31

ジルコニア微粉末の焼結挙動に及ぼす添加物の効果：
アルミナとシリカ添加の比較
松

井

光

二

Effects of additives on sintering behavior of fine zirconia powder: comparison between alumina and silica additions
Koji MATSUIThe sintering behavior of 3 mol％ Y2O3−doped ZrO2 powders with and without a small amount of Al2O3 or SiO2
was investigated to clarify the difference in the effects of their additives on the initial sintering stage. The
shrinkage behavior of powder compact was measured under constant rates of heating (CRH). The sintering rate
remarkably increased by a small amount of Al2O3 or SiO2 addition, and the enhanced sintering effect by Al2O3
addition was greater than that by SiO2 one. The apparent activation energy (nQ) and apparent frequency factor(βn0 ), where the n is the order depending on diffusion mechanism, were estimated at the initial sintering stage byapplying the sintering−rate equation to the CRH data. The diffusion mechanism changed from grain−boundary
diffusion to volume diffusions by Al2O3 or SiO2 addition. Both nQ and βn0 increased in the order of Al2O3 ＞ SiO2
＞ no additions. It is, therefore, concluded that the sintering rate increases in the order of Al2O3 ＞ SiO2 ＞ noadditions because the increase in βn0 rather than nQ is predominant.

rate equations that is applicable to shrinkage data

1．Introduction

measured under constant rates of heating (CRH), andDue to its excellent mechanical properties (i.e., highestimated the activation energy for causing diffusion atstrength and fracture toughness), Y 2 O 3 −stabilized

the initial sintering stage in Y2O3−stabilized ZrO2. Wang

tetragonal ZrO2 polycrystal (Y−TZP) has become an

and Raj2), 3) have investigated the densification behavior

important structural ceramic, finding use in optical fiberin Al2O3, ZrO2/Al2O3 composite, and ZrO2 containing

connectors, grinding media, and precision parts. The

Y 2O 3 by the CRH method. Thus far, the author hasmechanical properties strongly depend on the

kinetically investigated the initial sintering mechanism

microstructure in Y−TZP, which can be controlled byin Y 2O 3−doped ZrO 2 powders with and without 0.25

applying the sintering-acceleration effect of additive.

mass％ Al2O3, and clarified the following points. 4)-8)

The appearance of high−performance Y−TZP for newi) Al2O3 effect: Al2O3 changes the diffusion mechanism

application is desired to spread the zirconia−productfrom grain-boundary (GBD) to volume diffusions

market. To develop high-performance Y−TZP powder, it(VD) at the initial sintering stage.4)-6) As a result, theis needed to clarify the effect of various additives on thedensification is enhanced because the apparent

sintering mechanism of Y−TZP powder.

frequency-factor term in the rate constantincreases.6)

The initial sintering mechanism of various ceramic
powders has been investigated extensively.1)−14) For

ii) Specific surface−area effect: when the specificinstance, Young and Cutler1) have derived sintering−

surface area in Y2O3−doped ZrO2 powder increases,（ 31 ）

32

TOSOH Research ＆ Technology Review Vol.52（2008）

the sintering rate is enhanced because of anincrease in frequency-factor term.

7)

powder and colloidal SiO 2 or Al 2 O 3 powder with avibration mill. The atomic ratios of Al/(Zr＋Y) andiii) Y 2 O 3 concentration effect: the increase in Y 2 O 3

Si/(Zr＋Y) of these powders are 6×10−3 and 5×10−3,

concentration decreases the sintering rate because

respectively, which are roughly equal. For comparison

8)with the sintering behavior of 3YA and 3YC, the Y−TZP

of increasing the activation energy of diffusion.

Furthermore, the author also proposed an analyticalpowder without Al2O3 or SiO2 (3Y) was also treated bymethod that can experimentally determine the diffusion

wet−milling under the same process.

mechanism at the initial sintering stage using CRH

These powders were pressed uniaxially into a disk

data, and demonstrated the validity of the analyticalunder 70 MPa. The powder compacts were ca. 25mm

method using Y 2O 3−doped ZrO 2 powders with and×4mm in size, and sintered at 1100°−1500℃ for 2 h in4),5)

without 0.25 mass％ Al2O3.

air (heating rate 100℃/h).

From the industrial standpoint, it is important toclarify the roles of various additives which accelerate［2］Density and grain-size measurements

sintering in Y−TZP. Usually, not only Al 2O3 but also

The density of sintered bodies was measured using

SiO2 is well used as one of the additives. As described

the Archimedes method for the relative densities of ＞＿

in the above-mentioned, the effect of Al2O3 on the initial

80 ％, whereas calculated from the weight and the size

sintering stage in Y−TZP powder has been reported infor the relative densities of ＜ 80 ％. A scanning electron

previous papers.

4),6)-8)superplasticity

behavior

For the effect of SiO 2 , themicrostructure

Japan) was used to observe the microstructure anddevelopment in SiO 2 − doped Y− TZP has been

estimate the average grain sizes of sintered bodies.

15)-18)andmicroscopy (SEM; Model S−4500, Hitachi, Tokyo,

However, the effect of SiO2

SEM specimens were polished with a diamond paste ofaddition on the initial sintering stage of Y−TZP powder

3 μm, and then thermally etched in air for 1 h at a 50℃

has not been reported in the previous papers.

lower temperature than the sintering temperature of

Furthermore, although Al2O3 and SiO2 are well used aseach specimen. The average grain size was measured

additives which enhance the sintering rate in Y−TZP

by the Planimetric method.19)

investigated previously.

powder, the difference in their effects is not［3］CRH measurements

quantitatively discussed.
In the present study, Al2O3 and SiO2 as additives were

The powders were pressed uniaxially into a

noted, and the initial sintering behavior of Y−TZP

cylindrical disk under ca . 100 MPa and afterwards

powders with and without Al2O3 or SiO2 was analyzed

pressed isostatically at 200 MPa by cold isostaticby the CRH method.

pressing. The specimen was 6 mm

The difference in the enhanced

×15 mm. The

sintering effect of these additives was quantitatively

shrinkage of the powder compacts obtained withdiscussed based on the present analytical results.

sintering was measured using a dilatometer (Model DL
9700, ULVAC−RIKO, Yokohama, Japan). The shrinkage
measurements by the CRH technique were performed

2．Experimental procedurein the range from room temperature to 1500℃ at［1］Specimen preparation

heating rates of 5°,10°,15°,and 20℃/min in air. When

3 mol％ (5.2 mass％) Y−TZP powder with a specific2

the temperature reached 1500℃, the specimens were

surface area of 15 m /g (TZ−3Y grade, Tosoh, Tokyo,

cooled at a constant rate. The dilatometer was

Japan), colloidal SiO 2 (Snowtex, Nissan Chemical

calibrated using sapphire as a standard specimen.

Industries, Tokyo, Japan) with a particle size in the

Thermal expansion of each specimen was corrected byrange of 10−20 nm, and fine Al2O3 powder (Alu C grade,the same method in the previous paper. 4） It wasTokyo, Nippon Aerosil, Japan) with a specific surface

confirmed that the shrinkage proceeded isotropicallyarea of ca. 100 m /g were used as starting materials.

because for each measured specimen, the shrinkage

The Y−TZP powder with 0.25 mass％ Al2O3 (3YA) or

percentage of length axis was nearly equal to that of

SiO2 (3YC) was prepared by wet−milling the Y−TZP

diameter axis. Assuming isotropic shrinkage to powder2（ 32 ）

東ソー研究・技術報告 第52巻（2008）

compact, the density ρ(T) at a given temperature T is2)

Lfequation that is different from the type of Eq. (3) is also
derived from Eq. (2).5)

given by the following equation :d(ΔL/L 0) Kγs ΩD0R n nQ
nQ
＝ ka pcQ ・ RT 2 −n ・exp − RTdT

3

（ L（T））ρ

ρ（ T ）＝

33

(

（1）

)(

)

(

) （6）

where L and L(T) are the final length and the length atHere, c is the heating rate (i.e., c=dT/dt). For n=1/3

a T of the specimen, respectively. ρ indicates the final

and n =1/2, Eq. (6) corresponds to the sintering

density measured by the Archimedes method. The

equations derived by Young and Cutler.1) This rate

ρ(T) at a T was calculated using Eq. (1).

equation is also applied to CRH data.

S2 of the Arrhenius−type plot of ln[T［4］Analysis of initial sintering mechanism

expressed asby the same analytical method as the previous paper.

5)

The sintering-rate equation at the initial sintering stage
9)

△L
0

Here, the n is in the range of 0.31−0.50.

10)

In theprevious paper, the author has reported that the valuesof nQ in 3 mol ％ (5.3 mass％) Y−TZP powder with a

is given by the following equation :Kγs ΩD［（ L ） ］＝ kTa（7）nQ ＝−RS 2

diffusion at the initial sintering stage were determinedddtd(ΔL/L0)/dT ]against 1/ T , the apparent activation energy ( nQ ) is

The material−transport path and activation energy of

1/nUsing the slope
2−n

（2）

p

specific surface area of 6.7 m 2/g determined from
Arrhenius−type plots that correspond to n=1/3(=0.33)

where Δ L (＝ L 0 −L ) is the change in length of theand n =1/2(=0.50) were 219 and 217 kJ/mol,

specimen, L0 the initial length of the specimen, K therespectively.4) This result suggests that the nQ is nearlynumerical constant, γS the surface energy, Ω thethe same value in the n range of 0.31−0.50. Therefore,

atomic volume, D the diffusion coefficient, t the time, T

the S2 can be estimated as the approximate value fromabsolute temperature, k the Boltzmann s constant, a thethe Arrhenius−type plot of ln[ T 1.6 d(Δ L / L 0 )/d T ]particle radius, and the parameters of n and p the order

against 1/T that corresponds to n=0.4 that is the central

depending on the diffusion mechanism. The values of pvalue of n=0.31−0.50.5) Combining Eqs.(5) and (7), thefor GBD and VD are 4 and 3, respectively. The value offollowing equation is obtained5):

K includes the effective grain−boundary width innQ
S2n＝ Q ＝ S

GBD. 10) Equation (2) is applicable to the fractional（8）1

shrinkages of ＜ 4％ that satisfy the initial sintering

In the present analysis, the Q and n were determined in

condition.

the fractional shrinkage range of ＜4 ％ that satisfies

The following sintering−rate equation that is
applicable to CRH data is derived from Eq. (2).
dT

dρ

Q

［ （ dt （） dT ）］＝− RT ＋α（n, p）

ln T

5)

the initial sintering condition as with previous
papers.4),5) In previous papers, the author analyzed the（3）isothermal shrinkage behavior of Y−TZP powders withand without a small amount of Al2O3, and showed thatthe logarithm shrinkage−logarithm time (log−log) plots

whereKγ sΩD 0［ kα（n, p）＝ ln［ （ρ, n）］＋ ln］− p lna （4）in the shrinkage range of ＜ 4％ measured at various
constant

temperatures

becamethelinearHere, Q is the activation energy, R the gas constant,relationships.6)-8) At the initial sintering process withf (ρ, n ) the density function that depends on theparticle growth, it has been reported that the log−log

n , and D 0 the pre−exponential term defined asplot show no linear relationship. 11) Therefore, theD＝D0exp(−Q/RT ). Equation (3) corresponds to the

influence of particle growth on the Q and nQ is

sintering−rate equation derived by Wang and Raj.

2)

negligibly small in the shrinkage range of ＜ 4 ％.

Using the slope S 1 of the Arrhenius−type plot ofln[T(dT/dt)(dρ/dT )] against 1/T at the same density,
the Q is expressed as

Q ＝−RS 1（5）

3．Results［1］Densification and grain growth

On the other hand, the following sintering−rate（ 33 ）

Figure 1 shows the change of relative density in 3Y34

TOSOH Research ＆ Technology Review Vol.52（2008）

Relative density［％］

100

3Y

90

80

70

60

50
1100

1200

1300

1400

3YA

1500

Sintering temperature［℃］

Fig.１

Changes of relative density and average grain sizein 3Y, 3YA, and 3YC with sintering temperature.
The duration time at each sintering temperature is
2 h. (○), (□), and (△) indicate 3Y, 3YA, and 3YC,
respectively.

(without additive), 3YA (with 0.25 mass％ Al2O3), and
3YC (with 0.25 mass％

SiO 2 ) with sintering

3YC

temperature. The relative density of 3YC was higher
than that of 3Y at lower temperatures. The relative
density of 3YA was higher than that of 3YC at lower
temperatures, and attained 99.5 ％ at 1350℃. This
result reveals that the densification of Y−TZP powder isaccelerated by Al 2 O 3 or SiO 2 addition, and the
0.5μm

acceleration effect by Al2O3 addition is greater than thatby SiO 2 one. Figure 2 shows SEM images of theFig.２

polished and etched surfaces of 3Y, 3YA, and 3YC
sintered at 1500℃. The average grain sizes of 3Y, 3YA,

Scanning electron microscopy images of 3Y, 3YA,
and 3YC sintered at 1500℃. The duration time ateach sintering temperature is 2 h.

and 3YC were nearly equal, and in the range of 0.5−0.6
μm. This result suggests that at the sintering−hold［2］Densification behavior

time of 2h, the effect of Al2O3 or SiO2 addition did notappear for the grain−growth process.

Figure 3 shows the changes of the shrinkage of 3Y,
3YA, and 3YC with temperature in the course of 10℃/min heating. As can be seen in Fig. 3(a), the starting

5

100
（a）

−5
−10
−15

0.25

80
70
60
50

−20

900 1000 1100 1200 1300 1400 1500
Temperature［℃］

0.20
0.15
0.10
0.05

40

−25

Fig.３

（c）

90
dρ／dＴ［℃−1］

Relative density［％］

Expansion［％］

0

0.30
（b）

0.00
900 1000 1100 1200 1300 1400 1500
Temperature［℃］

900 1000 1100 1200 1300 1400 1500
Temperature［℃］

Temperature dependence of shrinkage, relative density (ρ), and densification rate (dρ/dT) of 3Y, 3YA, and 3YC in the courseof heating (10℃/min). (○), (□), and (△) indicate 3Y, 3YA, and 3YC, respectively.

（ 34 ）

東ソー研究・技術報告 第52巻（2008）35

temperature of 3Y, 3YA, and 3YC is nearly equal, andAl2O3 and SiO2 additions. The Q, nQ, and n of diffusion

the shrinkage increased in the order of 3YA ＞ 3YC ＞

at the initial sintering stage in 3YC can be estimated by

3Y at temperatures of ＞ 〜1100℃. Using the shrinkage

applying Eqs. (3), (6), and (8) to the results in Figs. 3

curves in Fig.3(a), the temperature changes of theand 4. Equation (3) is applied in the following way. For

relative density (ρ) and the densification rate (dρ/dT )each heating rate (dT/dt), both T and dρ/dT at thewere determined by Eq.(1) (Figs.3(b) and (c)). The ρ

same ρ were determined, and their values were

was high in the order of 3YA ＞ 3YC ＞ 3Y atplotted as T(dT/dt)(dρ/dT ) against 1/T (Fig. 5). Here,temperatures of ＞ 〜1100℃. The dρ/dT increased inthis analysis was performed in the ρ range of ＜＿ 54 ％

the order of 3YA ＞ 3YC ＞ 3Y when the temperaturethat corresponds to the fractional shrinkage range of ＜exceeded 〜1100℃, and the temperature of peak

4 ％. The plot at each ρ showed the linear relation

maximum of the dρ/dT curve became low in the orderbecause the values of correlation coefficient were in theof 3YA ＜ 3YC ＜ 3Y. The heating−rate dependence ofrange of −0.95 to −1.0. The Q at each ρ was determineddρ/d T in each specimen was also examined in thefrom the slope of the straight line (Fig. 6). The average

range of 5°−20℃/min. For all specimens, the dρ/dTcurves shifted to a higher temperature, as the heating

8
In｛T（dT/d t ）（dρ/dT ）［K/min］｝

rate increased. As an example, the heating−ratedependence of 3YC is shown in Fig. 4. The present
results reveal that Al2O3 or SiO2 addition enhances thedρ/dT, and the enhanced effect by Al2O3 addition isgreater than that by SiO2 one.
［3］CRH shrinkage analysis
In previous papers, the author reported the initialsintering mechanisms in Y−TZP powders with and

7

6

5

without 0.25 mass％ Al2O3.4),6)-8) However, the initial

6.8

7.0

has not been reported previously. Therefore, the initial

7.2

7.4

104/T［K−1］sintering mechanism of 3YC (with 0.25 mass％ SiO2)
Fig.５

sintering behavior of 3YC is analyzed to clarify thedifference between the sintering-acceleration effects by

Arrhenius−type plots of 3YC for the estimate ofactivation energies of sintering. (○), (□), (△), (◇),
and (●) represent 50％, 51％, 52％, 53％, and 54％
relative densities, respectively. The data of 50.5％,
51.5％, 52.5％, and 53.5％ relative densities were also
obtained in a similar manner.

0.20

1000

0.15

800

Activation energy［J/mol］

dρ／dT ［℃−1］

0.25

0.10

0.05

0.00
900

1000

1100

1200

1300

1400

400

200

1500

Temperature［℃］

Fig.４

600

0

Temperature dependence of densification rate (dρ/dT )of 3YC at various heating rates. (○), (□), and (△)
represent 5°, 10°,and 20℃/min heating rates. The datameasured at a 15℃/min heating rate were also obtainedin a similar manner.

50

51

52

53

54

Relative density［％］Fig.６（ 35 ）

Activation energies for diffusion of 3YC in the relative
density of 50-54％.

36

TOSOH Research ＆ Technology Review Vol.52（2008）

value of Q in this ρ range was 579 kJ/mol (standard

order of 3YC is n=1/2. Using each value of n, Eq. (9),deviation (σ)=28 kJ/mol). On the other hand, Eq.

and the shrinkage data (of < 4 ％) of 5−20℃/min(6) is applied in the following way. In the fractional

heating rates, the average values of Q were determinedshrinkage range of ＜4 ％, the nQ was determinedfrom the slopes of the straight lines in the plots offrom the slope of the straight line in the plot ofln[T 5/3d(ΔL/L0)/dT ] against 1/T for 3Y (n=1/3) andln[T 1.6d(ΔL/L0)/dT ] against 1/T using the shrinkageln[T 3/2d(ΔL/L0)/dT ] against 1/T for 3YA and 3YC

curve of each dT/dt. As an example, the plot at a dT/dt(n=1/2). Each β0 was determined from the intercept ofof 10℃/min is shown in Fig.5. It is seen that the plot

the plot using the determined Q. As an example, eachshows the linear relation. The linear relations wereplot at a 10℃/min heating rate is shown in Fig. 8.

obtained at all plots of 5−20℃/min heating rates.

Here, although the Q of 3YC has been already

Determining the average value of nQ, 280 kJ/mol (σ=6determined from the result in Fig. 6, it was re−kJ/mol) was obtained. Using the average values of both

estimated using Eq. (9) to strictly comparable with Qs

Q and nQ, the n was determined by Eq. (8), which wasof 3Y and 3YA. The obtained results are shown in Table

n =0.5(=1/2). According to two−sphere shrinkage

１. The Q increased in the order of 3Y ＞ 3YA ＞ 3YC.

models, the n ranges of GBD and VD are 0.31−0.33 andSimilarly, the β0 also increased in the order of 3Y ＞

10)

3YA ＞ 3YC. In previous papers, the author determined

0.40−0.50, respectively.

Therefore, the diffusion

mechanism of 3YC was assigned to VD. This diffusion
mechanism (i.e., n =1/2) agreed with that of Al 2O 3

5ln｛T 1.6 d（ΔL /L 0）/dT［K0.6］｝

addition in previous papers.4)-8)
To clarify the difference between Al 2O 3 and SiO 2
effects on the sintering rate, the Q and frequency
factor (that is defined by the following β0) of eachspecimen are estimated using Eq. (6).

Putting as

β0＝(KγSΩD0)/(kaP), Eq. (6) is expressed bynd(ΔL/L 0)RnQnQ＝β0n・ cQ ・ RT 2 −n ・exp − RT
dT

( )(

)

(

) （9）

4

3

2

1

0

In previous papers, the author reported that the orders−1

depending on the diffusion mechanism of Y−TZP

6.8

7.2

powder with and without 0.25 mass％ Al2O3 were n=1/2
and n=1/3, respectively,4),6-8) which can be applied as

Fig.７

the orders for 3Y and 3YA. In the above result, theln｛T 3/2d（ΔL/ L 0）/d T［K1/2］｝

ln｛T 5/3 d（ΔL /L 0）/d T［K2/3］｝

8.0

4

3

2

1

0

3

2

1

0

−1
6.8

7.2

7.6

8.0

8.4

Plot of ln［T 1.6 d(ΔL / L0 )/dT ］against 1/T of 3YC
in the course of heating (10℃/min).

4

5

8.4

104/T［K−1］

Fig.８

7.6
104/T［K−1］

6.8

7.2

7.6

8.0

8.4

104/T［K−1］

Plots of ln［ T 5／3d（ΔL ／L 0）／dT ］against 1/T in 3Y and ln［T 3／2d（ΔL ／L 0）／dT ］against 1/T in 3YC
in the course of heating (10℃/min). (○), (□), and (△) indicate 3Y, 3YA, and 3YC, respectively.

（ 36 ）

東ソー研究・技術報告 第52巻（2008）

TableⅠ

Apparent activation energies, apparent frequency factors, activation energies, and frequency factors for
diffusion at initial sintering stage
Activation energy＊Specimen

37

Q（kJ/mol）

Standard
deviation

Frequency factor＊ln［β0,（s−1）］

Apparent activationenergyStandard
deviationnQ（kJ/mol）

Standard
deviation

Apparent frequenecy
factorln［β0n,（s−n ）］Standard
deviation

17
6
0.5
1.5
681
227
17.1
3Y
51.4
4
0.3
0.6
589
7
295
23.0
3YA
46.1
6
0.5
1.1
558
13
279
21.5
3YC
43.0
＊
The values of Q and lnβ0 were calculated from the values of nQ and lnβ0n using n =1/3 for 3Y and n =1/2 for 3YA and 3YC.

the values of Q for sintering of 3 mol％ (5.1−5.3

According to Eq. (10), the magnitude of n should be

mass％) Y−TZP powders by the CRH and isothermal

considered when the diffusion mechanism changes.

shrinkage analyses to be 535−595 with 0.25 mass％

The nQ and apparent frequency factor (β0n ) were

Al 2 O 3 and 647− 694 kJ/mol without Al 2 O 3 . 4),6-8)

determined using the above Q and β0, respectively

Furthermore, the author also reported that the values(TableⅠ). Both nQ and β0n increase in the order ofof ln[β0 , (s −1 )] of 3Y and 3YA determined by the

3YA ＞ 3YC ＞ 3Y. As can be seen from the result in

isothermal shrinkage analysis were 48.2 and 45.7,

Fig. 3, when the temperature exceeded 〜1100℃, therespectively.6),8) Compared with these values, the values

sintering rate increases in the order of 3YA ＞ 3YC ＞

of Q and β0 in 3Y, 3YA, and 3YC determined by the

3Y. It is, therefore, concluded that the sintering ratepresent analytical method are considered to be

increases in the order of 3YA ＞ 3YC ＞ 3Y atreasonable. It is, therefore, clarified that the difference

temperatures of ＞ 〜1100℃ because the increase in β0nin the sintering rate by Al2O3 and SiO2 additions occurs

rather than nQ is predominant. Thus, the sintering rateby differences in both Q and β0 at the initial sintering

is controlled by not only the Q and β0 but also the n.

stage.
［2］Mechanism of GBD→VD change by additive
In the previous paper, the author reported that the

4．Discussion

mechanism of GBD→VD change by Al2O3 addition isreasonably explained as follows.6) Added Al2O3 particles［1］Effects of additives on the sintering rates
Based on the kinetic analysis, the effects of Al2O3 andbegin to dissolve in Y−TZP particles with neckSiO 2 additions on the initial sintering stage areformation and growth, and a part of the dissolved Al3＋

discussed as follows. The integral type of Eq. (6) isions segregates at grain boundaries formed between Y−

5)

TZP particles. When Al 3＋ ions segregate at grain

given by the following equation.
nKγΩD RnQ( ΔLL ) ＝( ka cQ ) T exp (− RT )RTnQ
＝β ( cQ ) exp(− RT )
s

0

n
0

p

0

boundaries, the grain− boundary energy (γ GB )ndecreases, which is led to the change of diffusion

n

（10）

mechanism from GBD to VD.

Under the same diffusion mechanism (namely, a

The similar discussion is available for the GBD→VD

constant n), the decrease in β0 under a constant Q andchange by SiO2 addition. Ikuhara et al.15) have reported

T or the increase in Q under a constant β0 and T leads

that the dynamic behavior with heating of 2.5 mol％ Y−

to the decrease in the ΔL/L0, which corresponds to the

TZP particles (Tosoh) to which SiO2 particles with thedecrease in the sintering rate. As shown in Fig. 3, thediameter of 10 nm (Nissan Chemical Industries) was

shrinkage, ρ, and dρ/dT of 3YA were greater than that

added was observed using in − situ observationof 3YC when the temperature exceeded 〜1100℃. As

technique by a transmission electron microscopy, andcan be seen the result in TableⅠ, both Q and β0 ofmost of the SiO 2 particles in the mixed state were

3YA are greater than those of 3YC. It is, therefore,

adsorbed into the ZrO2 grains at 〜1300℃. Furthermore,

concluded that the sintering rate of 3YA increases morethey showed that no amorphous layer exists along thethan that of 3YC at temperatures of ＞〜1100℃ because

grain−boundary faces in SiO2−doped Y−TZP, but Y3＋

the increase in β0 rather than Q is predominant.

and Si4＋ ions segregate at grain boundaries over widths（ 37 ）

38

TOSOH Research ＆ Technology Review Vol.52（2008）of 4−6 and 5−8 nm, respectively.16) Taking into account●●'''' ＋ 2VO
null ＝ VZrtheir observations,15),16) it is presumed that added SiO2

（13）
●●

particles begin to dissolve in Y−TZP particles with neck

This equilibrium reaction means that when theformation and growth, dissolved Si4＋ ions diffuse to Y−

concentration increases, the V ''''
Zr concentration

TZP particle interior within the range of solubility, anddecreases according to the law of mass action.

4＋

ions segregate at grain

Furthermore, when the VO concentration increases,

boundaries formed between Y−TZP particles. In the''''
because VO associates with VZrowing to coulombic

result in Fig. 1, a part of added SiO 2 has already''''attractive force, this associated VZrdoes not work for

dissolved in ZrO 2 particles at the temperature of 〜

the cation diffusion. The decrease in VZr'''' concentration

1100℃, which is consistent with their result. 15)''''
and the association between V Zr
and V O retard the

Furthermore, it has been reported that when Si4＋ ions

cation diffusion. When Al2O3 dissolves to Y−TZP, thesegregate at grain boundaries in 2.5 mol％ Y−TZP, thecation diffusion is depressed because the VO produces,

γ GB /γ S relatively decreases. 20) It is, therefore,

and the depressing effect appears as the increase in the

concluded that the diffusion mechanism changes fromQ of VD. The dissolution of SiO 2 to Y−TZP hardly

GBD to VD because the γGB decreases by the grain−influences the cation diffusion because the VO does nota part of the dissolved Si

boundary segregation of Si4＋ ions.

VO

●●

●●

●●

●●

●●

produce. Therefore, the Q of VD in Al2O3 addition islarger than that in SiO2 one. The present results reveal［3］Effects of additives on the activation energythat the Q of VD in 3YA is higher than that in 3YC

and frequency factor(TableⅠ), which is consistent with the conclusion in

Sakka et al. have reported that the cation diffusion in

the above discussion. Therefore, the Q difference of VD

the Y 2O 3− and CeO 2−(Zr 1−xHf x)O 2 systems is much

between 3YA and 3YC can be qualitatively interpreted21),22)

Therefore, theby the difference in the amount of VO that is produced

sintering rate in Y−TZP powders with and without aby the segregated dissolution of Al2O3 or SiO2 at Y−TZP

small amount of additive is controlled by the cation

grain boundaries.

smaller than the oxygen diffusion.

●●diffusion. In the previous paper, the author reported

The difference in β0 of VD between 3YA and 3YC

that the differences in both Q and β0 between 3Y and(i.e., β0 (3YA)＞β0 (3YC)) is discussed as follows.

3YA (i.e., Q(3Y)＞Q(3YA) and β0(3Y)＞β0(3YA)) can

According to the random walk theory, the D0 in thebe explained by the grain−boundary segregation of Y3＋

extrinsic region is given by the following equation.23)

ions and the difference in cation−vacancy (V ''''Zr )1
ΔS ≠D 0 ＝ 6 Bνλ2N V exp R(

concentrations between grain−boundary vicinity and

)

（14）

particle interior.6) The difference in Q of VD between

Here, B is the constant that depends on the crystal

3YA and 3YC (i.e., Q(3YA)＞Q(3YC)) is discussed as

structure, ν the frequency of atom, λ the space

follows. When Al2O3 dissolves to Y−TZP, the oxygen

between crystal planes, NV the vacancy concentration,

vacancy (VO ) is produced by the doping reaction in theand ΔS≠ the activation entropy. When the diffusion

following equation.

mechanism changes from GBD to VD by dissolution of

●●

ZrO2

×
' ＋ VO ＋ 2ZrO（11）Al2O3 ＋ 2Zr×2
Zr ＋ OO →2Al Zr●●

On the other hand, when SiO2 dissolves to Y−TZP, the''''Al 2 O 3 , the V Zr
concentration decreases (i.e., the●●decrease in NV) because the VO is formed. On the other
hand, the formation of VO causes the increase in γS.6)●●

VO is not produced as shown in the following doping-''''In the case of SiO2 addition, the VZrconcentration does

reaction equation.

not change and the γS change is small in comparison

●●

●●

ZrO2
×
SiO2 ＋ Zr ×
Zr → Si Zr ＋ ZrO2

with Al2O3 addition because the VO is not produced by（12）

the doping reaction of SiO2. Substituting Eq. (14) into

The cation diffusion proceeds primarily through cation

β0 ＝( KγS ΩD 0 )/( ka P ) and then taking β0 (3YA)/β0''''
vacancies (V Zr) of the schottky-defect type. The(3YC), one obtains

formation of schottky defect is expressed by thefollowing equilibrium reaction.
（ 38 ）

β0(3YA) 〜 γs(3YA)
β0(3YC) ＝ γs(3YC)

(

(3YA)
)( NN (3YC)
)
v

v

（15）

東ソー研究・技術報告 第52巻（2008）

39

Here, it is presumed that K, Ω, B, ν, λ, and ΔS≠ dosintering kinetics at CRH is very useful for

not vary by the difference in Al2O3 and SiO2 additions.

determination of the diffusion mechanism at the initial

The β0 of VD in 3YA is higher than that of VD in 3YC

sintering stage of ceramic powders.

(i.e., β0(3YA)/β0(3YC)＞1). The right side in Eq. (15)is the relations of γ S (3YA)/γ S (3YC)＞ 1 andNV(3YA)/NV(3YC)＜1. It is, therefore, concluded that

Acknowledgement

the β0 in 3YA becomes higher than that in 3YC

The author would like to express his sincere thanks

because the increase in γS rather than the decrease in

to Professor Junichi Hojo, Kyushu University, for his

NV by Al2O3 addition is predominant.

valuable advice in this work.

As discussed in Section IV. (1), the increase of
sintering rate by Al2O3 or SiO2 addition largely depends
on the increase of β0n, i.e., the increase in n with GBD→VD change, though the nQ also increases by Al2O3 orReferences
１）W.S.Young and I.B.Cutler, J.Am.Ceram.Soc., 53

SiO2 addition. Therefore, the kinetic process of VD isfaster than that of dominant GBD by the segregated(12), 659(1970)
２）J.Wang and R.Raj, J.Am.Ceram.Soc ., 73(5),dissolution of Al2O3 or SiO2 at Y−TZP grain boundaries.
Furthermore, the increase in βn0 depends primarily on

1172(1990)
３）J.Wang and R.Raj, J.Am.Ceram.Soc ., 74(8),the increase in γS, which corresponds to the increase in●●1959(1991)

VO . It is, therefore, explained that the sintering rate of４）K.Matsui, N.Ohmichi, M.Ohgai, N.Enomoto, and

3YA is greater than that of 3YC because of the increase

J.Hojo, J.Am.Ceram.Soc., 88(12), 3346(2005)●●

in VO produced by dissolution of Al2O3.

５）K.Matsui, K.Tanaka,, N.Enomoto, and J.Hojo,

J.Ceram.Soc.Jpn., 114(9), 763(2006)６）K.Matsui, T.Yamakawa, M.Uehara, N.Enomoto,

5．Conclusions

and J.Hojo, J.Am.Ceram.Soc., 91(6), 1888(2008)

In the present study, the effects of Al 2O3 and SiO2７）K.Matsui, A.Matsumoto, M.Uehara, N.Enomoto,

addition on initial sintering of Y− TZP powder

and J.Hojo, J.Am.Ceram.Soc., 90(1), 44(2007)

manufactured by the hydrolysis process were

８）K.Matsui, K.Tanaka, T.Yamakawa, M.Uehara,

investigated. The following conclusions were obtained:

N.Enomoto, and J.Hojo, J.Am.Ceram.Soc., 90(2),(1) The sintering rate of Y−TZP powder remarkably443(2007)

increased by a small amount of Al2O3 or SiO2 addition,９）Y.Moriyoshi and W.Komatsu, Yogyo-Kyokai-Shi,
79(10), 370(1971)

and the enhanced sintering effect by Al2O3 addition was
greater than that by SiO 2 addition. Al 2 O 3 or SiO 210）D.L.Johnson, J.Appl.Phys., 40(1), 192(1969)

addition affected little the grain−growth process at the11）Y.Moriyoshi and W.Komatsu, J.Am.Ceram.Soc .,53(12), 671(1970)

hold−time condition in the present sintering profiles.
(2) The nQ

and β0n at the initial sintering stage were

12）D.L.Johnson and I.B.Cutler, J.Am.Ceram.Soc .,46(11), 541(1963)

estimated by applying the sintering−rate equation to
the CRH data. The diffusion mechanism changed from
GBD to VD by Al2O3 or SiO2 addition. Both nQ13）D.L.Johnson and I.B.Cutler, J.Am.Ceram.Soc .,andβ0nincreased in the order of 3YA ＞ 3YC ＞ 3Y. It is,46(11), 545(1963)
14）H.Su and D.L.Johnson, J.Am.Ceram.Soc., 79(12),therefore, concluded that the sintering rate increases in
the order of 3YA ＞ 3YC ＞ 3Y because the increase in
β0n rather than

3211(1996).
15）Y.Ikuhara, Y.Nagai, T.Yamamoto, and T.Sakuma,

Interface Science, 7, 77(1999)

nQ is predominant. This enhanced

sintering mechanism is reasonably interpreted by the16）Y.Ikuhara, P.Thavorniti, and T.Sakuma, Acta.

Mater., 45(12), 5275(1997)

segregated dissolution of Al2O3 or SiO2 at Y−TZP grain17）J.Zhao, Y.Ikuhara, and T.Sakuma, J.Am.Ceram.

boundaries.
(3) The present analytical method derived based on the（ 39 ）

Soc., 81(8), 2087(1998)

40

TOSOH Research ＆ Technology Review Vol.52（2008）18）P.Thavorniti, Y.Ikuhara, and T.Sakuma, J.Am.

Ceram.Soc., 81(11), 2927(1998).
19）T.Yamaguchi, Ceramics Jpn., 19(6), 520(1984)20）Y.Ikuhara, T.Yamamoto, A.Kuwabara, H.Yoshida,
and T.Sakuma, Science and technology ofadvanced materials, 2, 411(2001)21）Y.Sakka, Y.Oishi, and K.Ando, J. Mater. Sci .,17(11), 3101(1982)
22）Y.Sakka, Y.Oishi, K.Ando, and S.Morita, J.Am.

Ceram.Soc., 74(10), 2610(1991).
23）H.Yanagita, K.Kawamoto, and M.Miyayama,
Kesshouchu deno Kakusan (Diffusion in
Crystal) ;pp.125-29 in seramikkusu no kagaku(Chemistry of Ceramics), Edited by H.Yanagita.
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著者

氏名 松 井 光 二
Koji MATSUI入社 昭和61年 4 月 1 日
所属 東京研究所
新材料・無機分野
主席研究員（ 40 ）