is an important topic. The current design method is to design the static suppression circuit at the input and output ends of the wafer, and to use an external static suppression element to protect the wafer. With the current trend in wafer design, it is increasingly infeasible to design ESD suppression lines in the wafer (because the cost is too high), and the use of external static suppression components is a future trend.

The composition of the prior overvoltage protection material almost contains conductive particles (powder), semiconductor particles (powder), insulating particles (powder is mixed with insulating adhesives, etc.), and conductive particles (powder), a mixture of insulating particles (powder) and insulating binders, etc. The conductivity of the conductive particles must be quite high (or low resistivity), at least between the particles and the particles, so insulation particles must be required. (powder) or semiconductor particles (powder), because without the barrier of insulating particles (or semiconductor particles), the conductive particles may be close to each other (adjacent) or only one layer of extremely thin insulating adhesive in the middle, so that In the off state, high resistance requirements are required. The main characteristic of overvoltage protection materials is that when the voltage is below the specified voltage or trigger voltage, the overvoltage protection material is in the off state. The higher the resistance, the better. When it exceeds this specific voltage or the trigger voltage, when the on-state is on, it is desirable that the resistance of the over-voltage protection material is reduced as low. Such a large current can pass this overvoltage protection material to protect the electronic components in the back stage. Previous overvoltage protection materials almost all contain conductive materials (particles), semiconductor materials (particles), insulating materials (particles) and insulation. Adhesive material. The material formula is complex, and the conductive materials (particles), semiconductor materials (particles) and insulating particles have different ratios or large differences, so they are not easily dispersed uniformly (insulation particles). The particles or semiconductor particles are not easily distributed evenly between the two conductive particles, so that the uniformity and reliability of the overvoltage protection material are poor.

US 5,781,395 discloses an overvoltage protection material which, by forming a conductive path between conductive particles and semiconductor particles, and separating the conductive particles from the semiconductor particles by smaller insulating particles, wants to make each conductive particle and semiconductor particles have a distance, Finally, the above particles are bonded together by insulating adhesive, but since the particle size and weight of the conductive particles and the semiconductor particles are much larger than the insulating particles and the insulating adhesive, it is desirable to have a uniformly distributed overvoltage protection material. It is very difficult, for example, if the conductive particles are poorly dispersed, the impedance of the overvoltage protection material is not very high in the off-state.

US 4,726,991 discloses an overvoltage protection material which produces non-linear properties by coating conductive particles with the surface of semiconductor particles with an inorganic insulating material, wherein the conductive particles and the semiconductor particles are both polyhedral or polyhedral, characterized by Many sharp corners are difficult to apply an inorganic insulator to the tip portion, so that each conductive particle cannot have a uniform insulating material, so that its proportion cannot be too high, and semiconductor particles are needed to separate the conductive particles. Otherwise, The impedance of this overvoltage protection material is not very high in the off-state, or a higher ratio (greater than 30%) of insulating binder (Binder) and semiconductor particles is one of its disadvantages, and It is a polyhedron, so there are many gaps between the particles and the particles, which cannot be closely adjacent. Therefore, under the same volume, it can not add a higher proportion of particles to the insulating adhesive like spherical particles. Another shortcoming.

The present invention relates to a nonlinear impedance material, and more particularly to a high resistivity powder, and comprising spherical high resistivity particles, the present invention Nonlinear impedance materials can be applied to overvoltage protection components, and can suppress transient high voltage surges and improve the reliability of electronic devices sensitive to voltage surges.

The nonlinear impedance material of the present invention comprises an insulating binder, a high resistivity powder. A high resistivity powder is a feature of the present invention which comprises at least one separate particles of conductive materials having an insulating layer on the surface of each particle. Thus each particle is almost insulative with respect to each other, and the particles of the particles have a resistivity of at least greater than 10 ⁶ ohms/cm. The content of the high resistivity powder may be between 10% and 90% by weight of the nonlinear impedance material, and the better choice is between 40% and 90% of the weight of the nonlinear impedance material, the best (the The selection is between 50% and 90% of the weight of the nonlinear impedance material, and the content of the insulating adhesive may be between 10% and 90% of the weight of the nonlinear impedance material. The preferred choice is between the nonlinear impedance. The weight of the material is 10%~60%, and the best choice is between 10% and 50% of the weight of the nonlinear impedance material. The insulating adhesive is mixed with the high-resistivity powder to form a mixture. The high-resistivity powder is evenly distributed inside the insulating adhesive. When the input voltage is greater than the trigger voltage of the nonlinear impedance material, the material is instantaneously high-impedance. Reduced to low impedance, to achieve overvoltage protection. Another feature of the present invention is a high resistivity powder comprising a spherical particle such that each of the high resistivity particles can achieve a relatively uniform high impedance while selecting particles of different particle sizes. Achieve the highest addition ratio, such as more than 90% by weight (because spherical particles have the smallest surface area at the same volume and particle size, a higher ratio can be added than other shaped particles), and It is necessary to add any semiconductor particles having an insulating layer, which is different from US 4,726,991.

1‧‧‧Overvoltage suppression components

10, 11‧‧‧Particles of conductive materials

20, 21‧‧‧ insulation

30‧‧‧Insulating adhesive

40‧‧‧Insulating particles

50‧‧‧Semiconductor particles

61‧‧‧First electrode

62‧‧‧second electrode

100, 101, 102‧‧‧ high resistivity powder

110‧‧‧Insert substrate

200, 201, 202, 203, 204‧‧‧ Nonlinear impedance materials

D‧‧‧ gap

FIG. 1 is a schematic structural view of an overvoltage suppressing element.

1A is a schematic view of a spherical high resistivity particle of the present invention.

1B is a schematic view of a polyhedron high resistivity particle of the present invention.

1C is a schematic view of two different high resistivity particles of FIGS. 1A & 1B of the present invention.

2A is a schematic view showing the relationship between a spherical high-resistivity powder and an insulating adhesive according to a first embodiment of the present invention.

2B is a schematic view showing the relationship between a polyhedral high resistivity powder and an insulating adhesive according to a second embodiment of the present invention.

2C is a schematic view showing a relationship between a high-resistivity powder of a spherical shape and a polyhedron and an insulating adhesive according to a third embodiment of the present invention.

3 is a schematic view showing a relationship between a spherical high-resistivity powder, insulating particles and an insulating binder according to a fourth embodiment of the present invention.

4 is a diagram showing a nonlinear impedance material according to a fifth embodiment of the present invention, which includes spherical high resistivity particles, a relationship between semiconductor particles and an insulating binder.

Figure 5 is a graph showing the relationship between the trigger voltage of a non-linear resistive material and the weight percent of a high resistivity powder in an insulating bond.

Figure 6 is a graph showing the relationship between the resistance of the nonlinear impedance material and the weight percent of the insulating binder in the nonlinear impedance material.

The present invention relates to a nonlinear impedance material that can be used in an overvoltage suppressor or an overvoltage protection component.

In order to further understand the features and technical contents of the present invention, please refer to the following related embodiments, and the detailed description is as follows:

1 is a cross-sectional view showing the structure of an overvoltage suppressor. The first electrode 61 can be connected to the input end of the signal, and the second electrode 62 can be connected to the ground of the circuit or system. The nonlinear impedance material 200 of the present invention can be filled in the gap D between the first electrode 61 and the second electrode 62 by printing, and the nonlinear impedance material of the present invention will follow the first electrode and the second electrode. The voltage difference between them changes the level of the impedance, so that when the voltage difference exceeds the trigger voltage of the nonlinear impedance material of the present invention, the impedance of the material is instantaneously reduced, and the instantaneous large current can be passed through the first The electrode, the nonlinear impedance material and the second electrode are introduced into the ground of the circuit or the system, so that an instantaneous large current (high voltage) can be avoided, and the electronic components at the rear end of the entire circuit are destroyed, so that the reliability of the electronic product can be improved.

The nonlinear impedance material of the present invention comprises a high resistivity powder and an insulating binder. 1A, 1B, and 1C are high resistivity particles of different shapes, different particle sizes, and different conductive materials. A schematic representation of a composition of high resistivity powder. Figure 1A shows a high-resistivity powder 100 composed of spherical high-resistivity particles, each of which has a spherical high-resistivity particle whose core is a spherical particle formed of at least one conductive material. 10, and on the surface of each of the spherical particles 10, there is an insulating layer 20. FIG. 1B illustrates a high-resistivity powder 101 composed of polyhedron high-resistivity particles, each of which is a high-resistivity particle of a polyhedron whose core is a polyhedral particle 11 formed of at least one electrically conductive material. And on the surface of each of the polyhedral particles 11, there is an insulating layer 21. Figure 1C shows some spheres and some polyhedrons, two different high-resistivity particles composed of high-resistivity particles 102, each of which has a spherical high-resistivity particle whose core is formed by at least one conductive material. The spherical particles 10, and on the surface of each of the spherical particles 10, have an insulating layer 20, each of the polyhedral high-resistivity particles, the core of which is a polyhedral particle 11 formed of at least one electrically conductive material, and On the surface of each of the polyhedral particles 11, there is an insulating layer 21.

2A illustrates a nonlinear impedance material 200 comprising a spherical high resistivity powder 100 and an insulating binder 30 in accordance with a first embodiment of the present invention. A spherical high-resistivity powder 100, each of which has a spherical high-resistivity particle whose core is a spherical particle 10 formed of a conductive material including a metal carbonyl (Metal Carbonyl), and in each spherical particle On the surface of 10, there is an insulating layer 20. The conductive material of the metal carbonyl group (Metal Carbonyl) comprises one of Carbonyl Iron, Carbonyl Nickel, Carbonyl Cobalt or a combination thereof. The material of the insulating layer 20 includes one of SiO ₂ , H ₃ PO ₄ , fumed silicon dioxide, haolin, kaolinite, various forms of silica, glass spheres, phosphoric acid, or a combination thereof. The spherical high-resistivity powder 100 in the present embodiment is a high-resistivity powder composed of carbonyl iron particles, and the material of the insulating layer 20 on the surface of the particles contains SiO ₂ and H ₃ PO ₄ . The spherical high-resistivity particles form a high-resistivity powder 100 having a resistivity of more than 10 ¹⁰ ohm/cm and an average particle diameter of less than 90 μm. The material of the insulating adhesive 30 includes thermoset polymers, thermoplastic polymers, epoxy, silicone, silicone rubber, rubber ( One of rubber, polyethylene, polypropylene, polyurethane, or a combination thereof, having a resistivity greater than 10 ¹⁰ ohms/cm. The insulating adhesive 30 in this embodiment is a silicone rubber having a resistivity greater than 10 ¹⁰ ohms/cm. The insulating adhesive 30 is mixed with the high resistivity powder 100 into a mixture, that is, the nonlinear resistive material 200. The high resistivity powder is uniformly and tightly distributed inside the insulating adhesive, when the input voltage is greater than the trigger voltage of the nonlinear resistive material 200 ( At Trigger Voltage), this material instantaneously drops from a high impedance at the off-state to a low impedance at the on-state, achieving overvoltage protection. The relationship between the weight percentage of the spherical high-resistivity powder in the insulating bond and the trigger voltage in this embodiment is the effect as shown in FIG. The trigger voltage in FIG. 5 is a non-linear impedance material modulated by a repeated input of a pulse of 1000 volts (Pulse) and passed through different weight percentages of high resistivity powder and different weight percentages of insulating adhesive. The measured result. There are two curves in Figure 5 that represent the same recipe and the same input voltage, but each measurement still has a different trigger voltage, but its trend is the same as the curve shown in Figure 5. , there will only be a percentage change in the interval. According to the results of Fig. 5, the weight percentage of the high resistivity powder has an influence on the trigger voltage from the ratio of 1%. When the ratio is higher, the trigger voltage will become lower and lower until it is added to 90%. The voltage is close to the lowest voltage, but the rate of change of the trigger voltage is greater when the high resistivity powder addition ratio is lower. When the average particle diameter of the high resistivity powder is less than 15 micrometers (um), the ratio of 90% may be the highest ratio, and when the ratio of the insulating binder is less than 10%, it is impossible to smoothly and uniformly mix into a mixture. That is, the insulating adhesive cannot cover every high resistivity particle. Of course, if the particle size of the high resistivity particles can be changed to a larger particle size, the ratio of addition can be greater than 90%. The non-linear impedance material of the present invention has another characteristic as shown in FIG. 6. Since the high-resistivity powder of the present invention is intrinsically high in each of the high-resistivity powders, the high-resistivity powder has a resistivity greater than 10 ¹⁰ ohm. /cm, even if the percentage of insulating binder is zero, the impedance of the nonlinear impedance material is still quite high (greater than 10 ¹⁰ ohm), which is different from previous technology or other patents, of course, the percentage of insulating cement Increasing, the impedance of the nonlinear impedance material in the off-state will be higher, as shown in Figure 6.

2B illustrates a nonlinear impedance material 201 comprising a polyhedron high resistivity powder 101 and an insulating binder 30 in accordance with a second embodiment of the present invention. The polyhedral high-resistivity powder 101, each of which is a polyhedral high-resistivity particle whose core is a polyhedral particle 11 formed of a conductive material including metal carbide (Metel Carbide), and is on the surface of each polyhedral particle 11 Above, there is an insulating layer 21. The metal carbide conductive material comprises titanium carbide, columbian carbide, tantalum carbide, tungsten carbide, zirconium carbide, boron carbide. One or a combination thereof. The insulating layer 21 is subjected to a high-temperature oxidation process, and the resulting metal oxide layer achieves an insulating effect. In the present embodiment, the polyhedral high-resistivity powder 101 is a high-resistivity powder 101 composed of titanium carbide particles, and the metal oxide layer is formed on the surface of the particle by the high-temperature oxidation process. The high resistivity powder 101 formed of the polyhedral high-resistivity particles has a resistivity of more than 10 ⁷ ohm/cm and an average particle diameter of less than 100 μm. The material of the insulating adhesive 30 includes thermoset polymers, thermoplastic polymers, epoxy, silicone, silicone rubber, rubber ( One of rubber, polyethylene, polypropylene, polyurethane, or a combination thereof, having a resistivity greater than 10 ⁷ ohms/cm. The insulating adhesive 30 in this embodiment is a silicone rubber. The insulating adhesive 30 is mixed with the high resistivity powder 101 into a mixture, that is, the nonlinear impedance material 201. The high resistivity powder is uniformly and tightly distributed inside the insulating adhesive, and the input voltage is greater than the trigger voltage of the nonlinear impedance material (Trigger Voltage), this material will instantly drop from the high impedance at the off-state to the low impedance at the on-state, achieving overvoltage protection. The relationship between the weight percentage of the polyhedron high resistivity powder in the insulating bond and the trigger voltage of this embodiment is similar to that shown in FIG. The result is similar to the result of Figure 5, which shows that the weight percentage of the high resistivity powder has an effect on the trigger voltage from a ratio of 1%. When the ratio is higher, the trigger voltage will become lower and lower until it is added to 90%. At the same time, the trigger voltage is close to the lowest voltage, but the rate of change of the trigger voltage is large when the high resistivity powder addition ratio is low. When the average particle diameter of the high resistivity powder is less than 15 micrometers (um), the ratio of 90% may be the highest ratio, and when the ratio of the insulating binder is less than 10%, it is impossible to smoothly and uniformly mix into a mixture. That is, the insulating adhesive cannot cover every high resistivity particle. Of course, if the particle size of the polyhedral high-resistivity particles can be changed to a larger particle diameter, the added ratio can also be greater than 90%, but the highest ratio is slightly lower than the spherical high-resistivity powder in the first embodiment. The nonlinear impedance material 201 of the present invention has another characteristic similar to that shown in FIG. 6. Because of the high resistivity powder of the present invention, each high resistivity particle itself is insulated, so the high resistivity powder has a resistivity greater than 10 ^{7 .} Ohm/cm, even if the percentage of insulating adhesive is zero, the impedance of the nonlinear impedance material is still quite high (greater than 10 ⁷ ohm), which is different from the previous technology or other patents, of course, the insulating adhesive As the percentage increases, the impedance of the nonlinear impedance material in the off-state will be higher, similar to that shown in Figure 6.

2C illustrates a nonlinear impedance material 202 according to a third embodiment of the present invention, including a spherical high resistivity powder in the first embodiment, and a polyhedron in the second embodiment. (polyhedron) a high resistivity powder and an insulating binder 30 (same as the first embodiment). The spherical high-resistivity powder 100 and the polyhedral high-resistivity powder 101 are mixed in any ratio, and the high-resistivity powder 102 and the insulating adhesive 30 are mixed to form a nonlinear impedance material 202, and the content of the high-resistivity powder 102 is Between 10% and 90% by weight of the nonlinear resistive material 202, the content of the insulating adhesive 30 is between 10% and 90% by weight of the nonlinear resistive material 202, and can also be similar to that described in the first embodiment. Electrical characteristics, that is, curves similar to the electrical characteristics of Figures 5 and 6.

3 illustrates a nonlinear impedance material 203 comprising a spherical high resistivity powder 100, an insulating powder (or particles 40) and an insulating binder 30, in accordance with a fourth embodiment of the present invention. In addition to the first embodiment, the nonlinear impedance material may further comprise an insulating particle or an insulating powder, the insulating powder comprising a metal oxide, a variant forms of silica, a fumed colloidal silica, an untreated fumed silica, cerium oxide, aluminum oxide, One of magnesium oxide, aluminum hydroxide, magnesium hydroxide or a combination thereof. The insulating powder has a size of each insulating particle of less than 10 micrometers (um) and a content of between 0.001% and 3% by weight of the nonlinear resistive material 203. The function of the insulating particles is mainly to separate or increase the distance between the high resistivity particles, thereby increasing the impedance of the nonlinear impedance material in off-state. Of course, the addition of the insulating particles also affects the trigger voltage. High or low, as long as the weight percentage of the addition is less than 3% of the weight percentage of the nonlinear impedance material, the electrical characteristics will still be similar to the curve trend of FIG. 5 & FIG. 6, so the legend is not shown. Of course, the second and third embodiments can also be like the fourth embodiment, and the second and third embodiments additionally comprise an insulating particle or an insulating powder, the weight percentage being less than 3% by weight of the nonlinear impedance material. Table I below shows examples of mixtures of nonlinear impedance materials of different weight ratios: In the fourth embodiment, a spherical high-resistivity powder (high-resistivity powder composed of carbonyl iron particles, the material of the insulating layer 20 on the surface of the particles contains SiO ₂ , H ₃ PO ₄ ), and an insulating powder are contained (for example: Untreated Fumed silica), a non-linear impedance material 203 in which an insulating adhesive (for example, silicone rubber) is mixed. The data in Table I shows that the addition of insulating powder is less than 3%, and the trigger voltage is still lower than 800V, which is still in line with the market demand for protection applications, but can improve the reliability of the impedance of the nonlinear impedance material in the off state. (ie, after a number of voltage surges, the high impedance state can still be maintained at the operating voltage of the line).

4 illustrates a nonlinear impedance material 204 comprising a spherical high resistivity powder 100, a semiconductor powder (or particles 50) and an insulating binder 30, in accordance with a fifth embodiment of the present invention. In addition to the first embodiment, the nonlinear impedance material of the fifth embodiment may further comprise a semiconductor powder or a semiconductor particle, the semiconductor powder comprising zinc oxide, calcium oxide, cerium oxide, vanadium oxide, titanium oxide, barium titanate, One of the tantalum carbides or a combination thereof. The function of the semiconductor particles is mainly to separate or increase the distance between the high resistivity particles, and to utilize the characteristics of the semiconductor, and also to reduce the trigger voltage of the nonlinear impedance material, and the different ratios added by the semiconductor particles can be used to adjust the non- The trigger voltage of the linear impedance material meets the requirements of different applications. As long as the weight percentage of the addition is less than 5% by weight of the nonlinear impedance material, the electrical characteristics will still be similar to the curve trend of FIG. 5 & FIG. 6, so the legend is not shown. Of course, the second and third embodiments can also be similar to the fifth embodiment. In the second and third embodiments, a semiconductor particle is further included, the semiconductor particles having a weight percentage less than 5% by weight of the nonlinear impedance material. Table II below shows examples of mixtures of nonlinear impedance materials of different weight ratios: In the fifth embodiment, a spherical high-resistivity powder (high-resistivity powder composed of carbonyl iron particles, the material of the insulating layer 20 on the surface of the particles contains SiO ₂ , H ₃ PO ₄ ), and a semiconductor powder are contained (for example: A non-linear impedance material 204 in which zinc oxide is mixed with an insulating adhesive (for example, silicone rubber). The data in Table II shows that the addition amount of semiconductor powder is 5%, the trigger voltage can be reduced by about 50~100V, which is still in line with the market demand for protection applications, and does not reduce the reliability of the impedance of the nonlinear impedance material in the off state. (ie, after a number of voltage surges, the high impedance state can still be maintained at the operating voltage of the line).

In the above five embodiments, the compatibility between the high resistivity powder and the insulating binder is very important, and if it can be ensured that the two materials can be combined better, in the process of the present invention, the first In the examples, coupling agents or plasticizers are added, the weight percentage of which is less than 3% by weight of the nonlinear impedance material, and it is found that the impedance of the nonlinear impedance material in off-state can be improved. High when no plasticizer or coupling agents are added.

Although the present invention has been disclosed in the above embodiments, it is not intended to limit the invention, and any one of ordinary skill in the art can make some modifications and refinements without departing from the spirit and scope of the invention. The scope of the present invention is defined by the scope of the appended claims, and the spirit of the invention is Similar changes to the description of the present invention and the contents of the drawings are included in the scope of the present invention.

200‧‧‧Nonlinear impedance material

10‧‧‧Particles of conductive materials

20‧‧‧Insulation

30‧‧‧Insulating adhesive

Claims (9)

A nonlinear impedance material comprising: a high resistivity powder comprising: at least one particle of a conductive material, and an insulating layer on a surface of each particle, the high resistance The resistivity of the powder is greater than 10 ⁷ ohms/cm, and the content is between 50% and 90% by weight of the nonlinear impedance material; an insulation binder. The non-linear impedance material according to claim 1, wherein the conductive material comprises one of carbonyl iron, nickel carbonyl, cobalt carbonyl or a combination thereof. The non-linear impedance material according to claim 1, wherein the particles of the conductive material are spherical. The non-linear impedance material according to claim 1, wherein the material of the insulating layer comprises phosphoric acid (H ₃ PO ₄ or phosphoric acid). The non-linear impedance material according to claim 1, wherein the insulating layer comprises an oxidized layer. The non-linear impedance material according to claim 1, wherein the insulating adhesive comprises thermoset polymers, thermoplastic polymers, silicone, and ruthenium rubber ( One of silicone rubber, rubber, polyethylene, polypropylene, polyurethane, or a combination thereof, having a resistivity greater than 10 ⁷ ohms/cm. A nonlinear impedance material as described in claim 1 of the patent application, further comprising an insulating powder Finally, each of the insulating particles has an average size of less than 10 micrometers (um) and a content of between 0.001% and 3% by weight of the nonlinearly resistive material. The non-linear impedance material according to claim 7, wherein the insulating powder comprises one of metal oxide, silica, cerium oxide, aluminum oxide, magnesium oxide, aluminum hydroxide, magnesium hydroxide or Its composition. The non-linear impedance material according to claim 1, further comprising a semiconductor powder, the content of which is between 0.001% and 5% by weight of the nonlinear impedance material.