Ceramic conductors

Ceramic conductors

Summary

4.1 High-temperature heating elements and electrodes

In the high-temperature, most materials either oxidize or melt.

For the high-temperature heating materials necessitate high resistivity.

Highly resistive elements necessitate high voltage power supplies which lead to difficulties in furnace design since refractories all become conductive when hot so that it is difficult to avoid current leakage and often an accompanying risk of thermal breakdown

(cf. Chapter 5, Section 5.2.2(b))

In case where reducing atmospheres can be tolerated, graphite or refractory metals such as molybdenum and tungsten can be used, while platinum and its alloys can be used safely in air up to 1500. Ceramics allow the use of air atmospheres at relatively low cost.

4.1.1 Silicon carbide (1) Silicon carbide (SiC) is a hard material and, because of a protective oxide laye

r, is stable in air up to 1650. - Application : Abrasive, furnace heating elements, varistors. - Making method : Heating a mixture of finely divided carbon and silicon to about 1000.

SiC is covalently bonded with a structure similar to that of diamond. - α-SiC : hexagonal form - β-SiC : cubic form (Pure cubic β-SiC is a semiconductor with a band gap of ap

proximately 2.2eV)

Important property : electrical conductivity, hardness (approximately 9 on the Mohs scale).

The colours arise from a variety of impurities including boron, aluminium, nitrogen and phosphorus, and are useful for selecting grades suitable for particular applications.

4.1.1 Silicon carbide (2) There are three principal methods of manufacturing SiC heating elements: 1. in situ formation of SiC from carbon and SiC. 2. reaction-bonding of SiC 3. pressureless sintering. SiC can be used up to about 1650 in air, because SiC rely on a thin native pa

ssivating silica film for their protection against oxidation.

Below approximately 800 the resistance from batch to batch is determined by impurities. About 600 SiC is an intrinsic semiconductor With a NTCR.

Fig.4.2 Resistance-temperature characteristics of SiC elements prepared by various methods.

4.1.2 Molybdenum disilicide

Molybdenum disilicide (MoSi2) has been developed as a heating element for use in air at temperature above 1500.

Resistivity : 2.5 X 10-7 Ωm (Room temperature) 4 X 10-6 Ωm (1800)

Fabrication of molybdenum disilicide (MoSi2)

-A mixture of fine MoSi2 powder with a carefully chosen clay is extruded into rods of suitable diameters for the thermal sections and heating zones.

-The rods are dried, sintered and cut to various lengths.

The best grade of MoSi2 element is capable of operating up to 1800.

4.1.3 Lanthanum chromite LaCrO3 combines a melt point of 2500 with high electronic conductivity (abo

ut 100Sm-1 at 1400) and resistance to corrosion.

LaCrO3 is one of the family of lanthanide perovskites RTO3, where R is a lanthanide ant T is a period 4 transition element.

- The cubic unit cell R occupies the cube corners, T occupies the cube centre and O occupies the face-centre positions.

- The coordination numbers of T and R are 6 and 8 respectively. Sintering takes place in a reducing atmosphere at temperatures close to 170

0 and is followed by an anneal in oxygen that establishes the high conductivity.

Satisfactory conductivity is maintained up to 1800 in air but falls off at low oxygen pressures so that the upper temperature limit is reduced to 1400 when the pressure is reduced to 0.1 Pa.

4.1.4 Tin oxide Tin oxide (SnO2) has found applications in high-Temperature conductor, ohmic re

sistors, transparent thin-film electrodes and gas sensors. Crystal structure : tetragonal rutile structure

It is wide band gap semiconductor which the band gap at 0K is 3.7eV, and therefore pure stoichiometric SnO2 is a good insulator at room temperature when its resistivity is probably of the order of 106Ωm.

A typical resistivity-temperature characteristic is shown in Fig.4.3.

An important application of SnO2 in ceramic form is in conducting electrodes for melting special glasses, such as those used for optical components and lead ‘crystal’ tableware.

SnO2 itself does not readily sinter to a dense ceramic and so sintering aids such as ZnO (r6(Zn2+)=75pm) and CuO (r6(Cu2+)=73pm) are added, together with group V elements such as antimony and arsenic to induce semiconductivity.

4.1.5 Zirconia

Zirconia (ZrO2) improve a defect (fragile, inconvenient resistivities). The ‘Nurnst filament’, one used as a light source, consisted of zirconia (ZrO

2) doped with thoria and ceria. Preheating the filament to about 600 using an external source reduces it

s resistance to a value which permits direct Joule heating to be effective whereupon temperatures of up to 1800 can be attained.

Its high negative temperature coefficient made it necessary to have a resistor, preferably with a high positive temperature coefficient, in series with it to limit current.

Zirconia always require preheating to reduce the resistance to a level at which Joule heating is effective.

Ones the element has reached temperatures exceeding about 700 it can be used as a susceptor and heated by eddy currents generated by an induction coil and a high-frequency (0.1-10MHz) power source.

4.2 Ohmic Resistors

Most resistors for electrical and electronic applications are required to be ohmic and to have small temperature coefficients of resistance.

The major requirement in electronics is for resistor in the range 103-108Ω, while materials with suitable electrical properties usually have resistivities less than 10-6Ωm.

Manufacturing method of material for thin and high-resistance is following. 1. Very thin conductive layers are deposited on an insulating substrate and larg

e length-to-width ratios are obtained by etching a suitable pattern. 2. The conductive material is diluted with an insulating phase. These methods are often combined.

4.2.1 Thin films

Thin films of thickness typically 10nm are readily formed in a vacuum chamber by evaporation, ‘sputtering’, or chemical vapor deposition (CVD).

Many metals and metal alloys can be evaporated from the molten state and condensed onto suitable substrates.

Ni-Cr alloys with resistivity values of about 10-6Ωm are deposited in thin-film form and provide a basis for the manufacture of high-value resistors.

Production thin-film oxide process is applied ‘sputtering’ for plazma. Thin-film is applied on glass and steatite substrates.

Fig 4.5 (a) thin-film resistors on glass and steatite substrates.

4.2.2 Thick films Rather thicker films with thicknesses typically in the range 10-15um are

made by what is termed the ‘thick-film’ or ‘silk-screen’ technique. Silk screening is a well-established method for printing artwork. The screen is held taut in a frame that is fixed 1-3mm above the surface to

be printed. A print of a stiff creamy consistency is swept across the screen by a hard rubber squeegee with sufficient pressure to force the screen, now loaded with paint, into contact with the underlying surface.

Examples of thick-film devices and circuits.

Fig. 4.5 (c) various thick-film resistor, (d) hybrid microcircuits

4.3Voltage-dependent resistors

4.3.1 Electrical characteristics and applications

There are a number of situations in which it is valuable to have resistor which offers a high resistance at low voltages and a low resistance at high voltages as is 속 case in the current-voltage characteristic shown in Fig. 4.7

Fig.4.8 Use of a voltage-dependent resistor to protect a circuit against transients

Fig.4.7 Typical current-voltage relation

for voltage-dependent resistor

Fig.4.8 Use of a voltage-dependent resistor to protect a circuit against transients

Electrical characteristics and applications 1

voltage-dependent resistor(VDR) 전압 의존 저항기 (VDR) 를 위한 세라믹의 마이크로 조직은 10 의 factor 에 의해 다른 입자 사이에서 두께를 바꾸는 입자간 layer(IGL) 와 함께 Fig.4.9.(a) 에 제시된 것처럼 시각화하게 될 수 있다 .

Anidealized structure which is useful for calculating average properties is shown in Fig.4.9(b), but is must be appreciated that in practice the high-current paths are likely to be via those particle separated 0nly by the thinnest IGLs, so that regions of high current density will form tortuous routes between the electrodes.

Fig.4.9 Illustrations of (a) actual and (b) idealized microstructure of a aristor


Fig.4.10 Proposed electronic structure at a junction between semiconducting ZnO grain(a) no voltage applied ; (b) with applied voltage


불연속의 conseqence 로서 결정의 표면에서 형성되는 전자 상태에 의해 ZnO varistor 의 VDR 결정된다 .

이 표면 에너지 준위는 접지로부터의 electron 을 위한 acceptor 로서 작용할지도 모르기 때문에 전하를 space charge 로 교체했다 .

따라서 이처럼 반대의 위치에 결정에 그 높은 저항은 어느 쪽의 방향이라도 전자 흐름에 제공될 것이다 (Fig.4.10(a))

전계를 가해준 상황은 Fig.4.10(b) 에 나타난다 . 직선형 부분이 그려질 수 있는 Fig.4.11. 에서 비전형적인 배리스터 전압 -

전류 특성은 관계를 보이게 된다 .

이 상수고 , falls off at low voltages . 과 는 10 의 factor 를 갖는 다른 전압에서의 전류이다 .

` 대신에

UkI 1

2

110logI

I21 II

IkU V

1k 1I 2I


Where =1/ and . 주어진 전압에서 저항은 (4.3)

그리고 분산시키는 힘 (4.4)

/1

1kkv

)1(

1

1 1 Uk

IkR v

11

UkIUP


VDR 의 성능 위의 어떤 제한은 그 2 차 peoperties 로부터 기인한다 . 그것은 전기 용량을 그 선도의 길이와 그 전극의 지역 때문에 그 전극과 그

유전율과 inductance 로부터 끌어내게 한다 . 동등한 회로가 Fig.4.12. 에 나타난다

VDR 가 전력 공급에서 우연히 발생할지도 모르는 매우 빠른 일시적 현상을 흡수하는 효율은 응답의 그 속력에 의존한다 .

전압에 민감한 물질의 응답 시간은 0.5 ns 일반적으로 adequate,being이다 .

VDR 와 평행의 커페시턴스는 부분적으로 일과성을 흡수할 것이고 , 전압에서 rise 를 지연시킬 것이다 , 이 의지를 제외하고 장치의 보호 작용에 영향을 미치지 마라 .


과도의 가파른 상승 시간이 초단파 전압 구성 요소의 앞에서 일어나기 때문에 VDR 와 직렬 , 유도에 인한 임피던스가 중요하기 때문에 , 유도는 중요한 효과를 가질 것이다 .

만일 Fig.4.13 에 나타나는 단순한 회선의 유도 L 이 100 Ω 의 직류 저항을 가지면 , 스위치가 있는 것을 통한 전류는 0.24 A 다

스위치를 open 것은 충전을 진동하고 있는 LC 루프로 설정한다 . 그리고 최고의 순간적인 전류는 0.24 A 다 .

최대 에너지가 콘덴서에서 저장해 두었든지 ( ) 는 유도기에 저장되는 그것과 같을 것임에 틀림없다 ( ) 그것은 따른다

⇒

2

2

1CU

2

2

1LI

2/1)(C

LIU V

k

IU 38)( /1

1

Fig.4.13 Use of a VDR to suppress sparking in a highly inductive circuit

Silicon carbide 1

입도가 범위 50~150 μm 에 보통 있다 .

SiC 분말은 점토와 다른 규산을 포함한 성분을 뒤섞이고 , 압착에 의해 형성되고 , 약 1300°C 에 소성된다 .

최종적인 본체는 규산을 포함한 glassy matrix 에 의해 밀접하게 결합되는 SiC 입자로 구성된다 .

최종적인 특성은 가공에 대한 경험적으로 상세한 설명에 의존

흑연과 같은 첨가제는 만들어 진다 . 그리고 점화 대기는 공기 , 질소 또는 수소이다 ..

금속 전극은 flame-spraying 에 의해 보통 적용된다 . 그리고 unit 은 그 다음 일련의 고 전류 파동의 특성을 안정시키는 극성 - 처리를 교체하는 것와 함께 받는다 .

Fig.4,14 section though a commercial SiC surge arrester.

Fig.4.15 (a)Silicon carbide varistor;(b)zinc oxide varistors (components supplied by power Developments Ltd); (c) packaged themistors. Inset, various NCT and PTC thermistors (components supplied by Bowthorpe thermistors.)

Silicon carbide 2

unit 은 보통 습도 (Fig.4.14) 의 입장을 방지하는 에폭시 수지가 있는 자기 셀에의 침입을 방지한다 ..

SiC unit 의 VDR 행위가 유도된다는 입자 (. 그것은 규석의 박층을 포함한다 ) 사이에 접촉면으로부터 없는 동안 비록 퍼텐셜 장벽을 통하여 운반자에 tunnelling 을 파는 것은 포함될 것 같지만 정밀한 메커니즘은 불확실하다 .

SiC varistors 의 범위는 Fig.4.15 에 나타난다

4.3.3 Zinc Oxide 산화 아연의 전기적 특성은 미량의 조성조건과 소결 조건에 의해 알 수 있다 . ZnO 는 이온을 포함하는 4 면체 interstice(틈 ) 을 절반가지는 6 각형의 밀집된 산소이온을 압축한 섬유아연석의 구조를 지닌다 .

산화 아연의 varistor 구성은 대략 1mol%의 수준에서 많은 dopants 를 포함한다 . 96.5 ZnO-0.5Bi2O3-1.0CoO-0.5Cr2O3 이다 .(mol.%에 ).

소결을 공기에서 대략 1250C 로 발생한다 .

2Zn

Ceramic conductors

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