Two types of chemical reactions for crystal growing are discussed. The first is a metathetical reaction to produce calcium tartrate tetrahydrate crystals, the second is a decomplexation reaction to produce cuprous chloride crystals

Rubin, B.

English

NASA Technical Reports Server

September 1970
	
Brief 70-10479 
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nology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Growing Single Crystals in Silica Gel 
Modern applications of materials in electronics 
often require single crystals with controlled purity 
and structural perfection. Various techniques exist 
for growing such crystals from a melt or from solu-
tion. Of these, the methods which require high tem-
peratures are likely to produce less perfect crystals, 
due to the higher frequency of dislocations produced 
by thermal vibration, the increased solubility of im-
purities, and the greater number of point imperfections 
caused by thermal stress during cooling. 
Results have been published of past research on 
the technique of growing crystals at room temper-
ature by controlled diffusion through a silica gel me-
dium. Although this technique is not new, the results 
demonstrate the effect of many variables on the growth 
of single crystals, and show the possibility of using 
the gel technique to grow single crystals of some ma-
terials potentially useful in electronics. 
The first results describe the effects of various 
parameters on the formation of silica gel from a 
tartaric acid solution of sodium metasilicate. In the 
pH range from 3.5 to 8.0, the time required for the 
gel to set is proportional to the hydrogen ion concen-
tration. At room temperature, gelation occurs almost 
immediately at a pH of 7 or more; at a pH of 3.5, it 
takes about three weeks. 
The time required for gelation is also strongly de-
pendent on the temperature,. When the pH 3.5 solu-
tion is incubated at a temperature of 40°C, the gelation 
time is reduced to three days. 
Most important, however, is the observation that 
the nature of the gel itself depends upon pH. Gels 
formed at low acidity tend to be rigid and very vis-
cous, while those formed from more acid solutions 
are softer and more yielding. In order to minimize the
resistance of the medium to growing crystals, the more 
acid solutions are recommended. 
Two types of chemical reaction have been investi-
gated. The first is a metathetical (the ions of two 
soluble compounds exchange partners to produce at 
least one insoluble product) reaction used to produce 
calcium tartrate tetrahydrate crystals, of interest be-
cause of their dielectric properties. 
To grow these crystals, a solution of 21.6 grams of 
sodium metasilicate in 250 cc of water i titrated with 
1 normal tartaric acid until the pH of the mixture 
reaches 3.5. The resulting solution is poured into 
1 x 6-in, test tubes, filling each of them two-thirds full. 
The test tubes are then tightly sealed and incubated 
for three days at 40°C, until gelation is complete. A 
nutrient solution of calcium chloride is then suspended 
on top of the gel. After several hours crystal growth 
begins downward from the gel face and continues for 
several weeks until the reagents are exhausted. It has 
been found experimentally that a IN solution of cal-
cium chloride produces the greatest number of the 
largest (up to 6 x 11 mm) and clearest crystals. 
If doped crystals are desired, the nutrient solution 
may contain the dopant in any desired concentration. 
Neodymium doped calcium tartrate tetrahydrate crys-
tals have been prepared by adding neodymium chlo-
ride to the solution of calcium chloride that is sup-
ported on top of the gel. The light purple color of the 
product confirms the incorporation of neodymium 
into the growing crystals.. 
The second reaction studied is a decomplexation 
reaction (a normally insoluble compound is dissolved 
by formation of a complex, and the crystal is grown 
through the reverse reaction) used to produce cuprous 
chloride crystals.
(collirn end oserleal) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately ownedrights.
https://ntrs.nasa.gov/search.jsp?R=19700000462 2020-03-17T00:31:29+00:00Z
Category 02 
a 
In this case, the gel is prepared at a p1-I of 4 using 3. A report on this project has been published in the 
hydrochloric acid, and the nutrient, a saturated solu- AIChE Journal, Vol. 15 No. 2(March 1969) pp. 206-
tion of cuprous chloride in 6N HCI, is suspended on 209. 
top of the gel. The complex (HxCuClx + j ) diffuses 4. No	 further	 documentation	 is	 available.	 Specific 
into	 the	 gel,. dissociates,	 and	 crystals	 of	 cuprous questions, however, may be directed to: 
chloride (CuCI), tetrahedra up to 9 mm on edge, are Technology Utilization Officer 
grown near the gel-nutrient interface. Headquarters 
Notes: National Aeronautics 
1. The gel technique may be used to grow single and Space Administration 
crystals of many materials which decompose, dis- Washington, D. C. 20546 
sociate, or react with the container material at high Reference: B70-10479 
temperatures.	 It may be used to produce single Patent status: 
crystals	 of other	 materials	 where	 stoichiometric No patent action is contemplated by NASA. 
effects are important. Source:	 Bernard Rubin 
2. This technique may be of interest to persons in- Electronics Research Center 
volved	 in	 growing,	 determining	 the	 properties, (ERC-10306) 
and	 using	 single	 crystals.	 Potential	 applications 
of crystals grown by . controlled diffusion through 
silica gel are in the area of optical modulators and 
infrared window materials.


Growing single crystals in silica gel

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