http://repository.osakafu-u.ac.jp/dspace/

   

Title Titanium Oxide Rectifier

Author(s) Yoshida, Koji; Kawamura, Takao

Editor(s)

CitationBulletin of University of Osaka Prefecture. Series A, Engineering and nat

ural sciences. 1958, 6, p.75-84

Issue Date 1958-03-30

URL http://hdl.handle.net/10466/7895

Rights

75

Titanium Oxide Rectifier

Koji YosmDA* and Takao KAwAMuRA*

(Received February 10, 1958)

Abstract

A report is made in this paper of the new titanium oxide rectifier of the area contact

type which is more eMcient and has greater practical uses than the conventional ones of

the point contact type. When the electrode with area contact is formed on the surface

of titanium oxide that is made into a semi-conductor by a thermal means, it tends to

. short-circuit. But, if the surface of titanium oxide, that has been made into a semi- conductor, is subjected to a further intensifying oxidization, it never short-circuits and

then the electrode with area contact can be formed. As the means of such an additional

intensified oxidization, anodization in fused salt is found out to bring about very satis-

faCtOAtX S:eSUsgkStl the static characteristics of the titanium oxide rectifier are so erncient

that they are almost equal to two selenium rectifiers connected in series. Also satisfacto!y

are its dynamic characteristics. But, its most conspicuous feature, among.otPers, li.es in

the fact that it can stand well against the elevated temperatures: 2000C is its OPtlMUM temperature and no deterioration is detected below 3000C. Neither humidity can de-

teriorate it. However, it is admitted that, if the purity of the material titanium is further im-

proved and the proper and profitable addition of impurities becomes feasible, its eMciency

can reasonably be expected to greatly improve.

I. Introduction

. While the stoichiometric titanium dioxide is a white electric insulator, the imperfectly

oxidized titanium dioxide, which is deficient in oxlgen, makes the so-called N type semi-

conductor. First, test has been made of the rectifier of such a type as made with metal being

pressed down upon the lump of titanium dioxide which is made into a semi-conductor by

sintering it first and then reducing it with hydrogen. But, as soon as the metalic titanium

became available, use has been made of the metalic titanium plate whose surface is

imperfectly oxidized in order to make it a semi-conductor. R. Breckenridge and W. Hosler

made a point contact of meta1 upon the imperfectly oxidized titanium dioxide and tested

its rectifying eMciency. Its temperature characteristios have been found superb'). Mean-

while, as soon as the titanium plate of domestic origin became available, an additional

test has been made by the author, proving the similar static and temperature characteristics.

In such cases, however, the maximum reverse voltage has remained within some scores

of volts only, while the maximum load current has also been some scores of milliamperes.

In order to be useful for the power use, therefore, it is essential to have it made

into an area contact typ6. But, whenever the electrode is formed by means of the metal

spraying, vacuum evaporation or baking in order to achieve an area contact, such a dith-

* Department of Electrical Engineering, College of Engineering,

1

76 K, YosHIDA and T. KAwAMuRA ' culty as short-circuit has resulted. This problem, however, has been solved by additional

anodization of the surface of titanium oxide, making it possible to increase the reverse

voltage at the same time. The patent application was made for this method in Japan

during September, 1954, and it has been patented.

In the meantime, Battelle Memorial Institute in the United States made a similar

research under sponsorship of the U.S. Air Force. The results have been reported in the

Electrical Engineering of October, 19552). Their method is similar to the one patented by

the author. Since then, however, the author has contrived a new process to anodize

titanium oxide in fused salt, which has resulted in the further improvement on the con-

ventional'ones, as fully dealt in the later section of this paper, and its patent is now in

application. 4

II. Structure and Process

The structure of the titanium oxide rectifier is shown

in Fig. 1. When the surface of the metalic titanium (1)

is oxidized imperfectly by a thermal treatment, a layer

Fig. 1. Schematic sectional (2) of a semi-conductor is formed upon it. It is as thin view of a titanium-oxide rectifier. as about O･05mm. This surface of the semi-conductor ･ layer is then strongly oxidized further by an electro!ytic process in order to form an extremely thin film whose electric resistance is great, and

then upon its surface, such a metaI as lead that has a melting point above 3000C, is made

' closely stuck, so that the counter electrode is formed there.

(1) The metalic titanium plate. '

The metalic titanium plate of 99.5% purity that is obtainable in the market has been

used. As a result of spectrum analysis, the metalic impurities contained in it are W, Cu,

Fe and Si etc. It is further found that its preparatory polishing and acid washing are

the effective means of improving the reverse voltage.

(2) First oxidization.

In this paper, it is provisionally called the first oxidization to form the semi-conductor

layer by means of the imperfect oxidization of the surface of the metalic titanium. Such

oxidization can be achieved by two methods: either by means of heating the metalic

titanium plate in oxygen atmosphere and then reducing it in hydrogen atmosphere, or by

heating it in water vapour. But, in view of the results obtained by either of these methods

being alike, the latter one has always been used for the reason of simplicity of its process.

First, the metalic titanium plate is put into an oven which is heated at a temperature and

for a time that are appropriately preconceived as under-mentioned, while water vapour is

led into it at the same time. In the case of a point contact type, such a treatment at

7000C for 1-2 hours is found suthcient. But, in order to increase the backward resistance

in the'event of an area contact type, treatment at 9000C for 2hours and then at 1,OOOOC

for O.5-1 hour is considered appropriate. After the above treatment, it is left to get cool

without letting out water vapour.

7Vtanium Oxide Rectijier 77

If the perfectly oxidized titanium oxide plate is heated in vacuum, however, reduction

takes place and electric conductivity increases. This assures an easy adjugtment of this

conductivity. Moreover, it is particularly interesting to note that this treatmept is found

also effective for the titanium oxide plate that has already formed the electrode.

(3) Second oxidization.

When the counter electrode that has area contact is formed on the titanium oxide

plate that has undergone its first oxidization, it frequently short-circuits. However, if

further oxidization is made on its surface, such short-circuit never recurs. Such an addi-

ti6nal oxidization is called in this paper the second oxidization and, in order to achicve

it use has been made of anodization. ' When the surface of titanium oxide, that has already undergone the first oxidization,

is anodized, oxidization takes place first in a part that is imperfectly oxidized and then it

spreads out over the whole surface, so that an extremely thin and uniform film is formed

there. For the same purpose, however, the techniques of making the electrolytic condenser

can be utilized as well.

If anodization is made in the elcctrolyte made with ammonium phosphate added to

ethylen glycol, both the backward and forward resistance increase. But, the former

increases more than the latter, so that the rectifier with a greater rectification ratio can be

had. When anodization is made, formation reaches a balanced state at a certain point in

its progrees. As a result, oxidization ceases. But, if the sample is furtaer washed with

acid at this stage and the process be repeated again,

formation resumes its progress and the bath voltage "tsreaches 400V, while the current density will become r3 mAl cm2.

Fig.2 shows the static characteristics of the

sample thus obtained. The curve1represents the case

where it is not washed with acid and the curve II is

the case of its being washed with acid. But, these

characteristics can further be improved, if anodization

is made to take place in fused salt.

For that purpose, sodium nitrate is used for the

electrolyte. First, the curreftt fiow with density of

70mA/cm2 is turned on and the bath voltage goes

up, as the current decreases. After a few minutes,

it so decreases as one tenth of its initial value. Upon

the surface of titanium dioxide thus treated, the'counter

electrode is formed by baking liquid bright gold. The

static characteristics of such a sample are as followS:

the current density is about 1oo mA/cm2 with the forward

density of about 3-6mA/crn2 with the backward voltage

mixed salt of sodium nitrate and caustic alkali is used

jop

z

mo

too

v20 to

1 2 s

v

s

rle

1

ls

%'

Fig. 2. Effects of acid･treatrnent

on a titanium-oxide rectifier.

voltage of 2.5V and the current

of 25V. However, when'the

for the electrolyte, the forward

78 K. YosHIDA and T. KAwAMuRAcharacteristics are not much different from the case where the electrolyte is made of

sodium nitrate alone, whereas the backward characteristics are improved. Fig. 6 in the

following section illustrates the characteristics of the sample thus obtained. According to

the different modes of formation, the various characteristics can be obtained.

The reason why the backward characteristics improve, when such caustic alkali as

potassium hydroxide or sodium hydroxide is mixed into sodium nitrate may be accounted

for as follows: Fused potassium hydroxide or sodium hydroxide dissolves the oxide film.

As soon as the oxide film is formed by anodization, it is dissolved in part, so that the

film becomes smooth. Consequently, on the surface of titanium oxide thus treated, no local

concentration of the current will result and the backward characteristics:are improved.

ffwrt

･k

(a)

g･

ke:

(a)(c)(d)

os

ee

･wt

tl'/t"tt?wr'

x

.ewaj iee

i'ti'ee

ftx

ta

k"

(c)

{･

vl

thscg

vatsf

(b)

Fig. 3. Photo-electron-micrographs of

first oxidation only, (b) anodized inanodized in fused sodium nitrate,anodized in fused mixed salts of sodiurn

(d)

titanium-oxide surface.

ethylen glycol,

nitrate and potassium hydroxide.

Titanium Oxide Rectijier 79

Fig. 3 is the electron micrograph (exaggerated 10,OOO times) of the surface of titanium

oxide, where (a) represents the one that has undergone the first oxidization only, (b) the

one anodized in ethylen glycol mixed with ammonium phosFhate, (c) the one anodized with

sodium nitrate and (d) the one anodized with sodium nitrate mixed with potassium

hydroxide. In (d), the surface of titaniurn dioxide has become smooth and no wrinkle is

recognized any more. Therefore, it is plain that the backward voltage is greatly in-

fiuenced by the smoothness of the surface of titanium dioxide.

(4) Counter electrod'e.

The greatest feature of the titanium oxide rectifier, however, is that it can stand well

against the elevated temperature. Therefore, when making the counter electrode, it does

not make sense to use such an alloy of low melting point as used in the selenium rectifier.

At least, it should be a metal whose melting point is above 3000C. The surface of titanium

oxide is suMciently smooth and hard. Consequently, when the counter electrode is made

by means of metal spraying, it is inevitablethat adhesion between the sprayed metal and

the surface of titanium oxide is weak. However, if the gold film is formed first on the

surface of titanium oxide by baking liquid bright gold upon it and then a metal is sprayed

further upon it, adhesion becomes strong and the above mentioned defect can be avoided.

But, inasrnuch as the baked gold is extremely thin, resistance along the surface of gold

is so great that the current tends to localize, if baked that way only and nothing is added

thereupon. Besides, heat conduction deteriorates and local over heating is liable to occur

because of thin gold. Therefore, it is intended by metal spraying to supplement the

thiness of baked gold. However, in place of metal spraying, electric plating may also

be used.

ilL Static Characteristics

As is the case with the usual metal rectifiers, the various characteristics of titanium

oxide rectifier may be obtained depending on -%mtthe different treatments of the materials. iii(1) The case of the first oxidization only.

If the first oxidization is made at varying ,temperatures and for different times, the

various characteristics are obtained as show in

Fig. 4, where the curve 1 represents the char-

acteristics obtained by heating the titanium

plate at 7500C for 2hours and then again at

9000C for lhour, the curve ll shows those vobtained by heating at 8000C for 1 hour first

and then at 9000C for 1hour, the curve III is

for those resultant from heating at 8000C for

1hour and then at 9000C for 2hours and at Fig. 4. Effects of first oxidation on alast at 1,OOOOC for 10 minutes, while the curve titanium rectifier.

toe

v2ele

so.

.

rv

123

so1

I Ym

80 K. YosHmA

l:2:t2 l l

tloe･

so

vse2ole

.

'

'

'

f234

vsFig. 5. Effects

ethylen-glycol

of anodization in solution of

on a titanium-oxide rectifier.

va i 11

':l.I

lao

5a

v60SV4tl3o2oIO

:,

'

tt

'J''''''

tt'

l

12jvtmaem2

m il

Se

Fig. 6. Effects of anodization in fused sodium nitrate on a titanium-oxide rectifier.

ezlt

and

tt

t.

I

I

toe

so

v

20looseeo4a?ou

I

l23

v1Fig. 7. Static characteristics of

oxide rectifiers manufactured best conditions.

'Ttshi

titaninm-

under the

T. KAwAMURA

IV illustrates those obtained after heating

at 9000C for 2hours and then at,1,OOOOC

for 40minutes respectively. The electrode

is made by pressing down lead upon the

titanium plate. If treatment ismade at tem-

perature above 1,1000C, the t-itanium plate

will perfectly be oxidized and becomes

almost insulating.

(2) Anodization in glycol solution.

Fig. 5 shows the characteristics of the

sample with the counter electrode made by

baking liquid bright gold upon it after its

second oxidization is made in the solution of

ethylen glycol and ammonium phosphate,

where the curves 1 and II respectively shew

the formation made in half an hour with the

bath voltage 200V and 300V. At the end

of each formation, the electric density be-

comes 1-2mA/cm2.

(3) Anodization in fused salt.

Fig. 6 shows the characteristics of the

sample with the counter electrode made by

baking liquid bright gold upon it after its

second oxidization is made in fused salt. The

dotted line represents the characteristics of

the selenium rectifier, while the curve II is

the one resembling them and the curve III

shows the results obtained, when the reverse

voltage is made approximately twice as much

as that of the selenium rectifier. The char-

acteristics of the titanium oxide rectifier in

this instance are almost equal to two sele-

nium rectifiers connected in series. The

curve 1, on the other hand, shows the result

obtained by laying stress on the forward

characteristics in neglect of the backward

ones. Fig. 7 showsthe characteristics of the

titanium oxide rectifier made under the

optimum conditions, where the curve 1 places

emphasis upon the forward characteristics,

Titanium Oxide Rectijier 81

whereas the curve II lays stress on the backward characteristics respectively. These sam-

ples have broken each time either with 110V or 130V.

IV. Dynamic Characteristics

When the sample (the electrode area ef O.3 cm2) of Fig. 6 and II is made to operate

under the conditions of resistance load and half wave arrangement with the load current

of 60mA, 100mA, 150mA and 300mA respectively, the current and voltage wave forms

appear as shown in Figs. 8, 9, 10 and 11. In these diagrams, (a), (b), (c) and (d)

represent the cases where the A.C. potential of 15V, 30V, 45V and 60V is applied

respectively. All the values of the current and voltage shown there are their maximum

ones. With the current of 60mA, the backward current will be almost zero up to 45V

of the applied potential, while it is always perceived up to 30V with 100mA, up to 15V

with 150mA and for any potential with 300mA respectively. After all, the backward

current is more strongly influenced by the load current than the potential. This point is

hard to explain.

et . ' ' tt

ta) lsv cb) 3ev Ca) i5v (b) 3ov eC `' e,

ee. e t ' ' ' ' ' ' ' ' ' ' ' (c) 4sv (d) 60v ' . (c) 4sv (d) sou FiftnitO,'iiXSV:sfoOrmMAS OioaV8itcaugre.;efitanadt CtUhr5

Fig. 8. Wave forms of voltage,eand cur- applied voltage as indicated. rent, i of 60 rnA load current at the ' applied voltage as indicated,

d- i

Ca) lsv itb} 3ov (a) tSv <b) 3ov

e ',.

(c) 4sv (a)Fig. 9. Wave forms of voltage, e

rent, i of 100 rnA load current applied voltage as indicated.

s

60v

and cur-

at theFig. 11.

rent, i applied

(C) 4SV

Wave forms of voltage, e

of 300mA load currentvoltage as indicated.

and cur-

at the

82 K. YosmDA and T. KAwAMuRA V. Temperature Characteristics

Generally, the metal rectifiers' show the tendency that, whenever temperature goes up,

both the backward and forward currents increase, while the puncture voltage drops.

With the titanium oxide rectifier, however, the higher the ambient temperature, the larger

the forward current, whereas the backward current shows only a small change. This is

one of the unique features of this titanium oxide rectifier, making it possible to stand

well against the elevated temperature or the overload.

Fig. 12 shows the temperature characteristics of the titanium oxide rectifier with the

electrode made by backing liquid bright gold

MA on the titanium oxide plate after its anodiza- so tion in mixed salt. D.C. potential has been

G applied, as indicated in the diagram, to theotrsov"Ng-oh

-ao::vv-NgMoarc

2S

aS

1,o･

wt

Fig. 12. Effects of temparature change on backward and forward currents of a titanium oxide rectifier at the applied voltage as indicated.

ts

'

"N.St,o

e; 2eO3co4ca ･sw

' t.rov'e

'elbeo'

'rh...'

,s'

}l''k'

2oeC 2ov lo mA 400'C 20V letnA

j2oec 2ov lomA

2000C 20V JOmA

3eoOc 2ov jomA

Fig. 13. 0scillograms

of 10mA at the indicated.

4SO'C 20V to,n,n

soo'C 2ov romA

of load current

temperatures as

(℃"j

sample that has the electrode area of O.2cm2

and both the backward and forward currents

have been measured as the temperature went

up from OOC to 5000C. It is plain from it

that the forward current sharply increases, as

the temperature goes up, while the backward

current shows the tendency to remain almost

stable up to 1000C and to reach its minimum

value at 2000C, but, sharply increasing at any

temperature higher than 2000C.

Fig. 13 shows the results of measuring

the dynamic characteristics at the same time

as the above-mentioned experiment. This

rneasurement has been made with the applied

voltage of 20V and the current of 10mA,

while an effort has been made to prevent the

temperature from rising on account of the

!oad current. It is found that the wave forms

of the rectified current is normal up to 4000C.

But, it begins to deteriorate at 4500C. If fur-

ther heated up to 5000C, however, the for-

ward resistance gradually increases and no

use can further be made of the rectifier.

Presumably, the advanced oxidization of the

surface of the rectifier may be responsible for

it. As for deterioration caused by the tem-

perature rise, a description is made in the

later section.

In any case, the fact, that even the silicon

' ' TVlaninm Oxide Rectijier ' 83

rectifier, whose temperature characteristics are considered the best among the various

rectifiers, can boast of the allowable maximum temperature of 1200C only, speaks eloquently

for the marked superiority of the temperature characteristics of the titanium oxide rectifier.

VI. Deterioration

' In or.der to measure deterioration resultant from the varying temperature, the sample

is heated up to 1000C and then cooled down to 200C. Again it is heated up to 2000C

and then cooled down to 200C. In a like manner, it is repeatedly heated up to 3000C,4ooOC and soOOC in turh.

Fig. 14 shows the static characteristics

observed at 200C in each cycle of the above

changes in temperature. Both the backward

and forward characteristics change only

slightly up to 3000C. At the same time, the

dynamic characteristic have been measured

and the results are shown in Fig. 15. Asa

results, it may be concluded that at 2000C the

characteristics reach their lx)st conditions and

no deterioration may likely to occur up to

3000C.

As for the long-time deterioration, how-

ever, no definite conclusion can be made yet,

because of its trial make having being com-

pleted only a short time ago. But, in view

of tlie chemical stability of titanium oxide, its

deterioration is supposed to be small. Until

the present, it has been left alone for 6

months, but, with no sign of deterioration yet.'

On the other hand, deterioration due to

humidity may well be considered nil, as might

be judged with good reasons from its manu-

facturing process. Contrary to supposition,

the forward current tends to increase in high

humidity, while the backward current remark-

ably decreases. For instance, when a small

volume of water or liquid sodium hydroxide

is painted on the surface of titanium oxide,

both the backward and forward characteristics

improve, but, they again return to their initial

VII.

By means of the second oxidization, area

I 200CmA

Hloooc.2oec so "rri

m2ooec-2oocIV3000C.200C

'

V4000C-200C v,

VI5000C-200C pt

so4e Ja2o

,2S

le

r,rv I2 3v

'

i''

v,nLO

states

Conclusion

contact

inAFig. 14. Static characteristics of a titanium-

oxide rectifier at the indicated cycles of･ temperature.

--.-t- --. -:

mo'c 2ete lr-t--------------T----"--t----it------N--tl4

L.- -.---- T-1 2oo'C 2oic 1l-"'-'-J"-'-'- T---'-.'..--.-..--.-.`--.---II

:

L.- "..- "- 3ca `C 20'C i/-""-""-''"'---''--""''-'`-'`''-""'---'1:i･

L-. -...-- ---; uaoOc 20'C l1-----"----m-------t------"------------------tl,

L..-

sve'c

Fig. 15. 0scillograms of load current at the indicated cycles of temperature.

after drying.

has become practical for the titanium

84 K. YosHmA and T. KAwAMuRAoxide rectifier, so that it may be put to the various practical uses. Further, by means of

anodization in fused salt, the characteristics have been so greatly improved that they can

rival those of the selenium rectifier. Besides, its manufacturing process is so simple that

it is suitable for a mass production. Indeed, in consideration of the fact that the satis-

factory characteristics have been attained despite of the purity of titanium oxide being

same as that of the materials obtainable in the commodity market, it is expected that its

manufacture will soon be started. At present, success has been made only in its trial

make which is as small as1cm2. But, it is true that, with the improved purity of the

material metal and the enlarged area of the sample, the far better backward characteristics

will become obtainable in future.

Its backward voltage is twice as great as the selemium rectifier. In addition, the

allowable current density is several times as great. And, if for the sarne capacity, the

measurement of its structure is expected to be far smaller than the selenium rectifier,

But, the detailed experiment in this connection has not been made yet.

The study on the titanium oxide rectifier has been started only some time ago, and

there is a variety of the problems to be faced. Among them, first and foremost, the study･

should be aimed at the improved purity of titanium oxide, the addition of activators and

the rectifying mechanism etc. According as the answers are found for them, its marked

eMciency will further be irnproved.

Literature

1) R. Breckenridge and W. Hosler: Journal of Research of the Bureau of Standard Vol. 49 (1952).

2) H. Corton, T. Shilliday and F. Eggleston: Electrical Engineering Vol. 74 No. 10 (1955).