Oxide Crystal Growing Facility
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
|CZ01||1600 ℃||Iridium||Φ10 × 10 cm3||~ 3 kg||For CaMoO4|
|CZ02||1600 ℃||Platinum||Φ10 × 10 cm3||~ 2.5 kg||For Li2MoO4|
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.
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.
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)
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.
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).
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|
|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|
|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]|
Table I-8: HPGe result of Li2MoO4 crystal and powder (unit is mBq/kg)
With 99.95 % MoO3
With sublimed MoO3
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
Na2Mo2O7 crystal growth
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.
Table 1: ICP-MS result of grown Na2Mo2O7 crystal
|Crystal No.||Element [ppt]|
|LMO1 from NIIC||476,184||8,713||5,711||<23||<23|
|LMO2 from NIIC||729,837||9,353||6,615||<23||<23|