EP3143621B1

EP3143621B1 - Metal matrix composite materials for acoustic applications

Info

Publication number: EP3143621B1
Application number: EP15792910.0A
Authority: EP
Inventors: Gordon B. SIMMONS; Andrew D. Tarrant; Peter John Andrews
Original assignee: Materion Corp
Current assignee: Materion Corp
Priority date: 2014-05-15
Filing date: 2015-05-15
Publication date: 2021-08-25
Anticipated expiration: 2035-05-15
Also published as: EP3143621A1; WO2015175897A1; EP3143621A4

Description

BACKGROUND

This disclosure relates generally to members having a high stiffness such as used in acoustical transducers. In particular, this disclosure relates to a lightweight, high stiffness speaker diaphragm member and a method for making the same.
Acoustic speaker diaphragms (also referred to as domes or membranes) are well-known in the prior art and are used to match the relatively low acoustic impedance of air to the relatively high mechanical impedance of a voice coil (e.g., acoustic drive system). The driver for an acoustic speaker generally can deliver a force of large magnitude but delivers a relatively small displacement. An acoustic speaker diaphragm having a large area results in a large volume of air being moved by a driver having a relatively small displacement. A speaker diaphragm is generally constructed of a material as stiff and as light as possible so that minimal mechanical energy is used to accelerate the diaphragm mass thereby providing rapid response of the diaphragm to driver inputs.
Speaker diaphragms can be made from a variety of materials ranging from inexpensive paper-based materials to various metals. A particularly popular metal for high-end speaker diaphragms is beryllium, which has a very high specific stiffness but is also very expensive. While beryllium diaphragms are popular for many high-end acoustic speakers, the cost generally prohibits its use in low and mid-end speakers. US 2008/124566 A1 describes audio components having high rigidity and low mass as well as methods of manufacturing such components. These components have a foil body formed of a metal or metal alloy matrix that is embedded with ultra-hard particles or grit, such as diamond and/or cBN. JP H 06 276 596 A discloses a diaphragm being formed by using material where the high hardness grain of at least one material of diamond, c-BN, SiC and B4C is dispersed in low hardness matrix components. Thereby, a diaphragm for speaker having high rigidity, high yield, long life and a high tone characteristic could be obtained. In US 6 284 014 B1 an extrudable and weldable matrix alloy composite comprising a base material metal of about 50 to 99.9% by weight, boron carbide or silicon carbide of about 0.1 to 50% by weight, less than about 3.0% by weight of at least one metal having an intermetallic phase temperature lower than the melting point of the base material metal and a reinforcement agent of up to about 5% by weight, is disclosed. JP S 54 111 818 A describes a diaphragm of low density and high elasticity for acoustic transducers by molding the mixture of boron compound powder and aluminum (or magnesium) powder to a desired shape and sintering the same. FR 2 607 741 discloses a process for obtaining composite materials, in particular with an aluminum alloy matrix, by powder metallurgy, in which a powdery reinforcing product belonging to the group consisting of oxides, carbides, nitrides. Their compound is added to the powder alloy mixtures in the form of grains or short fibers, in an amount such that the volume fraction of the addition is between 5 and 40% and then grinds all the powders. The grinding is maintained for a period sufficient to obtain a distribution of the reinforcing product over the entire mass of metal used within each alloy grain but without exceeding the time beyond which the reduction ratio of the particle size of the product reinforcement is greater than 5. US 6398843 B1 describes a method of manufacture of a dispersion-strengthened material comprising the steps of: (a) mixing the powdered or aluminum alloy matrix with ceramic particles added as a separate phase to the matrix and having a diameter of less than 30 nm wherein the ceramic particle content is in the range 1 to 25 volume percent; (b) blending of the resultant mixture to produce an essentially uniform dispersion of ceramic particles; and (c) consolidating the resultant blend to produce a solid material.

BRIEF DESCRIPTION

The invention is defined in the appended claims. In accordance with one aspect of the present disclosure, a loudspeaker diaphragm for use in producing sound from an electrical signal comprises a shaped metal matrix sheet of substantially uniform thickness wherein the metal matrix sheet comprises ceramic particles from 0.1 micrometers to 20 micrometers in size distributed in a metal matrix, the sheet having a substantially uniform thickness of 4 micrometers to 1,000 micrometers, wherein the shaped metal matrix sheet has an elastic modulus to weight ratio greater than 35 GPa/g/cc; and wherein the metal matrix is formed from aluminum or an aluminum alloy; and wherein the ceramic particles are selected from the group consisting of silicon carbide, titanium carbide, boron carbide, silicon nitride, titanium nitride, and zirconium oxide; and wherein the metal matrix sheet comprises from about 1 vol% to about 55 vol% of the ceramic particles.
The metal matrix can be aluminum metal. The shaped metal matrix sheet can be rolled to thickness. The metal matrix is formed from aluminum or an aluminum alloy.
In some embodiments, the metal matrix can be formed from an aluminum alloy comprising at least one element selected from the group consisting of chromium, copper, lithium, magnesium, manganese, zinc, iron, nickel, silver, scandium, vanadium and silicon.
In other embodiments, the metal matrix can be formed from an aluminum alloy comprising from about 91.2 wt% to about 98.6 wt% aluminum, from about 0.15 wt% to about 4.9 wt% copper, from about 0.1 wt% to about 1.8 wt% magnesium, and from about 0.1 wt% to about 1 wt% manganese.
In additional embodiments, the metal matrix can be formed from an aluminum alloy comprising from about 91.2 wt% to about 94.7 wt% aluminum, from about 3.8 wt% to about 4.9 wt% copper, from about 1.2 wt% to about 1.8 wt% magnesium, and from about 0.3 wt% to about 0.9 wt% manganese.
Alternatively, the metal matrix can be formed from an aluminum alloy comprising from about 92.8 wt% to about 95.8 wt% aluminum, from about 3.2 wt% to about 4.4 wt% copper, from 0 to about 0.2 wt% iron, from about 1.0 to about 1.6 wt% magnesium, from 0 to about 0.6 wt% oxygen, from 0 to about 0.25 wt% silicon, and from 0 to about 0.25 wt% zinc.
Sometimes, the metal matrix can be formed from an aluminum alloy comprising from about 95.8 wt% to about 98.6 wt% aluminum, from about 0.8 wt% to about 1.2 wt% magnesium, and from about 0.4 wt% to about 0.8 wt% silicon.
The metal matrix sheet may comprise from about 1 vol% to about 45 vol% of the ceramic particles. The metal matrix sheet can be shaped in the form of a dome or cone. The ceramic particles may have an average particle size from 2 micrometers to 20 micrometers.
In accordance with another aspect, a sheet material for use in loudspeaker diaphragms for use in producing sound from an electrical signal comprises a metal matrix sheet of substantially uniform thickness wherein the metal matrix sheet comprises ceramic particles from 0.1 micrometers to 20 micrometers in size distributed in a metal matrix wherein the substantially uniform thickness is of 4 micrometers to 1,000 micrometers.
The metal matrix can be aluminum metal. The metal matrix sheet can be rolled to thickness.
In accordance with another aspect, a method of making a loudspeaker diaphragm for use in producing sound from an electrical signal comprises providing a mass of metal particles, providing a mass of ceramic particles of a selected size distribution, mixing the metal particles and ceramic particles to achieve a uniform distribution of ceramic particles, forging a billet from the mixture, and rolling the billet into a sheet with a substantially uniform thickness and forming a loudspeaker diaphragm from the sheet. The substantially uniform thickness is 4 micrometers to 1,000 micrometers. The sheet comprises the ceramic particles distributed in a metal matrix and has an elastic modulus to weight ratio greater than 35 GPa/g/cc. The metal matrix is formed from aluminum or an aluminum alloy. The ceramic particles are selected from the group consisting of silicon carbide, titanium carbide, boron carbide, silicon nitride, titanium nitride, and zirconium oxide. The sheet comprises from about 1 vol% to about 55 vol% of the ceramic particles.
The ceramic particles can have a size from 0.1 micrometers to 20 micrometers. The metal can be an aluminum metal.
These and other non-limiting characteristics of the disclosure are more particularly disclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same.

Figure 1 is a perspective view of a speaker including a diaphragm in accordance with the present disclosure.
Figure 2 is a flowchart of an exemplary method in accordance with the present disclosure.

DETAILED DESCRIPTION

A more complete understanding of the components, methods/processes, and devices, disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and are, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments.
Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.
The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.
As used in the specification and in the claims, the term "comprising" may include the embodiments "consisting of" and "consisting essentially of." The terms "comprise(s)," "include(s)," "having," "has," "can," "contain(s)," and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that require the presence of the named components/steps and permit the presence of other components/steps. However, such description should be construed as also describing compositions or processes as "consisting of" and "consisting essentially of" the enumerated components/steps, which allows the presence of only the named components/steps, along with any impurities that might result therefrom, and excludes other components/steps.
Numerical values in the specification and claims of this application should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.
All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of "from 2 to 10" is inclusive of the endpoints, 2 and 10, and all the intermediate values).
The term "about" can be used to include any numerical value that can vary without changing the basic function of that value. When used with a range, "about" also discloses the range defined by the absolute values of the two endpoints, e.g. "about 2 to about 4" also discloses the range "from 2 to 4." The term "about" may refer to plus or minus 10% of the indicated number.
The present disclosure also refers to particles as having an average particle size. The average particle size for particles is defined as the particle diameter at which a cumulative percentage of 50% by volume of the particles are attained. In other words, 50 vol% of the particles have a diameter above the average particle size, and 50 vol% of the particles have a diameter below the average particle size.
With reference to Figure 1 , an exemplary speaker 110 is illustrated. The speaker 110 includes a speaker housing 112 in which various speaker electronics (e.g., voice coil, etc.) are supported. A loudspeaker diaphragm 114 is secured to the housing 112. The loudspeaker diaphragm, in accordance with the present disclosure, is comprised of a metal matrix composite material.
Desirably, the loudspeaker diaphragm has a high modulus of elasticity (i.e. Young's modulus) and a low weight, or put another way a high elastic modulus to weight ratio. Beryllium has the desired high ratio, but is very expensive, and thus not suitable for low-end to medium-end speakers.
Metal matrix composites are composite materials including a metal matrix and a reinforcing material (e.g., a ceramic material) dispersed in the metal matrix. The metal matrix phase is typically continuous whereas the reinforcing dispersed phase is typically discontinuous. The reinforcing material may serve a structural function and/or change one or more properties of the material. Metal matrix composites can provide combinations of mechanical and physical properties that cannot be achieved through conventional materials or process techniques. These property combinations make metal matrix composites particularly useful in applications where weight, strength, and stiffness are important, such as loudspeaker diaphragms.
Powder metallurgy is a process by which powdered materials are compacted into a desired shape and sintered to produce desired articles. Powder metallurgy allows for a faster quenching rate of the metal from the melt which typically results in smaller grain sized, increased solid solubility of most solute elements, and reduced segregation of intermetallic phases. These results may lead to beneficial properties in the produced articles, such as high strength at normal and elevated temperatures, high modulus values, good fracture toughness, low fatigue crack growth rate, and high resistance to stress corrosion cracking.
In accordance with the present disclosure, the loudspeaker diaphragm 114 is formed from a shaped metal matrix sheet having a substantially uniform thickness of 4 micrometers to 1,000 micrometers. The metal matrix sheet comprises ceramic particles distributed in a metal matrix phase. The ceramic particles have an average particle size from 0.1 micrometers to 20 micrometers. The loudspeaker diaphragm has an elastic modulus to weight ratio which is greater than 35 GPa/g/cc. The metal matrix is formed from aluminum or an aluminum alloy. The ceramic particles are selected from the group consisting of silicon carbide, titanium carbide, boron carbide, silicon nitride, titanium nitride, and zirconium oxide. The metal matrix sheet comprises from about 1 vol% to about 55 vol% of the ceramic particles. In one embodiment, a 4:1 (w/w) ratio of metal matrix to ceramic particles yields an elastic modulus to weight ratio of 39 GPa/g/cc. In another embodiment, a 2:5 (w/w) ratio of metal matrix material to ceramic particles yields an elastic modulus to weight ratio of 48 GPa/g/cc. It will be appreciated that a loudspeaker diaphragm made in accordance with the present disclosure can have a modulus of elasticity to weight ratio approaching that of a beryllium loudspeaker diaphragm, while costing approximately 60% less to manufacture.
Adding the ceramic particles to the metal matrix increases the modulus of elasticity over that of the matrix metal without ceramic particle reinforcement, while maintaining the relatively low weight of the matrix material. In this manner, the less expensive matrix material can achieve properties that approach beryllium or other high-end speaker diaphragm materials.
Various metals can be used for the matrix metal. For example, aluminum and titanium are relatively lightweight metals that are suitable for use in conjunction with aspects of the present disclosure. The matrix metal is aluminum or an aluminum alloy. The aluminum alloy may include at least one element selected from chromium, copper, lithium, magnesium, nickel, and silicon. It is noted that "aluminum," as used here, refers to aluminum with only impurities present, i.e. pure aluminum, whereas the term "aluminum alloy" is used to refer to alloys containing majority aluminum with a significant amount of another element.
In some embodiments, the aluminum alloy is a 1000 series alloy (≥ 99 wt% aluminum), a 2000 series alloy (including copper as an alloying component), a 3000 series alloy (including manganese as an alloying component), a 4000 series alloy (including silicon as an alloying component), a 5000 series alloy (including magnesium as an alloying component), a 6000 series alloy (including magnesium and silicon as alloying components), a 7000 series alloy (including zinc as an alloying component), or an 8000 series alloy (e.g., aluminum-lithium alloys).
The metal particles may have an average particle size in the range of from about 5 µm to about 150 µm, including from about 15 µm to about 75 µm, about 20 µm to about 50 µm, from about 20 µm to about 40 µm, from about 20 µm to about 30 µm, and about 75 µm.
In some embodiments, the aluminum alloy includes from about 91.2 wt% to about 94.7 wt% aluminum, from about 3.8 wt% to about 4.9 wt% copper, from about 1.2 wt% to about 1.8 wt% magnesium, and from about 0.3 wt% to about 0.9 wt% manganese.
In other embodiments, the aluminum alloy includes from about 95.8 wt% to about 98.6 wt% aluminum, from about 0.8 wt% to about 1.2 wt% magnesium, and from about 0.4 wt% to about 0.8 wt% silicon. In additional embodiments, these aluminum alloys can also include from about 0.15 wt% to about 0.4 wt% copper, and 0.04 wt% to about 0.35 wt% chromium.
The aluminum alloy may be 2009. The composition of 2009 aluminum alloy is as follows:

Component Wt%

Aluminum Remainder

Copper 3.2-4.4

Iron Max 0.2

Magnesium 1.0-1.6

Oxygen Max 0.6

Silicon Max 0.25

Zinc Max 0.25

The aluminum alloy may be 2090. The composition of 2090 aluminum alloy is as follows:

Component	Wt%
Aluminum	93.2-95.6
Chromium	0.05 max
Copper	2.4-3.0
Iron	0.12 max
Lithium	1.9-2.6
Magnesium	0.25 max
Manganese	0.05 max
Other, each	0.05 max
Other, total	0.15 max
Silicon	0.10 max
Titanium	0.15 max
Zinc	0.10 max
Zirconium	0.08-0.15

The aluminum alloy may be 2099. The composition of 2099 aluminum alloy is as follows:

Component	Wt%
Aluminum	92.51-95.35
Beryllium	0.0001 max
Copper	2.4-3.0
Iron	0.07 max
Lithium	1.6-2.0
Magnesium	0.10-0.50
Manganese	0.10-0.50
Other, each	0.05 max
Other, total	0.15 max
Silicon	0.05 max
Titanium	0.10 max
Zinc	0.40-1.0
Zirconium	0.05-0.12

The aluminum alloy may be 2124. The composition of 2124 aluminum alloy is as follows:

Component	Wt%
Aluminum	91.2-94.7
Chromium	Max 0.1
Copper	3.8-4.9
Iron	Max 0.3
Magnesium	1.2-1.8
Manganese	0.3-0.9
Other, each	Max 0.05
Other, total	Max 0.15
Silicon	Max 0.2
Titanium	Max 0.15
Zinc	Max 0.25

The aluminum alloy may be 2618. The composition of 2618 aluminum alloy is as follows:

Component	Wt%
Aluminum	Balance
Copper	1.9-2.7
Iron	0.9-1.3
Magnesium	1.3-1.8
Nickel	0.9-1.2
Silicon	Max 0.25
Titanium	0.04-0.1
Others, each	Max 0.05
Others, total	Max 0.15

The aluminum alloy may be 6061. The composition of 6061 aluminum alloy is as follows:

Component	Wt%
Aluminum	95.8-98.6
Chromium	0.04-0.35
Copper	0.15-0.4
Iron	Max 0.7
Magnesium	0.8-1.2
Manganese	Max 0.15
Other, each	Max 0.05
Other, total	Max 0.15
Silicon	0.4-0.8
Titanium	Max 0.15
Zinc	Max 0.25

The aluminum alloy may be 6082. The composition of 6082 aluminum alloy is as follows:

Component *	Wt%
Aluminum	95.2-98.3
Chromium	Max 0.25
Copper	Max 0.1
Iron	Max 0.5
Magnesium	0.6-1.2
Manganese	0.4-1.0
Other, total	Max 0.15
Silicon	0.7-1.3
Titanium	Max 0.1
Zinc	Max 0.2

The ceramic particles are selected from silicon carbide, titanium carbide, boron carbide, silicon nitride, titanium nitride, zirconium oxide, aluminum oxide, aluminum nitride, and titanium oxide. Particularly desired are silicon carbide and boron carbide.
The ceramic particles may have an average particle size in the range of from about 0.1 µm to about 20 µm, including from about 0.2 µm to about 20 µm, and from about 1 µm to about 4 µm. In other embodiments, the ceramic particles have an average particle size in the range of from about 0.2 µm to about 0.4 µm, or from about 0.5 µm to about 0.9 µm. It should be noted again that these are averages; some particles will be larger and some will be smaller.
The ceramic particles may make up to about 55 vol% of the overall metal matrix composite. The composite includes from about 1 vol% to about 55 vol% of the ceramic dispersed phase, including from about 20 to about 50 vol%, from about 35 to about 45 vol%, and about 40 vol%. Desirably, the resulting composite has both a high elastic modulus and sufficient rolling/forming capability to make the dome/cone.
Figure 2 is a flow chart illustrating an exemplary method 200 of the present disclosure. The method includes providing metal particles (e.g., aluminum or aluminum alloy particles) 205 and providing ceramic particles 210 to a high energy mixing stage 220. Such particles are in the form of powders.
The metal and ceramic powders should be mixed 220 with a high energy technique to distribute the ceramic reinforcement particles into the metal matrix. Suitable techniques for this mixing include ball milling, mechanical attritors, teamer mills, rotary mills and other methods to provide high energy mixing to the powder constituents. Mechanical alloying should be completed in an atmosphere to avoid excessive oxidation of powders preferable in an inert atmosphere using nitrogen or argon gas. The processing parameters should be selected to achieve an even distribution of the ceramic particles in the metallic matrix.
The powder from the high energy mixing stage is degassed to remove any retained moisture from the powder surface, this may be completed at between 120°C to 500°C.
A forging step 230 may also be performed to increase density and produce a billet 240. The hot compacting may be performed at a temperature in the range of from about 400 ºC to about 600 ºC, including from about 425 ºC to about 550 ºC and about 500 ºC. Hot compaction may include the use of hot die compaction, hot isostatic pressing or hot extrusion typically at pressures of between 30 to 150 MPa.
The billet may be subsequently rolled 250 to obtain a metal matrix sheet with a substantially uniform thickness. The formed sheet can then be used to produce speaker diaphragms using conventional speaker diaphragm manufacturing techniques. Powder metallurgy techniques are well-suited for this method.
Further details of exemplary processes for forming the billet are set forth in U.S. Patent No. 6,398,843 issued on June 4, 2002 , entitled "Dispersion-Strengthened Aluminum Alloy" and U.S. Patent No. 4,749,545 issued on June 7, 1988 , entitled "Preparation of Composites".
The present disclosure has been described with reference to exemplary embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

A loudspeaker diaphragm for use in producing sound from an electrical signal comprising a shaped metal matrix sheet;
wherein the metal matrix sheet comprises ceramic particles distributed in a metal matrix, the ceramic particles having an average particle size from 0.1 micrometers to 20 micrometers; and
wherein the metal matrix sheet has a substantially uniform thickness of 4 micrometers to 1,000 micrometers; and
wherein the shaped metal matrix sheet has an elastic modulus to weight ratio greater than 35 GPa/g/cc; and
wherein the metal matrix is formed from aluminum or an aluminum alloy; and
wherein the ceramic particles are selected from the group consisting of silicon carbide, titanium carbide, boron carbide, silicon nitride, titanium nitride, and zirconium oxide; and
wherein the metal matrix sheet comprises from about 1 vol% to about 55 vol% of the ceramic particles.
The loudspeaker diaphragm of claim 1, wherein the metal matrix is formed from an aluminum alloy comprising at least one element selected from the group consisting of chromium, copper, lithium, magnesium, manganese, zinc, iron, nickel, silver, scandium, vanadium and silicon.
The loudspeaker diaphragm of claim 1, wherein the metal matrix is formed from an aluminum alloy comprising from about 91.2 wt% to about 98.6 wt% aluminum, from about 0.15 wt% to about 4.9 wt% copper, from about 0.1 wt% to about 1.8 wt% magnesium, and from about 0.1 wt% to about 1 wt% manganese.
The loudspeaker diaphragm of claim 1, wherein the metal matrix sheet comprises from about 35 vol% to about 55 vol% of the ceramic particles.
The loudspeaker diaphragm of claim 1, wherein the ceramic particles are selected from the group consisting of silicon carbide and boron carbide; and
the metal matrix is formed from an aluminum alloy; and
the metal matrix sheet comprises from about 35 vol% to about 55 vol% of the ceramic particles.
A method of making a loudspeaker diaphragm for use in producing sound from an electrical signal according to claim 1, comprising:
mixing (i) metal particles with (ii) ceramic particles having an average particle size from 0.1 micrometers to 20 micrometers;

processing the mixture to achieve an even distribution of the ceramic particles;

forging a billet from the mixture;

rolling the billet into a sheet;

wherein the sheet comprises the ceramic particles distributed in a metal matrix; and

wherein the sheet has a substantially uniform thickness of 4 micrometers to 1,000 micrometers; and

wherein the sheet has an elastic modulus to weight ratio greater than 35 GPa/g/cc; and

wherein the metal matrix is formed from aluminum or an aluminum alloy; and

wherein the ceramic particles are selected from the group consisting of silicon carbide, titanium carbide, boron carbide, silicon nitride, titanium nitride, and zirconium oxide

wherein the sheet comprises from about 1 vol% to about 55 vol% of the ceramic particles;

forming a loudspeaker diaphragm from the sheet.
The method of claim 6, wherein the metal particles are an aluminum alloy comprising at least one element selected from the group consisting of chromium, copper, lithium, magnesium, manganese, zinc, iron, nickel, silver, scandium, vanadium and silicon.
The method of claim 6, wherein the metal particles are an aluminum alloy comprising from about 91.2 wt% to about 98.6 wt% aluminum, from about 0.15 wt% to about 4.9 wt% copper, from about 0.1 wt% to about 1.8 wt% magnesium, and from about 0.1 wt% to about 1 wt% manganese.
The method of claim 8, wherein the sheet comprises from about 35 vol% to about 55 vol% of the ceramic particles.
The method of claim 8, wherein the ceramic particles have an average particle size from 2 micrometers to 20 micrometers.