Oxide Crystal Growing Facility

Crystal Growers

So far, we have 2 Czochralski crystal growers (Table I-3, Figure I-23) for the AMoRE experiment. Their purpose is to grow high purity CaMoO4, Li2MoO4, and Na2Mo2O7 crystals from highly purified powder. Contamination is a very important issue for the AMoRE experiment, so we are using two growers for growing CaMoO4 (CZ01) and Li2MoO4 (CZ02) crystals separately. In the next year, a new Czochralski crystal grower will be installed for growing Na2Mo2O7 crystals. All growers are equipped with high-resolution load cells (can measure 1/10000 g of crystal weight) and computerized control systems. The growing process is controlled by a computer based on input crystal shape parameters.

Table I-3: Properties of growers

Grower Max temp’ Crucible Remarks
Material Dimension Max load
CZ01 1600 ℃ Iridium Φ10 × 10 cm3 ~ 3 kg For CaMoO4
CZ02 1600 ℃ Platinum Φ10 × 10 cm3 ~ 2.5 kg For Li2MoO4

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Figure I-23: The Czochralski crystal grower, CZ01 (left) and CZ02 (right)

CaMoO4 crystal growth

From 2014 to 2016, we tried to grow CaMoO4 crystals with the CZ01 grower. Raw CaMoO4 powder was synthesized by solid-phase reaction with CaCO3 and MoO3 powders (99.95 %, Alfa Aesar). Sintered powder was measured by an XRD, and the XRD analysis result (Figure I-24) matched well with reference CaMoO4 material.

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Figure I-24: XRD analysis result of sintered CaMoO4 powder

Six CaMoO4 crystals were successfully grown by the CZ01 grower with an iridium crucible. CUP-grown CaMoO4 crystals are shown in Figure I-25 with their properties listed in Table I-4. The crystals are rectangular because the crystal system is tetragonal. Oxygen deficiency causes the dark blue color. After annealing the crystals in the air (Figure 1-25, CZ01-1502 and 1604 after annealing), then the dark blue color was almost gone but a yellowish tint appeared because of impurities.

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Figure I-25: The grown CaMoO4 crystals by the CZ01 grower

Table I-4: Information of grown CaMoO4 crystals

Crystal No. Weight Total Length Body length Diameter Remarks
CZ01-1501 520 g 13 cm 7 cm 3.4 ~ 4.4 cm Used as seed
CZ01-1502 800 g 15 cm 7 cm 4.5 ~ 5.1 cm -
CZ01-1601 800 g 16 cm 7 cm 4.6 ~ 5.1 cm -
CZ01-1602 790 g 16 cm 7 cm 4.3 ~ 5.5 cm -
CZ01-1603 740 g 19 cm (plan) 10 cm (plan) 4.4 ~ 5.2 cm broken
CZ01-1604 1050 g 19 cm 10 cm 4.4 ~ 5.2 cm -

Sintered powder and CUP-grown crystals were measured with ICP-MS to study impurities. The results (see Table I-5) show that all impurities are reduced in CaMoO4 crystals. The reduction factors for K and Fe are about 100, and that for Th and U are about 20 and 70 respectively. Even though the crystals show good reduction factors, the final result is not good enough for the AMoRE experiment yet. At that time we used low-purity powder (99.95 %). Now we can use commercial MoO3 powders which have been further purified through sublimation by our chemical team. With the purified MoO3 powders, the impurity level of CaMoO4 crystals can be further reduced.

Table I-5: ICP-MS results of CaMoO4 crystals and sintered powder (unit is ppb)

C. No. K Cr Fe Zr Ir Pt Al Sr Ba Pb Th U
Powder 41,914 <30 132,737 893 <1 <0.3 44,008 68,289 2,382 228 32 629
1502 - - - 15.8 5.5 <10 <662 51,368 121 12.9 1.3 7.9
1601 471 <30 1,261 19.0 5.0 2.2 160 90,556 137 <1 1.5 9.0
1604 439 24 1,475 18.8 6.1 0.2 <60 59,413 116 6.6 1.3 9.3

Li2MoO4 crystal growth

Another candidate crystal for the AMoRE experiment is Li2MoO4 crystal. For crystal growth, we tried to sinter Li2CO3 and MoO3 powders by solid-phase reaction, as for the sintered CaMoO4 powder, but we could not obtain sintered Li2MoO4 powder. By mixing Li2CO3 and MoO3 powders and melting them in a platinum crucible in the CZ02 grower, we were able to grow Li2MoO4 crystal. Figure I-26 shows XRD analysis results of the CUP-grown Li2MoO4 crystal, and it is well matched with a reference crystal.

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Figure I-26: XRD analysis result of Li2MoO4 crystal

Figure I-27 and Table I-6 show information about CUP-grown Li2MoO4 crystals. We used Li2CO3 powder (99.998%, Alfa Aesar) for all the crystals. From CZ02-L1601 to CZ02-L1702, we used MoO3 powders of 99.95% purity from Alfa Aesar. Because of the low purity of the powders, the colors of the Li2MoO4 crystals are yellowish. From CZ02-L1703 to CZ02-L1707, we used sublimated MoO3 (> 99.999% purity) and the colors of crystals are less yellowish. The CZ02-L1801 crystal was double crystallized, meaning it was grown with material recycled from previous CUP-grown crystal (CZ02-L1704, L1705, L1706 and L1707).

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Figure I-27: The grown Li2MoO4 crystals

Table I-6: Information of grown Li2MoO4 crystals

Crystal No. Weight Total length Body length Diameter Remarks
L1601 470 g 14 cm 5 cm 4.8 ~ 5.2 cm Cracked
L1602 600 g 14 cm 7 cm 5.0 ~ 5.1 cm -
L1701 590 g 14 cm 7 cm 4.6 ~ 5.2 cm -
L1702 590 g 14 cm 7 cm 4.8 ~ 5.2 cm -
L1703 592 g 14 cm 7 cm 5.0 ~ 5.2 cm Sublimed MoO3
L1704 573 g 13 cm - 5.1 ~ 5.2 cm Sublimed MoO3
failed
L1705 648 g 15 cm 8 cm 4.8 ~ 5.1 cm Sublimed MoO3
L1706 657 g 15 cm 8 cm 4.8 ~ 5.1 cm Sublimed MoO3
L1707 641 g 14 cm - 5.0 ~ 5.1 cm Sublimed MoO3
failed
L1801 580 g 14 cm 7 cm 5.1 ~ 5.5 cm D/C

The CUP-grown Li2MoO4 crystals were measured with ICP-MS (Table I-7) and HPGe (Table I-8) in order to study general and radioactive impurities respectively. We did not sinter Li2MoO4 powder, so we cannot directly compare impurity levels of the powder with those of the crystal. ICP-MS results show all impurities significantly reduced. With the exception of Ba, all measured impurities of double-crystallized L1801 crystal were below the detection limit. Radioactivity of 40K in L1602 is about 30 mBq/kg, and the others are below detection limits. All the radioactive impurities are below the detection limits for both L1703, which was grown with sublimated MoO3, and for the double-crystallized L1801

Table I-7: ICP-MS results of grown Li2MoO4 crystals and powders

C. No. Element [ppt]
K Rh Zr Al Sr Ba Pb Th U
Li2CO3
99.998 %
314,524 <1,000 3,283 414,878 8,133 104,045 21,637 <10 615
MoO3
99.95 %
- - - - 23,714 2,829,035 107,027 91 7014
MoO3
sublimed
- - - - 1,670 255,652 66,893 <100 1,080
L1601 529,568 <550 <100 16,802 <15 5,612 <300 <15 <16
L1602 264,578 <550 <100 25,173 <15 5,167 <300 <15 <16
L1701 347,302 <550 <100 48,145 <15 5,445 <300 <15 <16
L1702 478,174 <550 <100 36,481 <15 6,243 <300 <15 <16
L1704
Sublimed
MoO3
107,897 <1,200 <100 39,212 758 7,080 <333 <10 <10
L1705
Sublimed
MoO3
<30,000 <1,000 <100 <20,000 1,005 28,474 <300 <9 <9
L1706
Sublimed
MoO3
38,000 <800 <300 <11,000 <50 7,579 <100 <8 <8
L1707
Sublimed
MoO3
39,000 <800 <300 <11,000 <50 6,300 <100 <8 <8
L1801
D/C
<30,000 <800 <300 <11,000 <50 4,744 <100 <8 <8

Table I-8: HPGe result of Li2MoO4 crystal and powder (unit is mBq/kg)

Crystal No. 238U 40K 228Ac 228Th
Li2CO3
99.998 %
0.95±0.22 9.0±3.4 1.4±0.64 0.41±0.22
MoO3
99.95 %
2,022±180 1,090±220 460±60 145.06±13.74
MoO3
Sublimed
189.0±17.3 73.8±17.6 44.9±7.8 18.1±2.5
L1602
With 99.95 % MoO3
<3.28 28.9±9.7 <2.63 <1.51
L1703
With sublimed MoO3
<0.55 <8.15 <2.10 <0.92
L1801
Double crystallization
<1.2 <13.8 <3.2 <1.3

Recently, we have tested lapping and polishing crystals and Figure I-28 shows polished 1cm3 Li2MoO4 crystals. We tried many different kinds of sanding and polishing materials. So far, using colloidal silica for final polishing shows very good polishing performance but some micro scratches of surface are remaining. We are planning to lap crystals with finer sanding papers

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Figure I-28: Lapped and polished 1 cm3 Li2MoO4 crystals

Na2Mo2O7 crystal growth

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Figure I-29: The grown Na2Mo2O7 crystal (CZ02-N1701, 260 g)

We initially tried to grow Na2Mo2O7 crystal with the CZ02 grower using 99.997 % Na2CO3 and sublimated MoO3, but were barely able to produce one cracked crystal (Figure I-29). Na2Mo2O7 crystal has multiple cleavage planes, so crystals cracked in every trial and fell down to the melt. The NIIC (Nikolaev Institute of Inorganic Chemistry), an institution collaborating with AMoRE on crystal growth, also tried to grow Na2Mo2O7 crystals more than 50 times, but they succeeded for only a few crystals. We will not have an additional Czochralski grower dedicated to Na2Mo2O7 crystal growth until 2019. This crystal is severely affected by thermal stress, so the new CZ03 grower will have reinforced thermal shielding to heat inside of the hot-zone. Figure I-30 and Table I-10 show XRD and impurities results of Na2Mo2O7 crystal. The XRD pattern is well matched with the reference. The impurities are quite reduced in CUP-grown crystal, but they are comparatively higher than impurities in the Li2MoO4 crystals. It looks like we need more purification of the raw powders or double crystallization.

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Figure I-30: XRD analysis result of grown Na2Mo2O7 crystal

Table 1: ICP-MS result of grown Na2Mo2O7 crystal

Crystal No. Element [ppt]
Sr Ba Pb Th U
Na2CO3 22,003 75,174 267,648 <52 <52
Sublimed MoO3 1,670 255,652 66,893 <100 1,080
N1701 3,928 11,908 31,333 <23 <23
LMO1 from NIIC 476,184 8,713 5,711 <23 <23
LMO2 from NIIC 729,837 9,353 6,615 <23 <23