11Development of highly transparent zirconia ceramics
Isao Yamashita ＊ 1
Masayuki Kudo ＊ 1
Koji Tsukuma ＊ 1

Highly transparent zirconia ceramics were developed and their optical and mechanical properties werecomprehensively studied. A low optical haze value (H <1.0 %), defined as the diffuse transmission divided by the total
forward transmission, was achieved by using high-purity powder and a novel sintering process. Theoretical in-linetransmission (74 %) was observed from the ultraviolet–visible region up to the infra-red region; an absorption edge
was found at 350 nm and 8 µm for the ultraviolet and infrared region, respectively. A colorless sintered body havings number (νd = 27.8) was obtained. A remarkably large dielectrica high refractive index (n d = 2.23) and a high Abbe’
constant (ε= 32.7) with low dielectric loss (tanδ= 0.006) was found. Transparent zirconia ceramics are candidates forhigh-refractive index lenses, optoelectric devices and infrared windows. Transparent zirconia ceramics also possess
excellent mechanical properties. Various colored transparent zirconia can be used as exterior components and forcomplex-shaped gemstones.

fabricating transparent cubic zirconia ceramics. 9,13-19

1．Introduction

Transparent zirconia ceramics using titanium oxide asTransparent and translucent ceramics have beena sintering additive were firstly reported by Tsukuma.15

studied extensively ever since the seminal work on

However, the sintered body had poor transparency

1and low mechanical strength. In this study, highly

Subsequently, researchers have conducted manytransparent zirconia ceramics of high strength were2developed. Their optical and mechanical properties

Y2O3, MgAl2O4 (spinel), and Y3Al5O12 (YAG). Various

were comprehensively studied, and the effects of the

optical applications for transparent ceramics havemicrostructure on these properties were discussed.

translucent alumina polycrystal by Coble in the 1960s.

studies to develop transparent ceramics such as MgO,
3456

been proposed, e.g., infrared windows, Nd laser hostmaterials,7 armor applications,8 and optical lenses.9
One recent industrial development of transparent
ceramics was the application of a lens named Lumicera

2．Experimental
Sample preparation

for a slim digital camera developed by Tanaka.10 The use

High-purity 10 mol% yttria-doped zirconia powderof perovskite-based transparent ceramics having a highmade by hydrolysis process 20 was used as a starting

refractive index reduced the focal length. Reference 11powder. As a sintering additive, 10 mol% of titanium

12and open web information give comprehensive reviews

oxide with respect to zirconia was used. The startingof transparent and translucent ceramics.

powder was compacted into a green body, andWith their high refractive index, yttria-stabilized

transparent zirconia was obtained by a subsequentzirconia (YSZ) single crystals are widely used as

novel high-densification process. 21

artificial gemstones. YSZ ceramics crystallize into

Optical properties

cubic structure and are candidates for transparent

The sample material was ground to a thickness of 1

ceramics. Thus, adequate effort has been dedicated tomm and thoroughly polished on both sides to eliminate＊1

surface scattering in subsequent measurements. TheTokyo Research Laboratory

TOSOH Research & Technology Review Vol.56（2012）

12in-line transmission of the ultraviolet–visible (UV–VIS) region was measured using a double-beamspectrophotometer (V-650, JASCO Corporation, Tokyo,Japan) with a wavelength range of 200–800 nm. The
infrared (IR) spectrum was measured using a Fouriertransform infrared spectrometer (IR Prestige-21,Shimadzu, Japan) with a range of 3–9 µm.
As the absolute value of transmission includes theeffects of surface reflections, the optical haze value

H , defined as the diffuse transmission divided by thetotal forward transmission, is widely used to evaluate
the transparency of transparent materials. The opticalFig.１

haze value at the D65 sources was evaluated using

Transparent zirconia ceramics (20 mm in diameterand 1 mm in thickness).

a haze meter (NDH 5000, Nihon Denshoku, Tokyo,Japan). The refractive index was measured using aTable1

spectroscopic ellipsometer (M-2000V, J. A. Woollam

Mechanical and optical properties oftransparent zirconia ceramics

DensityThree−point bending strength
Hardness［Hv10］
Young s modulus
Poisson s ratio
Fracture toughness
Dielectric constant［1 MHz］
Dielectric loss［1 MHz］
In−line transmission［1 mmt, D65］
Optical haze value［1 mmt, D65］
Refractive index［nd］
Abbe s number［νd］

Japan, Tokyo, Japan) from 370 to 1000 nm. The
refractive index was fitted to the Cauchy equation; theAbbe’s number νd was calculated using the data at n d(587.56 nm), n F (486.1 nm), and n C (656.3 nm). The
dielectric constant and the dielectric loss tangent at 1
MHz were also measured by the impedance method atthe room temperature.
Mechanical properties
The microstructure of the transparent zirconia was5.83 g ／ cm3250−300 MPa
1200200 GPa
0.32
1.8 MPa・m0.5
32.70.00674 ％
＜1 ％
2.23
27.8

estimated by scanning electron microscopy (SEM)(JSM-5400, JEOL Corporation, Tokyo, Japan), and thezirconia ceramics are tabulated in Table 1. The final

grain size was estimated by the intercept method. A

sintered body crystallizes into a single cubic phase withcorrection factor of 1.56 was used in the calculation.

a fluoride structure (Fm3m ). The lattice parameter as

The three-point bending strength was measured in line

determined by the Rietveld analysis is a = 0.51216 (1)with JIS-R-1601; a gauge length of 30 mm and a cross-nm. Because of the smaller Ti ionic radii, the latticehead speed of 0.5 mm/min were applied. The fracture

parameter of transparent zirconia is 0.5 % shorter than

toughness and hardness were estimated by indentationthat of 10 mol% of yttria-doped zirconia (a = 0.51447 (2)following JIS-R-1607. Young’s modulus and Poison’snm).

ratio were measured by the plus method in line with

Density of the sintered body is 5.83 g/cm 3 asascertained by the Archimedean method. Hardness,JIS-R-1602.

Young modulus, Poisson’s ratio, and fracture toughness

3．Results and discussion

were determined to be Hv = 1200, 200 GPa, 0.32, and 1.8
M·Pa m0.5, respectively. This set of values is comparable

Figure 1 shows the appearances of the transparent

to that of YSZ ceramics such as 8Y ceramics. 22 The

zirconia ceramics obtained. The sample size is 20 mm

three-point bending strength varies between 250 andin diameter and 1 mm in thickness. Although a slightly

300 MPa as a function of its microstructure that will beyellowish color has been reported for titania-dopeddiscussed later.

15transparent zirconia, a highly transparent and colorless

High in-line transmission with low optical hazesintered body was obtained in our experiment. The

was observed in the transparent zirconia ceramics.

optical and mechanical properties of the transparent

Figure 2 shows the wavelength dependence of in-80

60

40

200200Transparent zirconia ceramics
Single crystal
300

400

500

600700

第 56 巻（2012）1380

In−line transmission（％，1mmt）

In−line transmission（％, 1mmt）

東ソー研究・技術報告6040

2003800

4Fig.２56789Wavelength λ（µm）

Wavelength λ（nm）

In−line transmission of transparent zirconia ceramics
and YSZ single crystal. The solid line represents
transparent zirconia, the dashed line represents YSZ
single crystal.

Infrared transmission of transparent zirconia ceramics.

Fig.３

transparent up to the IR region. As the absorption edgeof the IR region is created by lattice vibration, the shiftline transmission of transparent ceramics and 10 mol%toward a higher wavelength might be attributed to theyttria-stabilized zirconia single crystal with a thickness

massive effect of heavy Zr, Y atoms.

of 1 mm. The transmission of transparent ceramics

A large dielectric constant ( ε= 32.7) with low

is comparable to that of a single crystal in the visible〜 26 %) resultingregion. A large reflection loss ( ＝dielectric loss (tanδ = 0.006) was found at 1 MHz. No
significant change was observed for the dielectricfrom a high refractive index was observed for bothconstant and dielectric loss up to 60 MHz. Figure 4

the single crystal and the ceramics. Theoretical in-line

shows the dielectric constant of typical transparent

transmission (74 %) was achieved for the transparent

inorganic materials at 1 MHz. It was found thatzirconia ceramics, with the optical haze value estimatedtransparent zirconia ceramics have a large dielectric

to be H < 1.0 % at the D65 sources. No remarkable

constant as compared with other transparent materials.

change with respect to wavelength, which causes a

The dielectric constant of electron polarization can becolor hue in ceramics, was observed in the visible

expressed by the square of its refractive index (n 2). Asregion.

the observed dielectric constant is much larger than theThe in-line transmission reached that of a single

estimated (n 2 = 4.79), another polarization mechanism

crystal in the visible region, whereas a large discrepancy

is suspected in transparent zirconia ceramics. It is well

was observed in the UV region. The absorption edge

known that zirconia is a typical ionic crystal. Therefore,of a single crystal was estimated to be 250 nm, which isionic polarization may be responsible for the large

almost comparable with the wavelength as calculated

dielectric constant.

from a band gap of 4.2 eV (295 nm).

23On the otherhand, a shift in the absorption edge toward a longer

The refractive index as estimated by ellipsometry canbe fitted to a Cauchy equation as follows.

wavelength was detected in the zirconia ceramics.
40

compared with that of zirconia. By substituting Ti atoms

35

into Zr, the Y cation site, the electronic band structureis changed.
The in-line transmission of the IR region is shownin Fig. 3. Transmission gradually decreases with anincrease in the wavelength, and an absorption edge isobserved near 8 µm. The absorption edges of typical

Dielectric constant（−）

Titania (rutile) has a narrow band gap of 3.2 eV as

302520151050transparent materials are 2.7 µm, 3.5 µm, and 6 µm forsoda-lime glass, quartz, and sapphire, respectively.
It is noteworthy that transparent zirconia is highly

Fig.４

QuartzSoda−lime−
glass

Sapphire

Transparent
zirconia

Dielectric constant of typical transparent inorganic
materials at 1 MHz.

TOSOH Research & Technology Review Vol.56（2012）

14n ＝A ＋BC2 ＋4（λ＝0.370−1.000 µm）λλ（1）, where A = 2.165, B = 0.01859, and C = 0.00070606.

σf＝σ0 ＋k（3）d√Here, σf and d are the bending strength and grain

Abbe’s number ν d is defined using the refractivesize, whereas k and σ0 are constant. A similar relationindex of the d-line (587.56 nm), F-line (486.1 nm), andwas reported for translucent alumina ceramics.24 Being

C-line (656.3 nm).

similar to Grifith’s expression, flaws having a similarν d ＝ n d−1n F−nC（2）grain size may be responsible for the fracture origin.
The relation between optical haze value H and grain

The Abbe’s number represents the dispersion ofsize is shown in Fig. 8. Transparency strongly depends

light − the larger the Abbe’s number, the smaller theon grain size, and transparency degrades as the grain

dispersion. Generally, a higher refractive index reduces

size decreases. Translucent ceramics having a non-the Abbe’s number. A larger Abbe’s number causes

cubic structure, such as alumina (hexagonal structure),

an aberration of the optical system. Thus, a higher

show similar behavior with respect to grain size. In-refractive index with a low Abbe’s number is required

line transmission increases with a decrease in grainfor optical lenses.

size, which is due to the effect of birefringence at theFigure 5 shows variations in the refractive index andgrain boundary.25, 26 As transparent zirconia ceramics

the Abbe’s number of optical glass and transparent

crystallize into a cubic structure, an absence ofzirconia ceramics. Commercial transparent ceramics

birefringence at the grain boundary must be considered.

(Lumicera, Murata Co., Japan) and glass (K-PSFn173,However, Fig. 8 implies another candidate for scattering

Sumita Optical Glass Inc., Japan) with a high refractive

origin at the grain boundary.

index were also plotted. Transparent zirconia ceramics
show an excellent relation of the refractive index withrespect to the Abbe’s number, i.e., a high refractive
index with a low Abbe’s number.
Figure 6 shows the microstructure of transparent
ceramics. A uniform grain structure can be observed,
and no abnormal grain growth was detected. It was
found that the three-point bending strength(σf) strongly
depends on the grain size (d ). The bending strength
increases with a decrease in the grain size. Figure 7
shows the relation between the bending strength andgrain size. The bending strength obeys empirical 1/d 0.5
rules 24 as follows:

Fig.６

×1000

10kV

Typical microstructure of transparent zirconia ceramics.

2.2

Transparent
Zirconia

2.1

Lumicera

K−PSFn173

2.0
1.9

Conventional
Optical glass

1.8

SF03

1.7
1.6
1.5

Bending strength σ f（MPa）

320

2.3

（17.7 µm）

907050Abbe s number v d3010Relation between refractive index (nd) and Abbe s number(v d) of optical glass and transparent ceramics.

（22.5 µm）

300（27.3 µm）
280

260 （69.8 µm）
0.1

1.4

Fig.５30μm

0.15

0.2

1／ 0.5（µm−0.5）

0.25

東ソー研究・技術報告第 56 巻（2012）

6100

t ＝1 mm

Haze value at 550 nm（％）

5432
10204060

10.1

rpore＝0.01 µm

0.010.180

0.1 µm10d （µm）

Fig.８

150.5 151050 100

500 1000

Vpore（ppm）

Effect of the grain size (d) on the optical haze value(H) of transparent zirconia ceramics.

Fig.10Relation between optical haze H and porosity Vpore asa function of the pore radius rpore at 550 nm, assumingt = 1 mm28. Haze value H is plotted from 0.01 µm to0.1 µm at 0.01 µm intervals.

Figure 9 shows a TEM micrograph of the grainboundary of a transparent zirconia ceramics. Residualof scattering must be negligible because scattering

pores selectively migrate at the grain boundaries. The

efficiency Q sca reaches zero for visible light.18 Thus, thepore size is estimated to be < 100 nm. Pores locatedelimination of pores with a visible wavelength scale isat the grain boundary can be easily eliminated during

essential for transparency.

densification through grain growth. On the otherFigure 11 shows various transparent zirconia

hand, pores entrapped in grains are not expected toceramics including colored zirconia ceramics. High27transparency with various colors can be obtained byLocalized residual pores on the grain boundary imply

the addition of pigments, such as lanthanoid oxide. The

well-controlled densification of transparent zirconia

hue can be changed by using a suitable set of pigments,ceramics.

thereby changing the concentration of the pigment, andbe removed-even by pressure-assisted sintering.

The effect of residual pores on transparency is shown28

the size of the sintered body. As transparent zirconia

Optical haze H is theoretically calculated

can be molded using a ceramic injection molding (CIM)

using a Mie scattering problem. According to thetechnique, a complex-shaped body is available. Exterior

in Fig. 10.

28

findings of this study, porosity with optical haze H = 1

parts and complex-shaped gem are suitable applications% is estimated to be V pore < 10 ppm when d pore=100 nm

for transparent zirconia ceramics.

is assumed, where V pore, d pore are porosity and pore size,respectively. Many small pores of size d pore <10 nm weredetected on the grain boundary. However, the effectConclusion
Highly transparent zirconia ceramics were

50 nm

2 cm

Fig.11
Fig.９

TEM micrograph of residual pores at grain boundary intransparent zirconia ceramics.

Various transparent zirconia ceramics. The small
convex−shaped ceramics were made by a ceramic−injection−molding technique. The others werefabricated by mechanical−pressing methods.

TOSOH Research & Technology Review Vol.56（2012）16

developed and their optical and mechanical properties
were comprehensively studied. Theoretical in-line
transmission (74 %) was achieved from UV–VIS up tothe IR region, and an absorption edge was found at 350

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compared with other transparent materials. Thus,transparent zirconia ceramics are candidates for highrefractive index lenses, optoelectric devices andinfrared windows. Transparent zirconia ceramics also
possess excellent mechanical properties. Various
colored transparent zirconia can be used as exterior
components and for complex-shaped gemstones.

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